![\"12.3.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Look-by-ali-yahya-155huuQwGvA-unsplash-scaled-3.jpg\")
Figure 12.3.1 \"Eye\" can see you.<\/em>[\/caption]\r\n\r\nWork Those Eye Muscles!<\/h1>\r\n<\/div>\r\nImagine the man in Figure 12.3.1 turns his eyes in your direction. This is a very small movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.\r\n\r\nWhat is Muscle Tissue?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2817\"]Muscle tissue[\/pb_glossary]<\/strong>\u00a0is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles\u00a0are\u00a0connective tissues, such as [pb_glossary id=\"4166\"]tendons[\/pb_glossary]\u00a0that attach\u00a0[pb_glossary id=\"2981\"]skeletal muscles[\/pb_glossary]\u00a0to\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and sheaths of [pb_glossary id=\"2815\"]connective tissues[\/pb_glossary]\u00a0that cover or line muscle tissues. Only muscle tissue per se<\/em>, has cells with the ability to contract.\r\n\r\nThere are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 12.3.2 shows how the three types of muscle tissues appear under magnification. When you read about each type below, you will learn why the three types appear as they do.\r\n\r\n[caption id=\"attachment_4170\" align=\"aligncenter\" width=\"535\"]![\"12.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Skeletal_Smooth_Cardiac-2.jpg\")
Figure 12.3.2 These magnified images show (a) skeletal muscle tissue, (b) smooth muscle tissue, and (c) cardiac muscle tissue.<\/em>[\/caption]\r\n\r\n\r\nSkeletal Muscle Tissue<\/h2>\r\n<\/div>\r\n[pb_glossary id=\"2981\"]Skeletal muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue that is attached to\u00a0bones\u00a0by\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary], <\/strong>which are bundles of [pb_glossary id=\"3970\"]collagen[\/pb_glossary] fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are [pb_glossary id=\"3004\"]voluntary[\/pb_glossary], or under conscious control of the [pb_glossary id=\"5933\"]central nervous system[\/pb_glossary] via the [pb_glossary id=\"3014\"]somatic nervous system[\/pb_glossary]. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.\r\n\r\n[caption id=\"attachment_4171\" align=\"aligncenter\" width=\"387\"]![\"12.4.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Anterior_and_Posterior_Views_of_Muscles-scaled-3.jpg\")
<\/a> Figure 12.3.3 Major skeletal muscles of the body. View this image full size here: http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/Anterior_and_Posterior_Views_of_Muscles-scaled.jpg<\/a>
<\/em>[\/caption]\r\nSkeletal Muscle Pairs<\/h2>\r\nTo move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.\u00a0 For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.\r\n\r\n[caption id=\"attachment_4172\" align=\"aligncenter\" width=\"500\"]![\"12.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Antagonistic-Muscle-Pair-by-CK-12-Foundation-2.jpg\")
Figure 12.3.4 Triceps and biceps muscles in the upper arm are opposing muscles that move the arm at the elbow in opposite directions.[\/caption]\r\nSkeletal Muscle Structure<\/h2>\r\nEach skeletal muscle consists of hundreds \u2014 or even\u00a0thousands<\/em> \u2014 of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called [pb_glossary id=\"4173\"]endomysium[\/pb_glossary]<\/strong>. The skeletal muscle fibres are bundled together in units called [pb_glossary id=\"4174\"]muscle fascicles[\/pb_glossary]<\/strong>, which\u00a0are surrounded by sheaths of connective tissue\u00a0called\u00a0[pb_glossary id=\"4175\"]perimysium[\/pb_glossary]<\/strong>. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called [pb_glossary id=\"4176\"]epimysium[\/pb_glossary]<\/strong>. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.\r\n\r\n[caption id=\"attachment_4177\" align=\"aligncenter\" width=\"675\"]![\"11.3.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Muscle_Fibes_large-2.jpg\")
Figure 12.3.5 Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibres make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle.<\/em>[\/caption]\r\n\r\nThe same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein [pb_glossary id=\"4181\"]actin<\/strong>[\/pb_glossary], which are anchored to structures called Z discs, and thick filaments of the protein [pb_glossary id=\"4182\"]myosin<\/strong>[\/pb_glossary]. The filaments are arranged together within a myofibril in repeating units called\u00a0[pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary],<\/strong> which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.\r\n\r\n[caption id=\"attachment_4183\" align=\"aligncenter\" width=\"801\"]![\"11.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Muscle_Fibers_small-2.jpg\")
Figure 12.3.6 Bundles of protein filaments form a myofibril, and bundles of myofibrils make up a single muscle fibre. I and A bands refer to the positioning of myosin and actin fibres in a myofibril. Sarcoplasmic reticulum is a specialized type of endoplasmic reticulum that forms a network around each myofibril. It serves as a reservoir for calcium ions, which are needed for muscle contractions. H zones and Z discs are also involved in muscle contractions, which you can read about in the concept Muscle Contraction.<\/em>[\/caption]\r\nSlow- and Fast-Twitch Skeletal Muscle Fibres<\/h2>\r\nSkeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.\r\n\r\n \t- [pb_glossary id=\"4184\"]Slow-twitch muscle fibres[\/pb_glossary]<\/strong> are dense with capillaries and rich in [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary] and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.<\/li>\r\n \t
- [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n\r\n
Smooth Muscle<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2982\"]Smooth muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue in the walls of internal organs and other internal structures such as\u00a0blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions.\u00a0When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are [pb_glossary id=\"3005\"]involuntary[\/pb_glossary], so they are not under conscious control. Instead, they are controlled by the [pb_glossary id=\"2935\"]autonomic\u00a0nervous system[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], [pb_glossary id=\"3056\"]neurotransmitters[\/pb_glossary], and other physiological factors.\r\nStructure of Smooth Muscle<\/h2>\r\nThe\u00a0cells\u00a0that make up smooth muscle are generally called\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. <\/strong>Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in [pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary]. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which in turn contain bundles of [pb_glossary id=\"4182\"]myosin[\/pb_glossary] and [pb_glossary id=\"4181\"]actin[\/pb_glossary] filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.\r\n\r\n[caption id=\"attachment_4186\" align=\"aligncenter\" width=\"1024\"]![\"11.3.7](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Smooth_Muscle_Contraction-2.jpg\")
Figure 12.3.7 The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue.<\/em>[\/caption]\r\n\r\n\r\n\r\nFunctions of Smooth Muscle<\/span>\r\n\r\n<\/div>\r\nUnlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of [pb_glossary id=\"4187\"]elastin[\/pb_glossary], [pb_glossary id=\"3970\"]collagen[\/pb_glossary], and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.\r\n
What is Muscle Tissue?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2817\"]Muscle tissue[\/pb_glossary]<\/strong>\u00a0is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles\u00a0are\u00a0connective tissues, such as [pb_glossary id=\"4166\"]tendons[\/pb_glossary]\u00a0that attach\u00a0[pb_glossary id=\"2981\"]skeletal muscles[\/pb_glossary]\u00a0to\u00a0[pb_glossary id=\"5913\"]bones[\/pb_glossary]\u00a0and sheaths of [pb_glossary id=\"2815\"]connective tissues[\/pb_glossary]\u00a0that cover or line muscle tissues. Only muscle tissue per se<\/em>, has cells with the ability to contract.\r\n\r\nThere are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 12.3.2 shows how the three types of muscle tissues appear under magnification. When you read about each type below, you will learn why the three types appear as they do.\r\n\r\n[caption id=\"attachment_4170\" align=\"aligncenter\" width=\"535\"] Figure 12.3.2 These magnified images show (a) skeletal muscle tissue, (b) smooth muscle tissue, and (c) cardiac muscle tissue.<\/em>[\/caption]\r\n\r\n\r\nSkeletal Muscle Tissue<\/h2>\r\n<\/div>\r\n[pb_glossary id=\"2981\"]Skeletal muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue that is attached to\u00a0bones\u00a0by\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary], <\/strong>which are bundles of [pb_glossary id=\"3970\"]collagen[\/pb_glossary] fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are [pb_glossary id=\"3004\"]voluntary[\/pb_glossary], or under conscious control of the [pb_glossary id=\"5933\"]central nervous system[\/pb_glossary] via the [pb_glossary id=\"3014\"]somatic nervous system[\/pb_glossary]. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.\r\n\r\n[caption id=\"attachment_4171\" align=\"aligncenter\" width=\"387\"]<\/a> Figure 12.3.3 Major skeletal muscles of the body. View this image full size here: http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/Anterior_and_Posterior_Views_of_Muscles-scaled.jpg<\/a>
<\/em>[\/caption]\r\nSkeletal Muscle Pairs<\/h2>\r\nTo move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.\u00a0 For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.\r\n\r\n[caption id=\"attachment_4172\" align=\"aligncenter\" width=\"500\"] Figure 12.3.4 Triceps and biceps muscles in the upper arm are opposing muscles that move the arm at the elbow in opposite directions.[\/caption]\r\nSkeletal Muscle Structure<\/h2>\r\nEach skeletal muscle consists of hundreds \u2014 or even\u00a0thousands<\/em> \u2014 of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called [pb_glossary id=\"4173\"]endomysium[\/pb_glossary]<\/strong>. The skeletal muscle fibres are bundled together in units called [pb_glossary id=\"4174\"]muscle fascicles[\/pb_glossary]<\/strong>, which\u00a0are surrounded by sheaths of connective tissue\u00a0called\u00a0[pb_glossary id=\"4175\"]perimysium[\/pb_glossary]<\/strong>. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called [pb_glossary id=\"4176\"]epimysium[\/pb_glossary]<\/strong>. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.\r\n\r\n[caption id=\"attachment_4177\" align=\"aligncenter\" width=\"675\"] Figure 12.3.5 Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibres make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle.<\/em>[\/caption]\r\n\r\nThe same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein [pb_glossary id=\"4181\"]actin<\/strong>[\/pb_glossary], which are anchored to structures called Z discs, and thick filaments of the protein [pb_glossary id=\"4182\"]myosin<\/strong>[\/pb_glossary]. The filaments are arranged together within a myofibril in repeating units called\u00a0[pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary],<\/strong> which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.\r\n\r\n[caption id=\"attachment_4183\" align=\"aligncenter\" width=\"801\"] Figure 12.3.6 Bundles of protein filaments form a myofibril, and bundles of myofibrils make up a single muscle fibre. I and A bands refer to the positioning of myosin and actin fibres in a myofibril. Sarcoplasmic reticulum is a specialized type of endoplasmic reticulum that forms a network around each myofibril. It serves as a reservoir for calcium ions, which are needed for muscle contractions. H zones and Z discs are also involved in muscle contractions, which you can read about in the concept Muscle Contraction.<\/em>[\/caption]\r\nSlow- and Fast-Twitch Skeletal Muscle Fibres<\/h2>\r\nSkeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.\r\n\r\n \t- [pb_glossary id=\"4184\"]Slow-twitch muscle fibres[\/pb_glossary]<\/strong> are dense with capillaries and rich in [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary] and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.<\/li>\r\n \t
- [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n\r\n
Smooth Muscle<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2982\"]Smooth muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue in the walls of internal organs and other internal structures such as\u00a0blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions.\u00a0When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are [pb_glossary id=\"3005\"]involuntary[\/pb_glossary], so they are not under conscious control. Instead, they are controlled by the [pb_glossary id=\"2935\"]autonomic\u00a0nervous system[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], [pb_glossary id=\"3056\"]neurotransmitters[\/pb_glossary], and other physiological factors.\r\nStructure of Smooth Muscle<\/h2>\r\nThe\u00a0cells\u00a0that make up smooth muscle are generally called\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. <\/strong>Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in [pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary]. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which in turn contain bundles of [pb_glossary id=\"4182\"]myosin[\/pb_glossary] and [pb_glossary id=\"4181\"]actin[\/pb_glossary] filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.\r\n\r\n[caption id=\"attachment_4186\" align=\"aligncenter\" width=\"1024\"] Figure 12.3.7 The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue.<\/em>[\/caption]\r\n\r\n\r\n\r\nFunctions of Smooth Muscle<\/span>\r\n\r\n<\/div>\r\nUnlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of [pb_glossary id=\"4187\"]elastin[\/pb_glossary], [pb_glossary id=\"3970\"]collagen[\/pb_glossary], and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.\r\n
Skeletal Muscle Tissue<\/h2>\r\n<\/div>\r\n[pb_glossary id=\"2981\"]Skeletal muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue that is attached to\u00a0bones\u00a0by\u00a0[pb_glossary id=\"4166\"]tendons[\/pb_glossary], <\/strong>which are bundles of [pb_glossary id=\"3970\"]collagen[\/pb_glossary] fibres. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are [pb_glossary id=\"3004\"]voluntary[\/pb_glossary], or under conscious control of the [pb_glossary id=\"5933\"]central nervous system[\/pb_glossary] via the [pb_glossary id=\"3014\"]somatic nervous system[\/pb_glossary]. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42% skeletal muscles, and the average adult female is about 36% skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 12.3.3 below.\r\n\r\n[caption id=\"attachment_4171\" align=\"aligncenter\" width=\"387\"]<\/a> Figure 12.3.3 Major skeletal muscles of the body. View this image full size here: http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/Anterior_and_Posterior_Views_of_Muscles-scaled.jpg<\/a>
<\/em>[\/caption]\r\nSkeletal Muscle Pairs<\/h2>\r\nTo move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another, also called antagonistic muscle pairs.\u00a0 For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure 12.3.4. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles, also shown in Figure 12.3.4, are an example of a muscle pair where the muscles work in opposition to each other.\r\n\r\n[caption id=\"attachment_4172\" align=\"aligncenter\" width=\"500\"] Figure 12.3.4 Triceps and biceps muscles in the upper arm are opposing muscles that move the arm at the elbow in opposite directions.[\/caption]\r\nSkeletal Muscle Structure<\/h2>\r\nEach skeletal muscle consists of hundreds \u2014 or even\u00a0thousands<\/em> \u2014 of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called [pb_glossary id=\"4173\"]endomysium[\/pb_glossary]<\/strong>. The skeletal muscle fibres are bundled together in units called [pb_glossary id=\"4174\"]muscle fascicles[\/pb_glossary]<\/strong>, which\u00a0are surrounded by sheaths of connective tissue\u00a0called\u00a0[pb_glossary id=\"4175\"]perimysium[\/pb_glossary]<\/strong>. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called [pb_glossary id=\"4176\"]epimysium[\/pb_glossary]<\/strong>. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.\r\n\r\n[caption id=\"attachment_4177\" align=\"aligncenter\" width=\"675\"] Figure 12.3.5 Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibres make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle.<\/em>[\/caption]\r\n\r\nThe same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein [pb_glossary id=\"4181\"]actin<\/strong>[\/pb_glossary], which are anchored to structures called Z discs, and thick filaments of the protein [pb_glossary id=\"4182\"]myosin<\/strong>[\/pb_glossary]. The filaments are arranged together within a myofibril in repeating units called\u00a0[pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary],<\/strong> which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.\r\n\r\n[caption id=\"attachment_4183\" align=\"aligncenter\" width=\"801\"] Figure 12.3.6 Bundles of protein filaments form a myofibril, and bundles of myofibrils make up a single muscle fibre. I and A bands refer to the positioning of myosin and actin fibres in a myofibril. Sarcoplasmic reticulum is a specialized type of endoplasmic reticulum that forms a network around each myofibril. It serves as a reservoir for calcium ions, which are needed for muscle contractions. H zones and Z discs are also involved in muscle contractions, which you can read about in the concept Muscle Contraction.<\/em>[\/caption]\r\nSlow- and Fast-Twitch Skeletal Muscle Fibres<\/h2>\r\nSkeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.\r\n\r\n \t- [pb_glossary id=\"4184\"]Slow-twitch muscle fibres[\/pb_glossary]<\/strong> are dense with capillaries and rich in [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary] and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.<\/li>\r\n \t
- [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n\r\n
Smooth Muscle<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2982\"]Smooth muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue in the walls of internal organs and other internal structures such as\u00a0blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions.\u00a0When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are [pb_glossary id=\"3005\"]involuntary[\/pb_glossary], so they are not under conscious control. Instead, they are controlled by the [pb_glossary id=\"2935\"]autonomic\u00a0nervous system[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], [pb_glossary id=\"3056\"]neurotransmitters[\/pb_glossary], and other physiological factors.\r\nStructure of Smooth Muscle<\/h2>\r\nThe\u00a0cells\u00a0that make up smooth muscle are generally called\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. <\/strong>Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in [pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary]. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which in turn contain bundles of [pb_glossary id=\"4182\"]myosin[\/pb_glossary] and [pb_glossary id=\"4181\"]actin[\/pb_glossary] filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.\r\n\r\n[caption id=\"attachment_4186\" align=\"aligncenter\" width=\"1024\"] Figure 12.3.7 The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue.<\/em>[\/caption]\r\n\r\n\r\n\r\nFunctions of Smooth Muscle<\/span>\r\n\r\n<\/div>\r\nUnlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of [pb_glossary id=\"4187\"]elastin[\/pb_glossary], [pb_glossary id=\"3970\"]collagen[\/pb_glossary], and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.\r\n
![\"12.3](\"http:\/\/humanbiology.pressbooks.tru.ca\/wp-content\/uploads\/sites\/6\/2019\/06\/Anterior_and_Posterior_Views_of_Muscles-scaled.jpg\")
Figure 12.3.4 Triceps and biceps muscles in the upper arm are opposing muscles that move the arm at the elbow in opposite directions.[\/caption]\r\nSkeletal Muscle Structure<\/h2>\r\nEach skeletal muscle consists of hundreds \u2014 or even\u00a0thousands<\/em> \u2014 of skeletal muscle fibres, which are long, string-like cells. As shown in Figure 12.3.5 below, skeletal muscle fibres are individually wrapped in connective tissue called [pb_glossary id=\"4173\"]endomysium[\/pb_glossary]<\/strong>. The skeletal muscle fibres are bundled together in units called [pb_glossary id=\"4174\"]muscle fascicles[\/pb_glossary]<\/strong>, which\u00a0are surrounded by sheaths of connective tissue\u00a0called\u00a0[pb_glossary id=\"4175\"]perimysium[\/pb_glossary]<\/strong>. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibres. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called [pb_glossary id=\"4176\"]epimysium[\/pb_glossary]<\/strong>. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibres, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.\r\n\r\n[caption id=\"attachment_4177\" align=\"aligncenter\" width=\"675\"] Figure 12.3.5 Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibres make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle.<\/em>[\/caption]\r\n\r\nThe same bundles-within-bundles structure is replicated within each muscle fibre. As shown in Figure 12.3.6, a muscle fibre consists of a bundle of [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein [pb_glossary id=\"4181\"]actin<\/strong>[\/pb_glossary], which are anchored to structures called Z discs, and thick filaments of the protein [pb_glossary id=\"4182\"]myosin<\/strong>[\/pb_glossary]. The filaments are arranged together within a myofibril in repeating units called\u00a0[pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary],<\/strong> which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal and cardiac muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated, because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibres. Other components of a skeletal muscle fibre include multiple nuclei and mitochondria.\r\n\r\n[caption id=\"attachment_4183\" align=\"aligncenter\" width=\"801\"] Figure 12.3.6 Bundles of protein filaments form a myofibril, and bundles of myofibrils make up a single muscle fibre. I and A bands refer to the positioning of myosin and actin fibres in a myofibril. Sarcoplasmic reticulum is a specialized type of endoplasmic reticulum that forms a network around each myofibril. It serves as a reservoir for calcium ions, which are needed for muscle contractions. H zones and Z discs are also involved in muscle contractions, which you can read about in the concept Muscle Contraction.<\/em>[\/caption]\r\nSlow- and Fast-Twitch Skeletal Muscle Fibres<\/h2>\r\nSkeletal muscle fibres can be divided into two types, called slow-twitch (or type I) muscle fibres and fast-twitch (or type II) muscle fibres.\r\n\r\n \t- [pb_glossary id=\"4184\"]Slow-twitch muscle fibres[\/pb_glossary]<\/strong> are dense with capillaries and rich in [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary] and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.<\/li>\r\n \t
- [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n\r\n
Smooth Muscle<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2982\"]Smooth muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue in the walls of internal organs and other internal structures such as\u00a0blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions.\u00a0When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are [pb_glossary id=\"3005\"]involuntary[\/pb_glossary], so they are not under conscious control. Instead, they are controlled by the [pb_glossary id=\"2935\"]autonomic\u00a0nervous system[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], [pb_glossary id=\"3056\"]neurotransmitters[\/pb_glossary], and other physiological factors.\r\nStructure of Smooth Muscle<\/h2>\r\nThe\u00a0cells\u00a0that make up smooth muscle are generally called\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. <\/strong>Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in [pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary]. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which in turn contain bundles of [pb_glossary id=\"4182\"]myosin[\/pb_glossary] and [pb_glossary id=\"4181\"]actin[\/pb_glossary] filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.\r\n\r\n[caption id=\"attachment_4186\" align=\"aligncenter\" width=\"1024\"] Figure 12.3.7 The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue.<\/em>[\/caption]\r\n\r\n\r\n\r\nFunctions of Smooth Muscle<\/span>\r\n\r\n<\/div>\r\nUnlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of [pb_glossary id=\"4187\"]elastin[\/pb_glossary], [pb_glossary id=\"3970\"]collagen[\/pb_glossary], and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.\r\n
- \r\n \t
- [pb_glossary id=\"4184\"]Slow-twitch muscle fibres[\/pb_glossary]<\/strong> are dense with capillaries and rich in [pb_glossary id=\"5783\"]mitochondria[\/pb_glossary] and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibres, slow-twitch fibres can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibres can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.<\/li>\r\n \t
- [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n
\r\nSmooth Muscle<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2982\"]Smooth muscle[\/pb_glossary]<\/strong>\u00a0is muscle tissue in the walls of internal organs and other internal structures such as\u00a0blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions.\u00a0When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are [pb_glossary id=\"3005\"]involuntary[\/pb_glossary], so they are not under conscious control. Instead, they are controlled by the [pb_glossary id=\"2935\"]autonomic\u00a0nervous system[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], [pb_glossary id=\"3056\"]neurotransmitters[\/pb_glossary], and other physiological factors.\r\n
Structure of Smooth Muscle<\/h2>\r\nThe\u00a0cells\u00a0that make up smooth muscle are generally called\u00a0[pb_glossary id=\"4157\"]myocytes[\/pb_glossary]. <\/strong>Unlike the muscle fibres of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in [pb_glossary id=\"4179\"]sarcomeres[\/pb_glossary]. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain [pb_glossary id=\"4180\"]myofibrils[\/pb_glossary], which in turn contain bundles of [pb_glossary id=\"4182\"]myosin[\/pb_glossary] and [pb_glossary id=\"4181\"]actin[\/pb_glossary] filaments. The filaments cause contractions when they slide over each other, as shown in Figure 12.3.7.\r\n\r\n[caption id=\"attachment_4186\" align=\"aligncenter\" width=\"1024\"]
Figure 12.3.7 The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue.<\/em>[\/caption]\r\n\r\n\r\n\r\nFunctions of Smooth Muscle<\/span>\r\n\r\n<\/div>\r\nUnlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of [pb_glossary id=\"4187\"]elastin[\/pb_glossary], [pb_glossary id=\"3970\"]collagen[\/pb_glossary], and other stretchy fibres. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (see Figures 12.3.8 and 12.3.9), both of which must stretch considerably as they perform their normal functions.\r\n - [pb_glossary id=\"4185\"]Fast-twitch muscle fibres[\/pb_glossary]<\/strong> contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fibre can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibres can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibres, fast-twitch fibres contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weightlifting.<\/li>\r\n<\/ul>\r\nProportions of fibre types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fibre than the other. Generally, an individual who has more slow-twitch fibres is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibres is better suited for activities requiring short bursts of power.\r\n
![Skeletal_Smooth_Cardiac<\/a>\u00a0by](\"https:\/\/commons.wikimedia.org\/wiki\/File:414_Skeletal_Smooth_Cardiac.jpg\"){kind=link}
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