Marvelous Muscles<\/h1>\r\n<\/div>\r\nDoes the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.\r\n\r\nWhat Is the Muscular System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"2907\"]muscular system[\/pb_glossary]<\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of [pb_glossary id=\"2981\"]skeletal muscles[\/pb_glossary], which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary], which makes up the walls of the heart, and [pb_glossary id=\"2982\"]smooth muscles[\/pb_glossary], which control movement in other internal organs and structures.\r\n\r\n[caption id=\"attachment_4155\" align=\"aligncenter\" width=\"304\"]![\"11.2.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bougle_whole2_retouched-2.png\")
Figure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em>[\/caption]\r\n\r\n\r\n\r\nMuscle Structure and Function<\/span>\r\n\r\n<\/div>\r\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0[pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]<\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.\r\n\r\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.\r\n\r\nSmooth muscles in the walls of blood vessels contract to cause [pb_glossary id=\"3942\"]vasoconstriction[\/pb_glossary], which may help conserve body heat. Relaxation of these muscles causes [pb_glossary id=\"2756\"]vasodilation[\/pb_glossary], which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0[pb_glossary id=\"2890\"]peristalsis[\/pb_glossary].<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.\r\n\r\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.\r\n\r\nMuscle Hypertrophy and Atrophy<\/h1>\r\n<\/div>\r\nMuscles can grow larger, or\u00a0[pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary].<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in [pb_glossary id=\"3409\"]testosterone[\/pb_glossary] that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.\r\n\r\nMuscles can also grow smaller, or\u00a0[pb_glossary id=\"4160\"]atrophy[\/pb_glossary]<\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).\r\n\r\n[caption id=\"attachment_4161\" align=\"aligncenter\" width=\"479\"]![\"12.1\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Daniel_Tani_iss016e027910-2.jpg\")
Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em>[\/caption]\r\n\r\nMany diseases, including [pb_glossary id=\"5605\"]cancer[\/pb_glossary] and [pb_glossary id=\"5615\"]AIDS[\/pb_glossary], are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0[pb_glossary id=\"4163\"]sarcopenia[\/pb_glossary].<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.\r\n\r\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.\r\n\r\nInteractions with Other Body Systems<\/h1>\r\n<\/div>\r\nMuscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0[pb_glossary id=\"4164\"]neuromuscular junction[\/pb_glossary]<\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.\r\n\r\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary] (smooth muscle) or specialized cells in the heart (cardiac muscle). [pb_glossary id=\"5661\"]Hormones[\/pb_glossary] and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.\r\n\r\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0[pb_glossary id=\"4165\"]musculoskeletal system[\/pb_glossary]<\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary]<\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a [pb_glossary id=\"3949\"]joint[\/pb_glossary]. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.\r\n\r\n\r\n12.2 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\nReferences<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\nTED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\n![\"12.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Natalia_Zabolotnaya_2012b-2.jpg\")
Figure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n\nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\nFigure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n\nMuscle Structure and Function<\/span><\/p>\n<\/div>\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\nMuscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\nFigure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\nMany diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n\n\n\n12.2 Summary<\/span><\/h1>\n<\/header>\n\n\n- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n12.2 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n\n
What Is the Muscular System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"2907\"]muscular system[\/pb_glossary]<\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of [pb_glossary id=\"2981\"]skeletal muscles[\/pb_glossary], which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary], which makes up the walls of the heart, and [pb_glossary id=\"2982\"]smooth muscles[\/pb_glossary], which control movement in other internal organs and structures.\r\n\r\n[caption id=\"attachment_4155\" align=\"aligncenter\" width=\"304\"] Figure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em>[\/caption]\r\n\r\n\r\n\r\nMuscle Structure and Function<\/span>\r\n\r\n<\/div>\r\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0[pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]<\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary]. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.\r\n\r\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.\r\n\r\nSmooth muscles in the walls of blood vessels contract to cause [pb_glossary id=\"3942\"]vasoconstriction[\/pb_glossary], which may help conserve body heat. Relaxation of these muscles causes [pb_glossary id=\"2756\"]vasodilation[\/pb_glossary], which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0[pb_glossary id=\"2890\"]peristalsis[\/pb_glossary].<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.\r\n\r\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.\r\n\r\nMuscle Hypertrophy and Atrophy<\/h1>\r\n<\/div>\r\nMuscles can grow larger, or\u00a0[pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary].<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in [pb_glossary id=\"3409\"]testosterone[\/pb_glossary] that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.\r\n\r\nMuscles can also grow smaller, or\u00a0[pb_glossary id=\"4160\"]atrophy[\/pb_glossary]<\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).\r\n\r\n[caption id=\"attachment_4161\" align=\"aligncenter\" width=\"479\"] Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em>[\/caption]\r\n\r\nMany diseases, including [pb_glossary id=\"5605\"]cancer[\/pb_glossary] and [pb_glossary id=\"5615\"]AIDS[\/pb_glossary], are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0[pb_glossary id=\"4163\"]sarcopenia[\/pb_glossary].<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.\r\n\r\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.\r\n\r\nInteractions with Other Body Systems<\/h1>\r\n<\/div>\r\nMuscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0[pb_glossary id=\"4164\"]neuromuscular junction[\/pb_glossary]<\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.\r\n\r\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary] (smooth muscle) or specialized cells in the heart (cardiac muscle). [pb_glossary id=\"5661\"]Hormones[\/pb_glossary] and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.\r\n\r\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0[pb_glossary id=\"4165\"]musculoskeletal system[\/pb_glossary]<\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary]<\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a [pb_glossary id=\"3949\"]joint[\/pb_glossary]. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.\r\n\r\n\r\n12.2 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\nReferences<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\nTED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\nFigure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n\nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\nFigure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n\nMuscle Structure and Function<\/span><\/p>\n<\/div>\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\nMuscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\nFigure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\nMany diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n\n\n\n12.2 Summary<\/span><\/h1>\n<\/header>\n\n\n- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n12.2 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n\n
Muscle Hypertrophy and Atrophy<\/h1>\r\n<\/div>\r\nMuscles can grow larger, or\u00a0[pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary].<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in [pb_glossary id=\"3409\"]testosterone[\/pb_glossary] that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.\r\n\r\nMuscles can also grow smaller, or\u00a0[pb_glossary id=\"4160\"]atrophy[\/pb_glossary]<\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).\r\n\r\n[caption id=\"attachment_4161\" align=\"aligncenter\" width=\"479\"] Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em>[\/caption]\r\n\r\nMany diseases, including [pb_glossary id=\"5605\"]cancer[\/pb_glossary] and [pb_glossary id=\"5615\"]AIDS[\/pb_glossary], are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0[pb_glossary id=\"4163\"]sarcopenia[\/pb_glossary].<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.\r\n\r\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.\r\n\r\nInteractions with Other Body Systems<\/h1>\r\n<\/div>\r\nMuscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0[pb_glossary id=\"4164\"]neuromuscular junction[\/pb_glossary]<\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.\r\n\r\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary] (smooth muscle) or specialized cells in the heart (cardiac muscle). [pb_glossary id=\"5661\"]Hormones[\/pb_glossary] and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.\r\n\r\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0[pb_glossary id=\"4165\"]musculoskeletal system[\/pb_glossary]<\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary]<\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a [pb_glossary id=\"3949\"]joint[\/pb_glossary]. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.\r\n\r\n\r\n12.2 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\nReferences<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\nTED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\nFigure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n\nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\nFigure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n\nMuscle Structure and Function<\/span><\/p>\n<\/div>\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\nMuscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\nFigure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\nMany diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n\n\n\n12.2 Summary<\/span><\/h1>\n<\/header>\n\n\n- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n12.2 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n\n
Interactions with Other Body Systems<\/h1>\r\n<\/div>\r\nMuscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0[pb_glossary id=\"4164\"]neuromuscular junction[\/pb_glossary]<\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.\r\n\r\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary] (smooth muscle) or specialized cells in the heart (cardiac muscle). [pb_glossary id=\"5661\"]Hormones[\/pb_glossary] and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.\r\n\r\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0[pb_glossary id=\"4165\"]musculoskeletal system[\/pb_glossary]<\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary]<\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a [pb_glossary id=\"3949\"]joint[\/pb_glossary]. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.\r\n\r\n\r\n12.2 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\nReferences<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\nTED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\nFigure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n\nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\nFigure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n\nMuscle Structure and Function<\/span><\/p>\n<\/div>\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\nMuscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\nFigure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\nMany diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n\n\n\n12.2 Summary<\/span><\/h1>\n<\/header>\n\n\n- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n12.2 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n\n
12.2 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\nReferences<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\nTED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\nFigure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n\nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\nFigure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n\nMuscle Structure and Function<\/span><\/p>\n<\/div>\nMuscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\nSkeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\nCardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\nMuscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\nFigure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\nMany diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\nIn both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\nInvoluntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\nMuscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n\n\n\n12.2 Summary<\/span><\/h1>\n<\/header>\n\n\n- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n12.2 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n\n
- \r\n \t
- The [pb_glossary id=\"2907\"]muscular system[\/pb_glossary] consists of all the muscles of the body. There are three types of muscle: [pb_glossary id=\"2981\"]skeletal muscle[\/pb_glossary] (which is attached to bones and enables [pb_glossary id=\"3004\"]voluntary[\/pb_glossary] body movements), [pb_glossary id=\"5925\"]cardiac muscle[\/pb_glossary] (which makes up the walls of the heart and makes it beat), and [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\r\n \t
- Muscles are organs composed mainly of muscle cells, which may also be called [pb_glossary id=\"4156\"]muscle fibres[\/pb_glossary] or [pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in [pb_glossary id=\"5549\"]ATP[\/pb_glossary].<\/li>\r\n \t
- Muscles can grow larger, or [pb_glossary id=\"4159\"]hypertrophy[\/pb_glossary]. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or [pb_glossary id=\"4160\"]atrophy[\/pb_glossary]. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\r\n \t
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. [pb_glossary id=\"3005\"]Involuntary[\/pb_glossary] contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the [pb_glossary id=\"5899\"]autonomic nervous system[\/pb_glossary], hormones, or other factors.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 12.2 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- What is the muscular system?<\/li>\r\n \t
- Describe muscle cells and their function.<\/li>\r\n \t
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\r\n \t
- Define muscle hypertrophy and muscle atrophy.<\/li>\r\n \t
- What are some possible causes of muscle hypertrophy?<\/li>\r\n \t
- Give three reasons that muscle atrophy may occur.<\/li>\r\n \t
- How do muscles change when they increase or decrease in size?<\/li>\r\n \t
- How do changes in muscle size affect strength?<\/li>\r\n \t
- Explain why astronauts can easily lose muscle mass in space.<\/li>\r\n \t
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\r\n \t
- [h5p id=\"600\"]<\/li>\r\n \t
- Name two systems in the body that work together with the muscular system to carry out movements.<\/li>\r\n \t
- Describe one way in which the muscular system is involved in regulating body temperature.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 12.2 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4\r\nHow your muscular system works - Emma Bryce, TED-Ed, 2017.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=emb_logo\r\n
3D Medical Animation - Peristalsis in Large Intestine\/Bowel || ABP \u00a9, AnimatedBiomedical, 2013.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=emb_logo\r\n
Muscle matters: Dr Brendan Egan at TEDxUCD, TEDx Talks, 2014.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 12.2.1<\/strong>\r\n\r\nNatalia_Zabolotnaya_2012b<\/a> by Simon Q<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/deed.en) license.\r\n\r\n\r\nFigure 12.2.2<\/strong>\r\n\r\nBougle_whole2_retouched<\/a> by Bougl\u00e9, Julien<\/a> from the National LIbrary of Medicine (NLM)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 12.2.3<\/strong>\r\n\r\nDaniel_Tani_iss016e027910<\/a> by NASA\/ International Space Station Imagery<\/a> on Wikimedia Commons\u00a0is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n
References<\/h2>\r\n
AnimatedBiomedical. (2013, January 30). 3D Medical animation - Peristalsis in large intestine\/bowel || ABP \u00a9. YouTube. https:\/\/www.youtube.com\/watch?v=Ujr0UAbyPS4&feature=youtu.be<\/p>\r\n
Bougle\u0301, J. (1899). Le corps humain en grandeur naturelle : planches colorie\u0301es et superpose\u0301es, avec texte explicatif. J. B. Baillie\u0300re et fils. In Historical Anatomies on the Web<\/em>. http:\/\/www.nlm.nih.gov\/exhibition\/historicalanatomies\/bougle_home.html<\/p>\r\n
TED-Ed. (2017, October 26). How your muscular system works - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=VVL-8zr2hk4&feature=youtu.be<\/p>\r\n
TEDx Talks. (2014, June 27). Muscle matters: Dr Brendan Egan at TEDxUCD. YouTube. https:\/\/www.youtube.com\/watch?v=LkXwfTsqQgQ&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 15). Natalya Zabolotnaya. In\u00a0Wikipedia.<\/i>\u00a0https:\/\/en.wikipedia.org\/w\/index.php?title=Natalya_Zabolotnaya&oldid=962630409<\/p>\r\n\r\n<\/div>","rendered":"
<\/p>\n
Figure 12.2.1 Natalia Zabolotnaya, 2012 Olympics.<\/em><\/figcaption><\/figure>\n \nMarvelous Muscles<\/h1>\n<\/div>\n
Does the word\u00a0muscle<\/em> make you think of the well-developed muscles of a weightlifter, like the woman in Figure 12.2.1? Her name is Natalia Zabolotnaya<\/a>, and she\u2019s a Russian Olympian. The muscles that are used to lift weights are easy to feel and see, but they aren\u2019t the only muscles in the human body. Many muscles are deep within the body, where they form the walls of internal organs and other structures. You can flex your biceps at will, but you can\u2019t control internal muscles\u00a0like\u00a0these. It\u2019s a good thing that these internal muscles work without any conscious effort on your part, because movement of these muscles is essential for survival. Muscles are the organs of the muscular system.<\/p>\n
\nWhat Is the Muscular System?<\/h1>\n<\/div>\n
The\u00a0muscular system<\/a><\/strong> consists of all the muscles of the body. The largest percentage of muscles in the muscular system consists of skeletal muscles<\/a>, which are attached to bones and enable voluntary body movements (shown in Figure 12.2.2). There are almost 650 skeletal muscles in the human body, many of them shown in Figure 12.2.2. Besides skeletal muscles, the muscular system also includes cardiac muscle<\/a>, which makes up the walls of the heart, and smooth muscles<\/a>, which control movement in other internal organs and structures.<\/p>\n
Figure 12.2.2 Many of the skeletal muscles in the human muscular system are shown in this drawing of the human body.<\/em><\/figcaption><\/figure>\n \nMuscle Structure and Function<\/span><\/p>\n<\/div>\n
Muscles are organs composed mainly of muscle cells, which are also called\u00a0muscle fibres<\/a> <\/strong>(mainly in skeletal and cardiac muscle) or\u00a0myocytes<\/a><\/strong>\u00a0(mainly in smooth muscle). Muscle cells are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP<\/a>. The sliding filaments increase the tension in \u2014 or shorten the length of \u2014 muscle cells, causing a contraction. Muscle contractions are responsible for virtually\u00a0all<\/em>\u00a0the movements of the body, both inside and out.<\/p>\n
Skeletal muscles are attached to bones of the skeleton. When these muscles contract, they move the body. They allow us to use our limbs in a variety of ways, from walking to turning cartwheels. Skeletal muscles also maintain posture and help us to keep balance.<\/p>\n
Smooth muscles in the walls of blood vessels contract to cause vasoconstriction<\/a>, which may help conserve body heat. Relaxation of these muscles causes vasodilation<\/a>, which may help the body lose heat. In the organs of the digestive system, smooth muscles squeeze food through the gastrointestinal tract by contracting in sequence to form a wave of muscle contractions called\u00a0peristalsis<\/a>.<\/strong>\u00a0Think of squirting toothpaste through a tube by applying pressure in sequence from the bottom of the tube to the top, and you have a good idea of how food is moved by muscles through the digestive system. Peristalsis of smooth muscles also moves urine through the urinary tract.<\/p>\n
Cardiac muscle tissue is found only in the walls of the heart. When cardiac muscle contracts, it makes the heart beat. The pumping action of the beating heart keeps blood flowing through the cardiovascular system.<\/p>\n
\nMuscle Hypertrophy and Atrophy<\/h1>\n<\/div>\n
Muscles can grow larger, or\u00a0hypertrophy<\/a>.<\/strong>\u00a0This generally occurs through increased use, although hormonal or other influences can also play a role.\u00a0The increase in testosterone<\/a> that occurs in males during puberty, for example, causes a significant increase in muscle size. Physical exercise that involves weight bearing or resistance training can increase the size of skeletal muscles in virtually everyone. Exercises (such as running) that increase the heart rate may also increase the size and strength of cardiac muscle. The size of muscle, in turn, is the main determinant of muscle strength, which may be measured by the amount of force a muscle can exert.<\/p>\n
Muscles can also grow smaller, or\u00a0atrophy<\/a><\/strong>, which can occur through lack of physical activity or from starvation. People who are immobilized for any length of time \u2014 for example, because of a broken bone or surgery \u2014 lose muscle mass relatively quickly. People in concentration or famine camps may be so malnourished that they lose much of their muscle mass, becoming almost literally just \u201cskin and bones.\u201d Astronauts on the International Space Station may also lose significant muscle mass because of weightlessness in space (see Figure 12.2.3).<\/p>\n
Figure 12.2.3 It is important for astronauts to exercise on board the International Space Station to help counter the loss of muscle mass that occurs because they are weightless without Earth\u2019s gravity.<\/em><\/figcaption><\/figure>\n Many diseases, including cancer<\/a> and AIDS<\/a>, are often associated with muscle atrophy. Atrophy of muscles also\u00a0happens\u00a0with age. As people grow older, there is a gradual decrease in the ability to maintain skeletal muscle mass, known as\u00a0sarcopenia<\/a>.<\/strong>\u00a0The exact cause of sarcopenia is not known, but one possible cause is a decrease in sensitivity to growth factors that are needed to maintain muscle mass. Because muscle size determines strength, muscle atrophy causes a corresponding decline in muscle strength.<\/p>\n
In both hypertrophy and atrophy, the number of muscle fibres does not change. What changes is the size of the muscle fibres. When muscles hypertrophy, the individual fibres become wider. When muscles atrophy, the fibres become narrower.<\/p>\n
\nInteractions with Other Body Systems<\/h1>\n<\/div>\n
Muscles cannot contract on their own. Skeletal muscles need stimulation from motor neurons in order to contract. The point where a motor neuron attaches to a muscle is called a\u00a0neuromuscular junction<\/a><\/strong>. Let\u2019s say you decide to raise your hand in class. Your brain sends electrical messages through motor neurons to your arm and shoulder. The motor neurons, in turn, stimulate muscle fibres in your arm and shoulder to contract, causing your arm to rise.<\/p>\n
Involuntary contractions of smooth and cardiac muscles are also controlled by electrical impulses, but in the case of these muscles, the impulses come from the autonomic nervous system<\/a> (smooth muscle) or specialized cells in the heart (cardiac muscle). Hormones<\/a> and some other factors also influence involuntary contractions of cardiac and smooth muscles. For example, the fight-or-flight hormone adrenaline increases the rate at which cardiac muscle contracts, thereby speeding up the heartbeat.<\/p>\n
Muscles cannot move the body on their own. They need the skeletal system to act upon. The two systems together are often referred to as the\u00a0musculoskeletal system<\/a><\/strong>. Skeletal muscles are attached to the skeleton by tough connective tissues called\u00a0tendons<\/a><\/strong>. Many skeletal muscles are attached to the ends of bones that meet at a joint<\/a>. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move. The skeletal system provides a system of levers that allow body movement. The muscular system provides the force that moves the levers.<\/p>\n
\n\n\n 12.2 Summary<\/span><\/h1>\n<\/header>\n
\n- \n
- The muscular system<\/a> consists of all the muscles of the body. There are three types of muscle: skeletal muscle<\/a> (which is attached to bones and enables voluntary<\/a> body movements), cardiac muscle<\/a> (which makes up the walls of the heart and makes it beat), and smooth muscle<\/a> (which is found in the walls of internal organs and other internal structures and controls their movements).<\/li>\n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
- Muscles can grow larger, or hypertrophy<\/a>. This generally occurs through increased use (physical exercise), although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy<\/a>. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size \u2014 but not the number \u2014 of muscle fibres changes. The size of muscles is the main determinant of muscle strength.<\/li>\n
- Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon. Involuntary<\/a> contractions of cardiac and smooth muscles are controlled by special cells in the heart, nerves of the autonomic nervous system<\/a>, hormones, or other factors.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
\n\n 12.2 Review Questions<\/span><\/h1>\n<\/header>\n
\n- \n
- What is the muscular system?<\/li>\n
- Describe muscle cells and their function.<\/li>\n
- Identify three types of muscle\u00a0tissue\u00a0and where each type is found.<\/li>\n
- Define muscle hypertrophy and muscle atrophy.<\/li>\n
- What are some possible causes of muscle hypertrophy?<\/li>\n
- Give three reasons that muscle atrophy may occur.<\/li>\n
- How do muscles change when they increase or decrease in size?<\/li>\n
- How do changes in muscle size affect strength?<\/li>\n
- Explain why astronauts can easily lose muscle mass in space.<\/li>\n
- Describe how the terms\u00a0muscle cells<\/em>,\u00a0muscle fibres<\/em>, and\u00a0myocytes<\/em>\u00a0relate to each other.<\/li>\n
- \n
\n - \n
- Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres<\/a> or myocytes<\/a>. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy in ATP<\/a>.<\/li>\n
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