Special and\u00a0General Senses<\/h1>\r\n<\/div>\r\nThe\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0[pb_glossary id=\"3122\"]Special senses[\/pb_glossary]<\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0[pb_glossary id=\"3123\"]General senses[\/pb_glossary],<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.\r\n\r\nSensory Receptors<\/h1>\r\n<\/div>\r\nA\u00a0[pb_glossary id=\"3006\"]sensory receptor[\/pb_glossary]<\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.\r\n\r\nThere are several different types of sensory receptors that respond to different kinds of stimuli:\r\n\r\n \t- [pb_glossary id=\"3124\"]Mechanoreceptors[\/pb_glossary]<\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\r\n \t
- [pb_glossary id=\"3125\"]Thermoreceptors[\/pb_glossary]<\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\r\n \t
- [pb_glossary id=\"3126\"]Nociceptors[\/pb_glossary]<\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\r\n \t
- [pb_glossary id=\"3127\"]Photoreceptors[\/pb_glossary]<\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\r\n \t
- [pb_glossary id=\"3128\"]Chemoreceptors[\/pb_glossary]<\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\r\n<\/ul>\r\n\r\n
Touch<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3129\"]Touch[\/pb_glossary]<\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.\r\n\r\n[caption id=\"attachment_3130\" align=\"aligncenter\" width=\"787\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Skin_TactileReceptors-2.png\")
Figure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3131\" align=\"alignright\" width=\"216\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Eye-by-Victor-Freitas-on-Unsplash-scaled-3.jpg\")
Figure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em>[\/caption]\r\n\r\nVision<\/span>\r\n\r\n<\/div>\r\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.\r\nHow the Eye Works<\/h2>\r\nFigure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye's functions are summarized in the following steps.\r\n\r\n \t- Light passes first through the\u00a0[pb_glossary id=\"5953\"]cornea[\/pb_glossary]<\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\r\n \t
- Next, light enters the interior of the eye through an opening called the\u00a0[pb_glossary id=\"3134\"]pupil[\/pb_glossary]<\/strong>. The size of this opening is controlled by the coloured part of the eye (called the [pb_glossary id=\"3135\"]iris[\/pb_glossary]<\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called [pb_glossary id=\"5895\"]aqueous humor[\/pb_glossary] and functions to maintain the shape of the eye.<\/li>\r\n \t
- The light then passes through the\u00a0[pb_glossary id=\"3137\"]lens[\/pb_glossary]<\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called [pb_glossary id=\"3138\"]vitreous humor[\/pb_glossary], which functions to maintain the shape of the eye.<\/li>\r\n \t
- The\u00a0[pb_glossary id=\"3139\"]retina[\/pb_glossary]<\/strong> contains two types of photoreceptors: rod and cone cells . [pb_glossary id=\"3140\"]Rod cells[\/pb_glossary]<\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0[pb_glossary id=\"3141\"]Cone cells[\/pb_glossary]<\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\r\n \t
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0[pb_glossary id=\"3142\"]optic disc[\/pb_glossary] <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_3132\" align=\"alignnone\" width=\"675\"]
![\"Diagram](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/EyeAnatomy_01-2.png\")
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em>[\/caption]\r\nColour Vision<\/h2>\r\nHumans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:\r\n\r\n[h5p id=\"565\"]\r\n\r\nFigure 8.7.5 RGB colours.\u00a0<\/em>\r\nRole of the Brain in Vision<\/h2>\r\nThe optic nerves from both eyes meet and cross just below the bottom of the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] in the brain. The information from both eyes is sent to the visual cortex in the [pb_glossary id=\"3090\"]occipital lobe[\/pb_glossary] of the [pb_glossary id=\"5941\"]cerebrum[\/pb_glossary], which is part of the [pb_glossary id=\"5937\"]cerebral cortex[\/pb_glossary]. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.\r\nVision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"]![\"Vision](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Glasses-by-dmitry-ratushny-wpi3sDUrSEk-unsplash-scaled-3.jpg\")
Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"]![\"Myopia](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Myopia_Diagram-2.jpg\")
Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"]![\"Hyperopia\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Hyperopia-2.gif\")
Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/AnatomyHumanEar-2.gif\")
Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Taste_bud_2_eng.svg_-2.png\")
Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"]![\"Olfactory](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Head_olfactory_nerve-2.jpg\")
Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\n![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Bee-Stereogram-2.jpg\")
Figure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Sensory Receptors<\/h1>\r\n<\/div>\r\nA\u00a0[pb_glossary id=\"3006\"]sensory receptor[\/pb_glossary]<\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.\r\n\r\nThere are several different types of sensory receptors that respond to different kinds of stimuli:\r\n\r\n \t- [pb_glossary id=\"3124\"]Mechanoreceptors[\/pb_glossary]<\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\r\n \t
- [pb_glossary id=\"3125\"]Thermoreceptors[\/pb_glossary]<\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\r\n \t
- [pb_glossary id=\"3126\"]Nociceptors[\/pb_glossary]<\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\r\n \t
- [pb_glossary id=\"3127\"]Photoreceptors[\/pb_glossary]<\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\r\n \t
- [pb_glossary id=\"3128\"]Chemoreceptors[\/pb_glossary]<\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\r\n<\/ul>\r\n\r\n
Touch<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3129\"]Touch[\/pb_glossary]<\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.\r\n\r\n[caption id=\"attachment_3130\" align=\"aligncenter\" width=\"787\"] Figure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3131\" align=\"alignright\" width=\"216\"] Figure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em>[\/caption]\r\n\r\nVision<\/span>\r\n\r\n<\/div>\r\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.\r\nHow the Eye Works<\/h2>\r\nFigure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye's functions are summarized in the following steps.\r\n\r\n \t- Light passes first through the\u00a0[pb_glossary id=\"5953\"]cornea[\/pb_glossary]<\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\r\n \t
- Next, light enters the interior of the eye through an opening called the\u00a0[pb_glossary id=\"3134\"]pupil[\/pb_glossary]<\/strong>. The size of this opening is controlled by the coloured part of the eye (called the [pb_glossary id=\"3135\"]iris[\/pb_glossary]<\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called [pb_glossary id=\"5895\"]aqueous humor[\/pb_glossary] and functions to maintain the shape of the eye.<\/li>\r\n \t
- The light then passes through the\u00a0[pb_glossary id=\"3137\"]lens[\/pb_glossary]<\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called [pb_glossary id=\"3138\"]vitreous humor[\/pb_glossary], which functions to maintain the shape of the eye.<\/li>\r\n \t
- The\u00a0[pb_glossary id=\"3139\"]retina[\/pb_glossary]<\/strong> contains two types of photoreceptors: rod and cone cells . [pb_glossary id=\"3140\"]Rod cells[\/pb_glossary]<\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0[pb_glossary id=\"3141\"]Cone cells[\/pb_glossary]<\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\r\n \t
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0[pb_glossary id=\"3142\"]optic disc[\/pb_glossary] <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_3132\" align=\"alignnone\" width=\"675\"] Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em>[\/caption]\r\n
Colour Vision<\/h2>\r\nHumans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:\r\n\r\n[h5p id=\"565\"]\r\n\r\nFigure 8.7.5 RGB colours.\u00a0<\/em>\r\nRole of the Brain in Vision<\/h2>\r\nThe optic nerves from both eyes meet and cross just below the bottom of the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] in the brain. The information from both eyes is sent to the visual cortex in the [pb_glossary id=\"3090\"]occipital lobe[\/pb_glossary] of the [pb_glossary id=\"5941\"]cerebrum[\/pb_glossary], which is part of the [pb_glossary id=\"5937\"]cerebral cortex[\/pb_glossary]. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.\r\nVision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"] Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"] Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Touch<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3129\"]Touch[\/pb_glossary]<\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.\r\n\r\n[caption id=\"attachment_3130\" align=\"aligncenter\" width=\"787\"] Figure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3131\" align=\"alignright\" width=\"216\"] Figure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em>[\/caption]\r\n\r\nVision<\/span>\r\n\r\n<\/div>\r\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.\r\nHow the Eye Works<\/h2>\r\nFigure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye's functions are summarized in the following steps.\r\n\r\n \t- Light passes first through the\u00a0[pb_glossary id=\"5953\"]cornea[\/pb_glossary]<\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\r\n \t
- Next, light enters the interior of the eye through an opening called the\u00a0[pb_glossary id=\"3134\"]pupil[\/pb_glossary]<\/strong>. The size of this opening is controlled by the coloured part of the eye (called the [pb_glossary id=\"3135\"]iris[\/pb_glossary]<\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called [pb_glossary id=\"5895\"]aqueous humor[\/pb_glossary] and functions to maintain the shape of the eye.<\/li>\r\n \t
- The light then passes through the\u00a0[pb_glossary id=\"3137\"]lens[\/pb_glossary]<\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called [pb_glossary id=\"3138\"]vitreous humor[\/pb_glossary], which functions to maintain the shape of the eye.<\/li>\r\n \t
- The\u00a0[pb_glossary id=\"3139\"]retina[\/pb_glossary]<\/strong> contains two types of photoreceptors: rod and cone cells . [pb_glossary id=\"3140\"]Rod cells[\/pb_glossary]<\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0[pb_glossary id=\"3141\"]Cone cells[\/pb_glossary]<\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\r\n \t
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0[pb_glossary id=\"3142\"]optic disc[\/pb_glossary] <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_3132\" align=\"alignnone\" width=\"675\"] Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em>[\/caption]\r\n
Colour Vision<\/h2>\r\nHumans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:\r\n\r\n[h5p id=\"565\"]\r\n\r\nFigure 8.7.5 RGB colours.\u00a0<\/em>\r\nRole of the Brain in Vision<\/h2>\r\nThe optic nerves from both eyes meet and cross just below the bottom of the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] in the brain. The information from both eyes is sent to the visual cortex in the [pb_glossary id=\"3090\"]occipital lobe[\/pb_glossary] of the [pb_glossary id=\"5941\"]cerebrum[\/pb_glossary], which is part of the [pb_glossary id=\"5937\"]cerebral cortex[\/pb_glossary]. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.\r\nVision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"] Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"] Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Figure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em>[\/caption]\r\n\r\nVision<\/span>\r\n\r\n<\/div>\r\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.\r\nHow the Eye Works<\/h2>\r\nFigure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye's functions are summarized in the following steps.\r\n\r\n \t- Light passes first through the\u00a0[pb_glossary id=\"5953\"]cornea[\/pb_glossary]<\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\r\n \t
- Next, light enters the interior of the eye through an opening called the\u00a0[pb_glossary id=\"3134\"]pupil[\/pb_glossary]<\/strong>. The size of this opening is controlled by the coloured part of the eye (called the [pb_glossary id=\"3135\"]iris[\/pb_glossary]<\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called [pb_glossary id=\"5895\"]aqueous humor[\/pb_glossary] and functions to maintain the shape of the eye.<\/li>\r\n \t
- The light then passes through the\u00a0[pb_glossary id=\"3137\"]lens[\/pb_glossary]<\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called [pb_glossary id=\"3138\"]vitreous humor[\/pb_glossary], which functions to maintain the shape of the eye.<\/li>\r\n \t
- The\u00a0[pb_glossary id=\"3139\"]retina[\/pb_glossary]<\/strong> contains two types of photoreceptors: rod and cone cells . [pb_glossary id=\"3140\"]Rod cells[\/pb_glossary]<\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0[pb_glossary id=\"3141\"]Cone cells[\/pb_glossary]<\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\r\n \t
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0[pb_glossary id=\"3142\"]optic disc[\/pb_glossary] <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_3132\" align=\"alignnone\" width=\"675\"] Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em>[\/caption]\r\n
Colour Vision<\/h2>\r\nHumans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:\r\n\r\n[h5p id=\"565\"]\r\n\r\nFigure 8.7.5 RGB colours.\u00a0<\/em>\r\nRole of the Brain in Vision<\/h2>\r\nThe optic nerves from both eyes meet and cross just below the bottom of the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] in the brain. The information from both eyes is sent to the visual cortex in the [pb_glossary id=\"3090\"]occipital lobe[\/pb_glossary] of the [pb_glossary id=\"5941\"]cerebrum[\/pb_glossary], which is part of the [pb_glossary id=\"5937\"]cerebral cortex[\/pb_glossary]. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.\r\nVision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"] Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"] Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em>[\/caption]\r\nColour Vision<\/h2>\r\nHumans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:\r\n\r\n[h5p id=\"565\"]\r\n\r\nFigure 8.7.5 RGB colours.\u00a0<\/em>\r\nRole of the Brain in Vision<\/h2>\r\nThe optic nerves from both eyes meet and cross just below the bottom of the [pb_glossary id=\"2937\"]hypothalamus[\/pb_glossary] in the brain. The information from both eyes is sent to the visual cortex in the [pb_glossary id=\"3090\"]occipital lobe[\/pb_glossary] of the [pb_glossary id=\"5941\"]cerebrum[\/pb_glossary], which is part of the [pb_glossary id=\"5937\"]cerebral cortex[\/pb_glossary]. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.\r\nVision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"] Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"] Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Vision Problems<\/h2>\r\n[caption id=\"attachment_3146\" align=\"alignright\" width=\"279\"] Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em>[\/caption]\r\n\r\nVision problems are very common. Two of the most common are [pb_glossary id=\"3144\"]myopia[\/pb_glossary] and [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary], and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.\r\nMyopia<\/h3>\r\n[caption id=\"attachment_3147\" align=\"alignleft\" width=\"398\"] Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3144\"]Myopia<\/strong>[\/pb_glossary] (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.\r\nHyperopia<\/h3>\r\n[caption id=\"attachment_3148\" align=\"alignright\" width=\"400\"] Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em>[\/caption]\r\n\r\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.\r\n<\/div>\r\nPresbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Presbyopia<\/h3>\r\nPresbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.\r\n\r\nHearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Hearing<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3151\"]Hearing[\/pb_glossary]<\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0[pb_glossary id=\"5977\"]ear[\/pb_glossary]<\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The [pb_glossary id=\"5949\"]cochlea[\/pb_glossary]<\/strong>\u00a0is a coiled tube filled with\u00a0liquid. The liquid moves in response to the vibrations, causing tiny\u00a0hair\u00a0cells(which are [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and \u201ctells\u201d us what we are hearing.\r\n\r\n[caption id=\"attachment_3154\" align=\"aligncenter\" width=\"480\"] Figure 8.7.9 Most of the structures of the ear are involved in hearing. Only the semicircular canals are not involved in hearing. Instead, they sense head position, which is used to monitor balance.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\nBalance<\/span>\r\n\r\n<\/div>\r\nThe ears are also responsible for the sense of balance.\u00a0[pb_glossary id=\"3155\"]Balance[\/pb_glossary]<\/strong>\u00a0is the ability to sense and maintain an appropriate body position. The [pb_glossary id=\"3156\"]semicircular canals[\/pb_glossary] inside the ear (see the figure\u00a0above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the\u00a0peripheral nervous system, which triggers contractions of\u00a0skeletal muscles\u00a0as needed to maintain balance.\r\n\r\nTaste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Taste and\u00a0Smell<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"3157\"]Taste<\/strong>[\/pb_glossary] and [pb_glossary id=\"3158\"]smell<\/strong>[\/pb_glossary]\u00a0are both abilities to sense chemicals, so both taste and olfactory (odor) receptors are [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain \u201ctells\u201d us what we are tasting or smelling.\r\n\r\nTaste receptors are found in tiny bumps on the tongue called\u00a0[pb_glossary id=\"3159\"]taste buds[\/pb_glossary].<\/strong>You can see a diagram of a taste receptor cell and related structures in Figure 8.7.10. Taste receptor cells make contact with chemicals in food through tiny openings called [pb_glossary id=\"3160\"]taste pores[\/pb_glossary]<\/strong>. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty \u2014 or savory \u2014 taste is called umami.\r\n\r\n[caption id=\"attachment_3161\" align=\"aligncenter\" width=\"532\"] Figure 8.7.10 Taste receptor cells are located in taste buds on the tongue. Basal cells are not involved in tasting, but differentiate into taste receptor cells. Taste receptor cells are replaced about every nine to ten days.<\/em>[\/caption]\r\n\r\n\r\n\r\n[caption id=\"attachment_3162\" align=\"aligncenter\" width=\"518\"] Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
Figure 8.7.11 The yellow structures inside this drawing of the nasal passages are an olfactory nerve with many nerve endings. The nerve endings sense chemicals in the air as it passes through the nasal cavities.<\/em>[\/caption]\r\n\r\n<\/div>\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nThe most common cause of blindness in the Western hemisphere is\u00a0age-related macular degeneration<\/a> (AMD).<\/strong> Approximately 1.4 million people in Canada have this type of blindness, and 196 million people are affected worldwide and is expected to increase to 288 millions people by the year 2040. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina\u2019s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.\r\n\r\nIn 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient\u2019s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from\u00a0[pb_glossary id=\"3163\"]stem cells[\/pb_glossary],<\/strong>\u00a0which are undifferentiated cells that\u00a0can\u00a0develop into other cell types.\u00a0Within\u00a0six months\u00a0of\u00a0the operation, the new cells were still surviving, and the doctor was hopeful that the patient\u2019s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery, and that it will prevent millions of patients from losing their vision.\r\n\r\n8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
8.7 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- \r\n
\r\n \t- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
\r\n \t- \r\n
\r\n \t- \r\n
\r\n \t- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\nReferences<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\nda Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\nFile:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\nSeeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\nSeeing Is Believing<\/span><\/p>\nAt first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\nThere are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
\n- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n\n
Touch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\nFigure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n\nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\nVision<\/span><\/p>\n<\/div>\nVision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\nHow the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
\n- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\nColour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\nRole of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\nVision Problems<\/h2>\nFigure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\nVision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\nMyopia<\/h3>\nFigure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\nMyopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\nHyperopia<\/h3>\nFigure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\nHyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
- \r\n \t
- The\u00a0human body\u00a0has two major types of senses: [pb_glossary id=\"3122\"]special senses[\/pb_glossary] and [pb_glossary id=\"3123\"]general senses[\/pb_glossary]. Special senses have specialized sense organs and include [pb_glossary id=\"3164\"]vision[\/pb_glossary] (eyes), [pb_glossary id=\"3151\"]hearing[\/pb_glossary] (ears), [pb_glossary id=\"3155\"]balance[\/pb_glossary] (ears), [pb_glossary id=\"3157\"]taste[\/pb_glossary] (tongue), and [pb_glossary id=\"3158\"]smell[\/pb_glossary] (nasal passages). General senses are all associated with [pb_glossary id=\"3129\"]touch[\/pb_glossary] and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin.<\/li>\r\n \t
- All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include [pb_glossary id=\"3124\"]mechanoreceptors[\/pb_glossary]\u00a0(mechanical forces), [pb_glossary id=\"3125\"]thermoreceptors[\/pb_glossary]\u00a0(temperature), [pb_glossary id=\"3126\"]nociceptors[\/pb_glossary]\u00a0(pain), [pb_glossary id=\"3127\"]photoreceptors[\/pb_glossary]\u00a0(light), and [pb_glossary id=\"3128\"]chemoreceptors[\/pb_glossary]\u00a0(chemicals).<\/li>\r\n \t
- Touch\u00a0is\u00a0the ability to sense pressure, vibration,\u00a0temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.<\/li>\r\n \t
- Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and \u201ctells\u201d us what we are seeing.<\/li>\r\n \t
- Common vision problems include [pb_glossary id=\"3144\"]myopia[\/pb_glossary] (nearsightedness), [pb_glossary id=\"3145\"]hyperopia[\/pb_glossary] (farsightedness), and [pb_glossary id=\"3165\"]presbyopia[\/pb_glossary] (age-related decline in close vision). Vision problems can be corrected with lenses (eyeglasses or contacts) or \u2014 in many cases \u2014 with laser surgery.<\/li>\r\n \t
- Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and \u201ctells\u201d us what we are hearing.<\/li>\r\n \t
- The ear is also the organ responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses about head position to the brain, which sends messages to\u00a0skeletal muscles\u00a0via the\u00a0peripheral nervous system. The\u00a0muscles\u00a0respond by contracting to maintain balance.<\/li>\r\n \t
- Taste and smell\u00a0are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food,\u00a0while\u00a0olfactory receptors in the nasal passages sense chemicals in the air.\u00a0Sense of smell contributes significantly to sense of taste.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 8.7 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- \r\n
- \r\n \t
- Compare and contrast special senses and general senses.<\/li>\r\n \t
- What are sensory receptors?<\/li>\r\n \t
- [h5p id=\"566\"]<\/li>\r\n \t
- Describe the\u00a0range of tactile stimuli detected in the sense of touch.<\/li>\r\n \t
- Explain how the eye collects and focuses light to form an image, and how it converts it to nerve impulses.<\/li>\r\n \t
- Identify two common vision problems,along with\u00a0their causes and their effects on vision.<\/li>\r\n \t
- [h5p id=\"567\"]<\/li>\r\n \t
- Explain how structures of the ear collect and amplify sound waves and transform them to nerve impulses.<\/li>\r\n \t
- What role does the ear play in balance? Which structures of the ear are involved in balance?<\/li>\r\n \t
- Describe two ways that the body senses chemicals. What are the special sense organs involved in these senses?<\/li>\r\n \t
- Explain why your skin can detect different types of stimuli, such as pressure and temperature.<\/li>\r\n \t
- Is sensory information sent to the central nervous system via efferent or afferent nerves?<\/li>\r\n \t
- Identify a mechanoreceptor used in two different human senses. Describe the type of mechanical stimuli that each detects.<\/li>\r\n \t
- If a person is blind, but their retina is functioning properly, where do you think the damage might be? Explain your answer.<\/li>\r\n \t
- When you see colours, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?<\/li>\r\n \t
- The auditory nerve carries _______________.<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n
- \r\n \t
- \r\n
- \r\n \t
- \r\n
- \r\n \t
- smell information<\/li>\r\n \t
- taste information<\/li>\r\n \t
- balance information<\/li>\r\n \t
- sound information<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 8.7 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?time_continue=4&v=rkRbebvoYqI&feature=emb_logo\r\nWhat color is Tuesday? Exploring synesthesia - Richard E. Cytowic, TED-Ed, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=emb_logo\r\n
What Is Vertigo & Why Do We Get It?, Seeker, 2016.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA\r\n
How do animals see in the dark? - Anna St\u00f6ckl, TED-Ed,\u00a0 2016.<\/p>\r\nhttps:\/\/youtu.be\/Y6e_m9iq-4Q\r\n
What are those floaty things in your eye? - Michael Mauser, TED-Ed, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 8.7.1<\/strong>\r\n\r\nBee Stereogram<\/a>\u00a0by\u00a0Be Mosaic<\/a> on Flickr<\/a> is used under a CC BY-NC-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/2.0\/) license.\r\n\r\nFigure 8.7.2<\/strong>\r\n\r\nSkin_TactileReceptors<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.3<\/strong>\r\n\r\nMacro shot photograph of someone's right eye <\/a>[photo] by Jordan Whitfield<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.4<\/strong>\r\n\r\nEyeAnatomy_01<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 8.7.5<\/strong>\r\n\r\nRGB colours [screenshots] from Microsoft Paint.\r\n\r\nFigure 8.7.6<\/strong>\r\n\r\nThrough the reading glasses<\/a>\u00a0[photo] by Dmitry Ratushny<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 8.7.7<\/strong>\r\n\r\nMyopia_Diagram<\/a> by National Eye Institute<\/a>\/ National Institutes of Health on Wikimedia Commons is used under a CC BY 2.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 8.7.8<\/strong>\r\n\r\nHyperopia<\/a> by National Institute of Health\/NIH<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.9<\/strong>\r\n\r\nAnatomyHumanEar<\/a> by unknown author from Occupational Safety & Health Administration<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 8.7.10<\/strong>\r\n\r\nTaste_bud_2_eng.svg<\/a> by Jonas T\u00f6le<\/a> on Wikimedia Commons is used under a CC0 1.0<\/a> Universal Public Domain Dedication license (https:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/deed.en).\r\n\r\n\r\nFigure 8.7.11\r\n<\/strong>\r\n\r\nHead_olfactory_nerve<\/a> by Patrick.lynch<\/a>, medical illustrator on Wikimedia Commons is used under a CC BY 2.5<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.5\/deed.en) license.\r\n
References<\/h2>\r\n
Age-Related Macular Degeneration. (n.d.). WebMD. https:\/\/www.webmd.com\/eye-health\/macular-degeneration\/age-related-macular-degeneration-overview#3 (Reviewed by Alan Kozarsky, MD on October 26, 2019)<\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436.<\/p>\r\n
da Cruz, L., Fynes, K., Georgiadis, O. et al. (2018, March 19). Phase 1 clinical study of an embryonic stem cell\u2013derived retinal pigment epithelium patch in age-related macular degeneration. Natural Biotechnology, 36<\/em>, 328\u2013337. https:\/\/doi.org\/10.1038\/nbt.4114<\/p>\r\n
File:Eye Diagram without text.gif. (2018, February 9).\u00a0Wikimedia Commons.<\/i> https:\/\/commons.wikimedia.org\/w\/index.php?title=File:Eye_Diagram_without_text.gif&oldid=286008241 (original image from\u00a0National Eye Institute\u00a0- modified by\u00a0User:Nordelch) [public domain (https:\/\/en.wikipedia.org\/wiki\/Public_domain)]<\/p>\r\n
Occupational Health and Safety Administration. (n.d.). Figure 7. Anatomy of the human ear [diagram]. In OSHA Technical Manual<\/em> (Section III, Chapter 5 - Noise). United States Department of Labour [online]. https:\/\/www.osha.gov\/dts\/osta\/otm\/new_noise\/<\/p>\r\n
Seeker. (2016, March 18). What is vertigo & why do we get it? YouTube. https:\/\/www.youtube.com\/watch?v=UL8YSLhqa5U&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, June 10). What color is Tuesday? Exploring synesthesia - Richard E. Cytowic. YouTube. https:\/\/www.youtube.com\/watch?v=rkRbebvoYqI&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, December 1). What are those floaty things in your eye? - Michael Mauser. YouTube. https:\/\/www.youtube.com\/watch?v=Y6e_m9iq-4Q&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, August 25). How do animals see in the dark? - Anna St\u00f6ckl\u200b. YouTube. https:\/\/www.youtube.com\/watch?v=t3CjTU7TaNA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\n
Figure 8.7.1 This stereogram contains a hidden image, BEE-lieve it or not.<\/em><\/figcaption><\/figure>\n Seeing Is Believing<\/span><\/p>\n
At first glance, Figure 8.7.1 appears to be just random dots of colour, but hidden within it is the three-dimensional shape of a bee. Can you see it among the dots? This figure is an example of a stereogram<\/a>, which is a two-dimensional picture that, when viewed correctly, reveals a three-dimensional object. If you can\u2019t see the hidden image, it doesn\u2019t mean that there is anything wrong with your eyes. It\u2019s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.<\/p>\n
\nSpecial and\u00a0General Senses<\/h1>\n<\/div>\n
The\u00a0human body\u00a0has two basic types of senses, called special senses and general senses.\u00a0Special senses<\/a><\/strong>\u00a0have specialized sense organs that gather sensory information and change it into\u00a0nerve impulses. Special senses include vision (for which the\u00a0eyes\u00a0are the specialized sense organs), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages).\u00a0General senses<\/a>,<\/strong>\u00a0in contrast, are all associated with the sense of touch. They lack special sense organs. Instead, sensory information about touch is gathered by the skin and other\u00a0body tissues, all of which have important functions besides gathering sense information. Whether the senses are special or general, however,\u00a0they all\u00a0depend on\u00a0cells called sensory receptors.<\/p>\n
\nSensory Receptors<\/h1>\n<\/div>\n
A\u00a0sensory receptor<\/a><\/strong>\u00a0is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a\u00a0nerve impulse. The nerve impulse then travels along the sensory (afferent) nerve to the\u00a0central nervous system\u00a0for processing and to form a response.<\/p>\n
There are several different types of sensory receptors that respond to different kinds of stimuli:<\/p>\n
- \n
- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Nociceptors<\/a><\/strong>\u00a0respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs, as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus.\u00a0Some detect damaging\u00a0heat\u00a0or cold, others detect excessive pressure, and still others detect painful chemicals (such as very hot spices in food).<\/li>\n
- Photoreceptors<\/a><\/strong>\u00a0detect and respond to light. Most photoreceptors are found in the\u00a0eyes\u00a0and are needed for the sense of vision.<\/li>\n
- Chemoreceptors<\/a><\/strong>\u00a0respond to certain chemicals. They are found mainly in taste buds on the tongue \u2014 where they are needed for the sense of taste \u2014 and in nasal passages, where they are needed for the sense of smell.<\/li>\n<\/ul>\n
\nTouch<\/h1>\n<\/div>\n
Touch<\/a><\/strong> is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure 8.7.2.<\/p>\n
Figure 8.7.2 Tactile receptors in the skin include free nerve endings, Merkel cells, Meissner\u2019s corpuscles, Pacinian corpuscles, root hair plexuses, and Ruffini corpuscles. Each type of sensory receptor responds to a different kind of tactile stimulus. For example, free nerve endings generally respond to pain and temperature variations, whereas Merkel cells are associated with the sense of light touch and the discrimination of shapes and textures.<\/em><\/figcaption><\/figure>\n \nFigure 8.7.3 The human eye is a sensory organ that collects and focusses light, forms images,\u00a0and changes them to nerve impulses.<\/em><\/figcaption><\/figure>\n Vision<\/span><\/p>\n<\/div>\n
Vision<\/strong>\u00a0(or sight) is the ability to sense light and see. The\u00a0eye<\/strong> is the special sensory organ that collects and focuses light and forms images. The eye, however, is not sufficient for us to see. The brain also plays a necessary role in vision.\u00a0 Vision is our primary sense and more than 50 per cent of the cerebral cortex is devoted to processing visual information.\u00a0 A person with normal colour vision can differentiate between hundreds of thousands of different colours, hues, and shades.<\/p>\n
How the Eye Works<\/h2>\n
Figure 8.7.4 (below) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image, and then changes the image to nerve impulses that travel to the brain. The eye’s functions are summarized in the following steps.<\/p>\n
- \n
- Light passes first through the\u00a0cornea<\/a><\/strong>, which is a clear outer layer that protects the eye and helps to focus the light by refracting (or bending) it.<\/li>\n
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- The light then passes through the\u00a0lens<\/a><\/strong>, which refracts the light even more and focuses it on the retina at the back of the eye, as an inverted image. Sitting behind the lens is a gelatinous fluid called vitreous humor<\/a>, which functions to maintain the shape of the eye.<\/li>\n
- The\u00a0retina<\/a><\/strong> contains two types of photoreceptors: rod and cone cells . Rod cells<\/a><\/strong>, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light.\u00a0Cone cells<\/a><\/strong>, which are found mainly in the center of the retina, are sensitive to light of different colours, and allow colour vision. The rods and cones convert the light that strikes them to nerve impulses.<\/li>\n
- The nerve impulses from the rods and cones travel to the optic nerve via the\u00a0optic disc<\/a> <\/strong>(also known as the optic nerve), which is a circular area at the back of the eye where the optic nerve connects to the retina.<\/li>\n<\/ol>\n
Figure 8.7.4 Trace the path of light through the eye as you read about in the five steps above.<\/em><\/figcaption><\/figure>\n Colour Vision<\/h2>\n
Humans have colour vision because we have three types of cone cells:\u00a0 blue, green and red.\u00a0 Each of these types of cone cell detects a specific wavelength of light, for which they are named.\u00a0 The combined stimulus\u00a0 is then perceived as a specific colour, based on the ratio of the amount stimulus coming from each of the three types of cone cells.\u00a0 Do you know what else uses these same three pieces of information to communicate colour?\u00a0 Your computer monitor!\u00a0 When working in a creative program, such as Paint, these three reference points of red (R), green (G), and blue (B), can be used to create any of the million colours the human eye can perceive, as illustrated in Figure 8.7.5. Take a look at each of the numerical values for red, green, and blue and what colour their combined values create:<\/p>\n
\n<\/div>\n<\/div>\nFigure 8.7.5 RGB colours.\u00a0<\/em><\/p>\n
Role of the Brain in Vision<\/h2>\n
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus<\/a> in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe<\/a> of the cerebrum<\/a>, which is part of the cerebral cortex<\/a>. The visual cortex is the largest system in the human brain, and is responsible for processing visual images. It interprets messages from both eyes and \u201ctells\u201d us what we are seeing.<\/p>\n
Vision Problems<\/h2>\n
Figure 8.7.6 The three vision problems described are typically solved by using glasses.<\/em><\/figcaption><\/figure>\n Vision problems are very common. Two of the most common are myopia<\/a> and hyperopia<\/a>, and they often start in\u00a0childhood\u00a0or adolescence. Another common problem, called presbyopia, occurs in most people, beginning\u00a0in middle\u00a0adulthood. In all three conditions, the eyes fail to focus images correctly on the retina, resulting in blurred vision.<\/p>\n
Myopia<\/h3>\n
Figure 8.7.7 In a patient who is nearsighted, the image is focused in front of the retina, resulting in distant objects appearing out of focus.<\/em><\/figcaption><\/figure>\n Myopia<\/strong><\/a> (or nearsightedness) occurs when the light that comes into the eye does not directly focus\u00a0on<\/em>\u00a0the retina, but\u00a0in front<\/em> of it, as shown in Figure 8.7.7. As a result, distant objects may appear out of focus, but the focus of close objects is not affected. Myopia may occur because the eyeball is elongated from front to back, or because the cornea is too curved. Myopia can be corrected with the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.<\/p>\n
Hyperopia<\/h3>\n
Figure 8.7.8 In a patient who exhibits hyperopia, the image focuses at a point somewhere behind the retina, causing close objects to appear blurry.<\/em><\/figcaption><\/figure>\n Hyperopia<\/strong>\u00a0(or farsightedness)\u00a0happens\u00a0when the light\u00a0coming\u00a0into the eye does not directly focus\u00a0on<\/em>\u00a0the retina but\u00a0behind<\/em> it, as shown in Figure 8.7.8. This causes close objects to appear out of focus, but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back, or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.<\/p>\n
<\/div>\nPresbyopia<\/h3>\n
Presbyopia<\/strong> is a vision problem associated with aging, in which the eye gradually loses its ability to focus on close objects. The precise\u00a0origin\u00a0of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, causing the\u00a0muscles\u00a0that control the lens\u00a0to\u00a0lose power as people grow older. The first signs of presbyopia \u2014 eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print \u2014 are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects, because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.<\/p>\n
\nHearing<\/h1>\n<\/div>\n
Hearing<\/a><\/strong>\u00a0is the ability to sense\u00a0sound waves, and the\u00a0ear<\/a><\/strong> is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear Figure 8.7.9). The sound waves strike the eardrum, and make it vibrate. The vibrations then travel through the three tiny bones (incus, malleus and stapes) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The
- Next, light enters the interior of the eye through an opening called the\u00a0pupil<\/a><\/strong>. The size of this opening is controlled by the coloured part of the eye (called the iris<\/a><\/strong>), which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.\u00a0 Filling the space between the cornea and the iris is a semi-gelatinous fluid called aqueous humor<\/a> and functions to maintain the shape of the eye.<\/li>\n
- Thermoreceptors<\/a><\/strong>\u00a0respond to variations in\u00a0temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.<\/li>\n
- Mechanoreceptors<\/a><\/strong>\u00a0respond to mechanical forces, such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear, where they are needed for the senses of\u00a0hearing and balance.<\/li>\n
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![Skin_TactileReceptors<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0809_Skin_TactileReceptors.png\"){kind=link}
![EyeAnatomy_01<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Blausen_0388_EyeAnatomy_01.png\"){kind=link}
![Myopia_Diagram<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Myopia_Diagram.jpg\"){kind=link}
![Hyperopia<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Hyperopia.gif\"){kind=link}
![AnatomyHumanEar<\/a> by unknown author from](\"https:\/\/commons.wikimedia.org\/wiki\/File:AnatomyHumanEar.gif\"){kind=link}
![Taste_bud_2_eng.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Taste_bud_2_eng.svg\"){kind=link}
![Head_olfactory_nerve<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Head_olfactory_nerve.jpg\"){kind=link}