Week 7

muscle disorders

During the course of a lifetime, nearly everyone will suffer from some type of muscular disorder. Muscular disorders cover a wide spectrum in terms of severity. Minor muscle irritation, inflammation, or injury may resolve without any medical care. However, many diseases affecting the neuromuscular system are extremely serious and eventually prove to be fatal. Spasms are sudden, involuntary muscular contractions, most often accompanied by pain. Spasms can occur in both smooth and skeletal muscles. A spasm of the intestinal tract is a "bellyache"; most such spasms are not serious. Multiple spasms of skeletal muscles are called convulsions. Cramps are strong, painful spasms, especially of the leg and foot, usually due to strenuous athletic activity. Cramps typically occur after a strenuous workout, and may even occur when sleeping. Facial tics, such as periodic eye blinking or grimacing, are spasms that can be controlled voluntarily but only with great effort. Muscles, joints, and their connective tissues are often subject to overuse injuries: strains, sprains, and tendinitis. A strain is caused by stretching or tearing of a muscle. A sprain is the twisting of a joint, leading to swelling and injury not only of muscles but also of ligaments, tendons, blood vessels, and nerves. The ankle and knee are two areas often subject to sprains. Tendinitis is inflammation of a tendon due to repeated athletic activity. The tendons most commonly affected are those associated with the shoulder, elbow, hip, and knee. Overuse injuries are often minor and can be treated with pain medication and rest. However, an individual should seek medical attention if the injured area is extremely painful, hot, or swollen, or if accompanied by a fever. Neuromuscular Diseases Neuromuscular disease can result from pathologic changes to the muscle itself. It can also result from excessive motor nerve stimulation, or from damage or destruction of the motor neurons that supply the muscle. The disease tetanus develops in persons who have not been properly immunized against the toxin of the tetanus bacterium. Tetanus toxin shuts down brain areas that normally inhibit unnecessary muscle contractions. As a result, excessive brain stimulation causes muscles to lock in a tetanic contraction (from which the disease gets its name). A rigidly locked jaw is one of the first signs of bacterial infection and toxin production. Though antibiotics will kill the bacteria, once the toxin is circulating in the bloodstream, it cannot be removed or neutralized. Because muscles can't relax, the patient cannot breathe or swallow, and death may occur due to respiratory failure. Immunization and periodic booster shots will prevent the toxin's effects (see Immunization: The Great Protector, pages 309-310). Fibromyalgia is a chronic condition whose symptoms include achy pain, tenderness, and stiffness of muscles. Its precise cause is not known, though 80-90% of sufferers are women. Substance P, a neurotransmitter (messenger chemical) of pain pathways in the brain, has been found in the bloodstream of affected individuals. Exercise seems to decrease blood levels of substance P. Therapeutic massage, over-the-counter pain medication, and muscle relaxants are also recommended. Muscular dystrophy is a broad term applied to a group of disorders that causes progressive degeneration and weakening of muscles. As muscle fibers die, fat and connective tissue take their place. Duchenne muscular dystrophy, the most common type, is inherited through a flawed gene carried by the mother. It is now known that the lack of a protein called dystrophin causes the condition. When dystrophin is absent, calcium leaks into the cell and activates an enzyme that dissolves muscle fibers. Treatment includes muscle injections with immature muscle cells that do produce dystrophin. Myasthenia gravis is an autoimmune disease characterized by weakness that especially affects the muscles of the eyelids, face, neck, and extremities. Muscle contraction is impaired because the immune system mistakenly produces antibodies that destroy acetylcholine receptors on the sarcolemma. (Recall that acetylcholine is the neurotransmitter released by motor neurons.) In many cases, the first signs of the disease are drooping eyelids and double vision. Treatment includes drugs that inhibit the enzyme that digests acetylcholine, thus allowing it to accumulate. Amyotrophic lateral sclerosis (ALS) is often called Lou Gehrig's disease, after its most famous victim, the 1930s-era baseball player. ALS sufferers experience the gradual death of their motor neurons, thus losing the ability to walk, talk, chew, swallow, etc. Intellect and sensation are not affected, however. Drugs can slow the disease's progression, but ALS is always fatal.

in the lab

Samples of intact muscle can be taken out of the body and studied in a laboratory, and these experiments have allowed scientists to make some important observations about how muscle fibers and entire muscles work. For example, an isolated muscle can be placed in a laboratory solution that provides the ATP, nutrients, and electrolytes it needs to survive and contract. Next, it is stimulated with an electric shock, whose voltage must be strong enough to make the muscle contract. If a contraction occurs, the electrical stimulus is called a threshold stimulus; if not, the stimulus is called a subthreshold stimulus. A single threshold stimulus causes the muscle to quickly contract and relax. This action—a single contraction that lasts only a fraction of a second—is called a muscle twitch. The mechanical force of contraction is recorded as a visual pattern called a myogram (Fig. 7.7a). A muscle twitch can be divided into three stages: the latent period, contraction period, and relaxation period. The latent period is the interval between the threshold stimulus and the onset of the muscle contraction. During this time, all of the events that lead up to cross-bridge formation are occurring in the cell. Acetylcholine diffuses across the neuromuscular junction and binds to receptors on the muscle, the action potential spreads over and throughout the muscle fibers, and calcium is released from the sarcoplasmic reticulum. The contraction period follows the latent period. The muscle physically shortens during this period as cross-bridges are formed, and thick and thin myofilaments slide past one another. Force generated by the muscle increases. Finally, during the relaxation period, muscle force decreases. The muscle returns to its former length as cross-bridges break and calcium is returned to the sarcoplasmic reticulum. If a muscle twitches and then is allowed to relax in between threshold stimuli, the resulting myogram shows a series of twitches Page 143with identical force (Fig. 7.7b). However, if the muscle is stimulated rapidly, the contraction force gradually increases, even if the stimulus voltage is exactly the same. The more rapidly the muscle is stimulated, the greater the contraction force becomes. This effect is called summation (Fig. 7.7c). Scientists believe that summation occurs because when the muscle is stimulated quickly enough, there is not enough time between stimuli to return all the calcium to the sarcoplasmic reticulum. With extra calcium in the sarcoplasm, more cross-bridges can be formed after each threshold stimulus. Further, repetitive muscle contraction also generates heat as ATP is broken down to release energy, and scientists believe that muscle enzymes may work more efficiently to cause contraction when muscle fibers are warmer. During summation, the muscle has less time to relax as the rate of stimulation increases, and the muscle force becomes more and more constant. When the muscle is stimulated very rapidly, the muscle has no time to relax at all. This effect is called tetanus,1 or a tetanic contraction. It can be seen on the myogram as a horizontal line, because muscle force is constant during this period. If stimulation continues at the same rate, tetanus will continue until the muscle fatigues. Fatigue occurs when the muscle relaxes even though stimulation continues, and on the myogram it can be seen when muscle force falls. There are several reasons why isolated muscles become fatigued. First, ATP is depleted during constant use of a muscle; the muscle essentially "runs out of energy." At the same time, repetitive use causes production of lactic acid by fermentation, which lowers the pH of the sarcoplasm and inhibits muscle function. In addition, the motor nerves that supply muscle can run out of their neurotransmitter, acetylcholine. However, intact muscles in the body rarely fatigue completely like an isolated muscle in the laboratory does. Muscles are well supplied with blood vessels to transport nutrients and remove lactic acid. Instead, in the body, fatigue is a gradual weakening that occurs after repetitive use. In addition, the brain itself may signal a person to stop exercising, even if the muscles are not truly fatigued. The mechanisms that cause this muscle fatigue are not well understood. People who train can exercise for longer periods without experiencing fatigue.

smooth muscle

Smooth muscle is located in the walls of hollow internal organs and blood vessels. Its involuntary contractions regulate blood flow in blood vessels and move materials through hollow organs such as the digestive organs, uterus, and urinary bladder. Smooth muscle Page 135cells are spindle-shaped: narrow, tapered cylindrical cells with pointed ends. Each cell is uninucleate (has a single nucleus). The cells are usually arranged in parallel lines, forming sheets. Smooth muscle does not have the striations (bands of light and dark) seen in cardiac and skeletal muscle. Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily.

muscle carpartment

The deep fascia that surrounds a group of several muscles is called a muscle compartment. It functions as a tight sleeve containing the muscles, nerves, and blood vessels. In a compartment syndrome, swelling inside a compartment increases, choking off the blood supply to the muscle. This condition can cause complete muscle destruction and may be fatal. What type of injury could cause a compartment syndrome? How could it be corrected?

filaments

Thick Filaments A thick filament is composed of several hundred molecules of the protein myosin. Each myosin molecule is composed of two protein strands, each shaped like a golf club. The straight portions of each strand coil around each other. Each myosin molecule ends in a double globular head. The paired myosin heads are slanted away from the middle of a sarcomere, toward the thin filaments surrounding them. One of the paired heads has an actin binding site that will link to actin in thin filaments during muscle contraction. The second is an enzyme that will break down cellular ATP to release the energy needed for the contraction. Thin Filaments A thin filament consists primarily of two strands of the globular protein actin, twisted around each other like intertwined bead necklaces. Double strands of tropomyosin coil over each actin strand. Troponin occurs at intervals on the tropomyosin strand (see Fig. 7.5a).

skeletal muscle fibers

fibers are cylindrical and have multiple nuclei around the periphery of the cell, just inside the plasma membrane. The fibers have alternating light and dark bands, and you will remember (from Chapter 4) that these are called striations. Skeletal muscles attach to the skeleton or directly to the skin (in the case of facial muscles). Skeletal muscle fibers can run the length of a muscle and therefore can be quite long. Skeletal muscle is voluntary because its contraction is always stimulated and controlled by the nervous system. (It's important to note, however, that though skeletal muscle is termed voluntary, it is not always consciously controlled. In Chapter 8, you'll learn about muscle reflexes, which happen without any conscious thought.) In this chapter, we will explore why skeletal muscle (and cardiac muscle) is striated.

--

Athletics and Muscle Contraction Athletes who excel in a particular sport, and much of the general public as well, are interested in staying fit by exercising. The Medical Focus on pages 155-156 outlines the importance of exercising throughout life, and gives suggestions for starting and staying with an exercise program. Exercise and Size of Muscles Muscles that are not used or that are used for only very weak contractions decrease in size, or atrophy. Atrophy can occur when a limb is placed in a cast or when the nerve serving a muscle is damaged. If nerve stimulation is not restored, muscle fibers are gradually replaced by fat and fibrous tissue. Unfortunately, atrophy can cause muscle fibers to shorten progressively, leaving body parts contracted in contorted positions called contractures. Forceful muscular activity over a prolonged period causes muscle to increase in size as the size of the individual myofibrils within the muscle fibers increases. Increase in muscle size, called hypertrophy, occurs only if the muscle contracts to at least 75% of its maximum tension. Some athletes take anabolic steroids, either testosterone or related chemicals, to promote muscle growth. This practice is quite dangerous and has many undesirable side effects, as discussed in the Medical Focus on pages 234-235. Slow-Twitch and Fast-Twitch Muscle Fibers We've seen that all skeletal muscle fibers metabolize both aerobically (using oxygen during cellular respiration) and anaerobically (without oxygen, using fermentation or creatine phosphate breakdown). However, some muscle fibers utilize one method more than the other to provide myofibrils with ATP. Slow-twitch and intermediate-twitch muscle fibers tend to be aerobic, and fast-twitch fibers tend to be anaerobic. Slow-twitch fibers are also referred to as type I fibers, intermediate-twitch fibers are type IIa, and fast-twitch fibers are called type IIb fibers. Slow-twitch fibers have motor units with a smaller number of fibers. These muscle fibers are most helpful in endurance sports such as long-distance running or swimming in a triathlon. Because they produce most of their energy aerobically, they tire only when their fuel supply is gone. Slow-twitch fibers have many mitochondria that can maintain a steady, prolonged production of ATP when oxygen is available. An abundant supply of myoglobin, the respiratory pigment found in muscles, gives these fibers a dark color. They are also surrounded by dense capillary beds for a continuous supply of blood and oxygen. Slow-twitch fibers have a low maximum tension, which develops slowly, but these muscle fibers are highly resistant to fatigue. Like slow-twitch fibers, intermediate-twitch fibers are well supplied with myoglobin and contain many mitochondria. Intermediate-twitch fibers have an extensive blood supply as well. However, they contract much more quickly and can be described as fast aerobic fibers. Activities that require moderate strength for shorter periods (such as walking, jogging, or biking) will employ these muscles. Page 145Fast-twitch fibers have fewer capillaries to supply blood and oxygen, and they tend to function anaerobically. These fibers are light in color because they have fewer mitochondria and little myoglobin. They provide explosions of energy and are most helpful in sports activities such as sprinting, weight lifting, swinging a golf club, or pitching a baseball. Fast-twitch fibers can develop maximum tension more rapidly, and their maximum tension is greater. However, their dependence on anaerobic energy leaves them vulnerable to an accumulation of lactic acid that causes them to fatigue quickly.

cardiac muscles

forms the heart wall. Its fibers are striated and cylindrical and have one or two central nuclei. Because they branch, cardiac muscle fibers are able to interlock at intercalated disks. Intercalated disks contain gap junctions and adhesion junctions, which permit contractions to spread quickly throughout the heart. Cardiac fibers relax completely between contractions, which prevents fatigue. Contraction of cardiac muscle fibers is rhythmical and occurs without requiring outside nervous stimulation. Thus, cardiac muscle contraction is involuntary. However, keep in mind that in order to maintain homeostasis, the nerves that supply the heart can increase or decrease both heart rate and strength of contraction. You'll learn more about cardiac muscle in Chapter 12

muscle fiber structure

A muscle fiber contains the usual cellular components, but special names have been assigned to some of these components (note that the terms used to describe muscle start with the prefixes myo- and sarco-; Table 7.1 and Fig. 7.3). Thus, the plasma membrane is called the sarcolemma; cytoplasm is the sarcoplasm; and the endoplasmic reticulum is the sarcoplasmic reticulum. A muscle fiber also has some unique anatomical characteristics. One feature is its T (for transverse) system; the sarcolemma forms T (transverse) tubules that penetrate, or dip down, into the cell so that they come into contact—but do not fuse with—expanded portions of the sarcoplasmic reticulum. The expanded portions of the sarcoplasmic reticulum are calcium storage sites. Calcium ions (Ca2+), as we will see, are essential for muscle contraction. The sarcoplasmic reticulum is a fine endomembrane network that surrounds hundreds and sometimes even thousands of myofibrils, which are bundles of myofilaments. Each myofibril is about 1 micrometer in diameter. Myofibrils are the parts of muscle fibers that contract. Any other organelles, such as mitochondria, are located in the sarcoplasm between the myofibrils. The sarcoplasm also contains glycogen, which provides stored energy for muscle contraction, and the red pigment myoglobin, which binds oxygen until it is needed for muscle contraction. Muscle is the only tissue that has both myoglobin for a supplemental oxygen supply and glycogen as a nutrient source. Thus, muscle is well-suited to produce the enormous amounts of ATP energy needed for muscle contraction.

muscles

All muscles, regardless of type, can contract—that is, shorten. When muscles contract, some part of the body or the entire body moves. As you learned in Chapter 4, humans have three types of muscles: skeletal, cardiac, and smooth (Fig. 7.1). The contractile cells of these tissues are elongated and therefore are called muscle fibers.

in the body

As you know, muscles in the body are stimulated to contract by motor nerves composed of motor neurons. The combination of the neuron and all of the muscle fibers it innervates is called a motor unit. Motor units function according to a property called the all-or-none law: since all the muscle fibers in a motor unit are stimulated at once by the same neuron, they all contract simultaneously in response to the neuron, or do not contract. It's interesting to note that the number of muscle fibers within a motor unit can be quite different. For example, in the ocular muscles that move the eyes, the innervation ratio is one motor axon per 23 muscle fibers, while in the gastrocnemius muscle of the leg, the ratio is about one motor axon per 1,000 muscle fibers. No doubt, moving the eyes requires finer control than moving the lower limbs. Changing the strength of a contraction occurs consciously or unconsciously throughout one's day. For example, think about the strength it might take to lift a pencil. Now imagine the muscular effort needed to lift a book bag loaded with an entire day's books and supplies. Increasing a muscle's contraction strength can be accomplished by a phenomenon known as recruitment. By increasing the intensity of nervous stimulation, the nervous system can activate, or recruit, more and more motor units, resulting in stronger and stronger muscle contraction. However, while some muscle fibers within a muscle are contracting, others in the same muscle are relaxing. This allows a muscle to sustain a contraction for a long period of time. In life, even when muscles appear to be at rest, they exhibit tone, in which some of their fibers are always contracting. Muscle tone is particularly important in maintaining Page 144posture. If all the fibers within the muscles of the neck, trunk, and lower limbs were to suddenly relax, the body would collapse.

cellular respiration

Cellular respiration completed in mitochondria usually provides most of a muscle's ATP. Glycogen and fat are stored in muscle cells. Therefore, a muscle cell can use glucose from glycogen and fatty acids from fat as fuel to produce ATP if oxygen is available: How A D P converts into A T P for energy [D] The red pigment myoglobin is an oxygen carrier manufactured by muscle cells, and it is similar to the hemoglobin protein of red blood cells. Its presence accounts for the reddish-brown color of skeletal muscle fibers. Myoglobin has a higher affinity for oxygen than hemoglobin. Therefore, myoglobin can pull oxygen out of blood and make it available to muscle mitochondria that are carrying on cellular respiration. Then, too, the ability of myoglobin to temporarily store oxygen reduces a muscle's immediate need for oxygen when cellular respiration begins. The end products of cellular respiration (carbon dioxide and water) Page 142are usually no problem in a healthy person's body. Carbon dioxide is exhaled by the lungs. Water simply enters the extracellular space, and any excess can be excreted by the kidneys. The by-product of muscular contraction—heat—keeps the entire body warm.

contraction

Contraction of Cardiac Muscle The events of contraction in cardiac muscle are very similar to those in skeletal muscle. However, in cardiac muscle, the calcium needed to bind to troponin comes from outside the cell as well as inside. After the action potential signal in cardiac muscle, calcium diffuses into the sarcoplasm and triggers the release of more calcium from the sarcoplasmic reticulum. Once calcium has bound to troponin, cross-bridges can form between activated myosin and actin, just as in skeletal muscle. Contraction of Smooth Muscle You'll recall that smooth muscle cells are spindle-shaped, that is, uninucleate, cylindrical cells with pointed ends, and that smooth muscle control is involuntary. Like skeletal muscle, smooth muscle contains thick and thin filaments. However, in smooth muscle these filaments are not arranged into myofibrils that create visible striations. Instead, thin filaments in smooth muscle are anchored directly to the sarcolemma or to protein molecules called dense bodies. Dense bodies are scattered through the sarcoplasm. When a smooth muscle cell contracts, its fibers shorten in all directions, causing the cylindrical cell to become more oval in shape. In turn, the entire smooth muscle shortens. Smooth muscle contraction occurs very slowly but can last for long periods of time without fatigue. Breakdown of ATP and CrossBridge Movement During Muscle Contraction Energy for Muscle Contraction ATP produced before strenuous exercise and found in the muscle cell sarcoplasm lasts a few seconds. Then, muscles must manufacture new ATP in three different ways: creatine phosphate Page 141breakdown, cellular respiration, and fermentation (Fig. 7.6). Creatine phosphate breakdown and fermentation are anaerobic, meaning that they do not require oxygen.

phosophate breakdown

Creatine Phosphate Breakdown Creatine phosphate is a high-energy compound built up when a muscle is resting. Creatine phosphate cannot participate directly in muscle contraction. Instead, it can recycle ADP inside the cell into ATP by transferring its phosphate to ADP, using the following reaction: The phosphate group is completely removed from creatine phosphate. The phosphate group is placed onto the ADP, or adenosine DI phosphate, molecule. This converts ADP to adenosine TRI phosphate, or ATP. The remaining creatine molecule can later be converted back to creatine phosphate. This reaction occurs in the midst of sliding filaments, and therefore is the speediest way to make ATP available to muscles. Creatine phosphate provides enough energy for only about eight seconds of intense activity, and then it is used up. Creatine phosphate is rebuilt when a muscle is resting by transferring a phosphate group from ATP back to creatine (Fig. 7.6a).

muscles of neck

Deep muscles of the neck (not illustrated) are responsible for swallowing. Superficial muscles of the neck move the head (Table 7.2 and Fig. 7.10). Page 148 Swallowing Swallowing is an important activity that begins after we chew our food. First, the tongue (a muscle) and the buccinators squeeze the food back along the roof of the mouth toward the pharynx. An important bone that functions in swallowing is the hyoid (see Figure 6.4). As you know, the hyoid is the only bone in the body that does not articulate (form a joint) with another bone. Muscles that lie superior to the hyoid, called the suprahyoid muscles, and muscles that lie inferior to the hyoid, called the infrahyoid muscles, move the hyoid. Because these muscles lie deep in the neck, they are not illustrated in Figure 7.10. The suprahyoid muscles pull the hyoid forward and upward toward the mandible. Because the hyoid is attached to the larynx, this pulls the larynx upward and forward. The epiglottis now lies over the glottis and closes the respiratory passages. Small palatini muscles (not illustrated) pull the soft palate backward, closing off the nasal passages. Pharyngeal constrictor muscles (not illustrated) push the bolus of food into the pharynx, which widens when the suprahyoid muscles move the hyoid. The hyoid bone and larynx are returned to their original positions by the infrahyoid muscles. Notice that the suprahyoid and infrahyoid muscles are antagonists. Muscles That Move the Head Two muscles in the neck are of particular interest: The sternocleidomastoid and the trapezius are listed in Table 7.2 and illustrated in Figure 7.10. Recall that flexion is a movement that closes the angle at a joint and extension is a movement that increases the angle at a joint. Recall that abduction is a movement away from the midline of the body, whereas adduction is a movement toward the midline. Also, rotation is the movement of a part around its own axis. Sternocleidomastoid muscles ascend obliquely from their origin on the sternum and clavicle to their insertion on the mastoid process of the temporal bone. Which part of the body do you expect them to move? When both sternocleidomastoid muscles contract, flexion of the head occurs. When only one contracts, the head turns to the opposite side. If you turn your head to the right, you can see how the left sternocleidomastoid shortens, pulling the head to the right. Each side of the trapezius muscle is triangular, but together, they take on a diamond or trapezoid shape. The origin of the trapezius is at the base of the skull. Its insertion is on the clavicles and scapula. You would expect the trapezius muscles to move the scapulae, and they do. They adduct the scapulae when the shoulders are shrugged or pulled back. The trapezius muscles also help extend the head, however. The prime movers for head extension are actually deep to the trapezius and not illustrated in Figure 7.10.

fermentation

Fermentation, like creatine phosphate breakdown, supplies ATP without consuming oxygen. Like creatine phosphate breakdown, fermentation is an anaerobic process, and it occurs in the sarcoplasm. Fermentation produces the ATP necessary for short bursts of exercise—for example, a 50-yard dash or a run around the bases in a baseball game. During fermentation, glucose is broken down to lactate (lactic acid): The accumulation of lactate in a muscle fiber makes the sarcoplasm more acidic, and eventually enzymes cease to function well. If fermentation continues longer than two or three minutes, cramping and fatigue set in. Cramping seems to be due to lack of ATP. As you recall, that is because ATP is needed to pump calcium ions back into the sarcoplasmic reticulum and to break the linkages between the actin and myosin filaments so that muscle fibers can relax.

role of mylofilaments

Figure 7.5a shows the placement of the three proteins that make up a thin filament. Thin filaments are composed of a double row of twisted actin molecules. Threads of tropomyosin wind around each actin filament, and troponin occurs at intervals along the tropomyosin threads. A myosin binding site can be found on each actin molecule; when muscle is relaxed, these binding sites are covered by tropomyosin. Calcium ions (Ca2+) that have been released from the sarcoplasmic reticulum combine with troponin. After binding occurs, the tropomyosin threads shift their position, and myosin binding sites on the actin molecules are exposed. Each one of the paired globular heads of a myosin thick filament has its own binding sites. One site binds to ATP and then functions as an ATPase enzyme, splitting ATP into ADP and a phosphate group ℗ (see Fig. 7.5b, step 1). The energy from this reaction activates the second binding site so that it will bind to actin. The ADP and ℗ remain on the myosin heads until the heads attach to actin (Fig. 7.5b, step 2). Now, ADP and ℗ are released, and this causes the myosin head to bend sharply toward the center of the sarcomere (Fig. 7.5b, step 3). This action of myosin is called the power stroke, and it pulls the thin filaments toward the middle of the sarcomere. When another ATP molecule binds to a myosin head, the head detaches from actin (Fig. 7.5b, step 4). The cycle begins again, and the thin filaments move nearer the center of the sarcomere each time the cycle is repeated. Some of the myosin heads remain attached to actin during the cycle while others form new bonds. Thus, the thin filaments don't slide back to their resting position while a Page 139contraction is occurring. Note that ATP has two roles in this process: first to energize myosin, and then to break the link between myosin and actin. (A good analogy for the sliding filament action of the myosin myofilaments in a sarcomere is a game of tug-of-war. The players on either side can be likened to myosin molecules. As their hands grab, release, and grab the rope again, each side is pulled toward the other and the distance between them is shortened.) As the thin filaments slide past the thick filaments toward the sarcomere's center, the entire sarcomere shortens (though the thick and thin filaments themselves remain the same length). This causes the I band to shorten and the H zone to almost or completely disappear (Fig. 7.3c and d). It's important to note that although the thin filaments slide past the thick filaments, the myosin filaments do the work with their power stroke. As sarcomeres shorten, the myofibril and ultimately the entire muscle fiber are shortened. Contraction continues until nerve signals stop and calcium ions are returned to their storage sites. The membranes of the sarcoplasmic reticulum contain active transport proteins that pump calcium ions back into the interior of the sarcoplasmic reticulum. Of course, this active transport process also requires ATP energy.

Intro

If you were asked to describe what skeletal muscles do, you'd probably talk about the conscious limb movements you need for everyday activities like walking, running, lifting, bending, twisting from side to side, performing sports activities, and so on. But here's another skeletal muscle function you might not have thought of: communication through facial expression. We use over 20 muscles just for facial expression (anatomists disagree on the exact number), and each performs a very precise movement. It's interesting to note that human infants mimic the expressions of their caregivers almost from birth. Expressing emotions—joy, fear, frustration, unhappiness, loneliness—without speaking is essential for the baby's survival, for without this ability the child couldn't communicate its needs. Further, facial expressions convey nonverbal messages between people of all races and cultures. Check out the expressions of the people pictured here—what do you think each is feeling? Then, study Figure 7.10 and see if you can determine which facial muscles are used to make each of these expressions.I

naming muscles

It's easier to learn and remember muscle names if you consider what each muscle's name means. The names of the various skeletal muscles are often combinations of the following terms used to characterize muscles: Size. For example, the gluteus maximus is the largest muscle that makes up the buttocks. The gluteus minimus is the smallest of the gluteal muscles. Other terms used to indicate size are vastus (huge), longus (long), and brevis (short). Shape. For example, the deltoid is shaped like a delta, or triangle, while the trapezius is shaped like a trapezoid. Other terms used to indicate shape are latissimus (wide) and teres (round). Direction of fibers. For example, the rectus abdominis is a longitudinal muscle of the abdomen (rectus means straight). The orbicularis oculi is a circular muscle around the eye. Other terms used to indicate direction are transverse (across) and oblique (diagonal). Location. For example, the frontalis overlies the frontal bone. The external obliques are located outside the internal obliques. Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath). You should also review these directional terms: anterior, posterior, lateral, medial, proximal, distal, superficial, and deep. Attachment. For example, the sternocleidomastoid is attached to the sternum, clavicle, and mastoid process. The brachioradialis is attached to the brachium (arm) and the radius. Number of attachments. For example, the biceps brachii has two attachments, or origins (and is located on the arm). The quadriceps femoris has four origins (and is located on the anterior femur). Action. For example, the extensor digitorum extends the fingers or digits. The adductor magnus is a large muscle that adducts the thigh. Other terms used to indicate action are flexor (to flex), masseter (to chew), and levator (to lift).

aging intro

Muscle mass and strength tend to decrease as people age. How much of this is due to lack of exercise and a poor diet is under careful study. Deteriorated muscle elements are replaced initially by connective tissue and, eventually, by fat. With age, degenerative changes take place in the mitochondria, and endurance decreases. Also, changes in the nervous and cardiovascular systems adversely affect the structure and function of muscles. Muscle mass and strength can improve remarkably if elderly people undergo a training program. Exercise at any age appears to stimulate muscle buildup. As discussed in the Medical Focus, Benefits of Exercise on pages 155-156, exercise has many other benefits as well. For example, exercise improves the cardiovascular system and reduces the risk of elevated blood sugar, metabolic syndrome, diabetes, and glycation. During glycation, excess glucose molecules stick to body proteins so that the proteins no longer have their normal structure and cannot function properly. Exercise burns glucose and, in this way, helps prevent muscle deterioration.

muscles thatmove leg, ankle and foot

Muscles That Move the Leg The muscles that move the leg originate from the pelvic girdle or femur and insert on the tibia. They are listed in Table 7.5 and illustrated in Figures 7.14 and 7.15. Before studying these muscles, review the movement of the knee when the leg extends and when it flexes (see Chapter 6, pp. 124-126). Quadriceps femoris group (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), also known as the "quads," is found on the anterior and medial thigh. The rectus femoris, which originates from the ilium, is external to the vastus intermedius, and therefore the vastus intermedius is not shown in Figure 7.14. These muscles are the primary extensors of the leg, as when you kick a ball by straightening your knee. They also stabilize the hip and assist in thigh flexion. Sartorius is a long, straplike muscle that has its origin on the iliac spine and then goes across the anterior thigh to insert on the medial side of the knee (Fig. 7.14). Because this muscle crosses both the hip and knee joint, it acts on the thigh in addition to the leg. The insertion of the sartorius is such that it flexes both the leg and the thigh. It also abducts and laterally rotates the thigh, enabling us to sit cross-legged, as tailors were accustomed to do in another era. Therefore, it is sometimes called the "tailor's muscle," and in fact, sartor means tailor in Latin. Hamstring group (biceps femoris, semimembranosus, semitendinosus) is located on the posterior thigh (Fig. 7.15). Notice that these muscles also cross the hip and knee joint because they have origins on the ischium and insert on the tibia. They flex and rotate the leg medially, but they also extend the thigh. Their strong tendons can be felt behind the knee. These same tendons are present in hogs and were used by butchers as strings to hang up hams for smoking—hence, the name. Notice that the quadriceps femoris group and the hamstring group are antagonistic muscles in that the quads extend the leg and the hamstrings flex the leg. Likewise, the quads assist in flexing the thigh, while the hamstrings extend the thigh. Muscles That Move the Ankle and Foot Muscles that move the ankle and foot are shown in Figures 7.16 and 7.17. Figure 7.16 Muscles of the anterior right leg. Module 6: Dissection Tibia and fibula Figure 7.17 Muscles of the lateral right leg. Module 6: Dissection Tibia and fibula Gastrocnemius is a muscle of the posterior leg, where it forms a large part of the calf. It arises from the femur; distally, the muscle joins the strong calcaneal tendon, which attaches to the calcaneus bone (heel). The gastrocnemius is a leg flexor, but its most important function is to act as a powerful plantar flexor of the foot that aids in pushing the body forward during walking or running. It is sometimes called the "toe dancer's muscle" because it allows a person to stand on tiptoe. Tibialis anterior is a long, spindle-shaped muscle of the anterior leg. It arises from the surface of the tibia and attaches to the bones of the ankle and foot. Contraction of this muscle causes dorsiflexion and inversion of the foot. Fibularis muscles (fibularis longus, fibularis brevis) are found on the lateral side of the leg, connecting the fibula to the metatarsal bones of the foot. These muscles evert the foot and also help bring about plantar flexion. Flexor (not shown) and extensor digitorum longus muscles primarily originate from the tibia and insert on the toes. They flex and extend the toes, respectively, and assist in other movements of the feet. Page 154

muscles that move...

Muscles That Move the Scapula Of the muscles that move the scapula, you know (from page 148) that the trapezius adducts the scapulae. Serratus anterior is located below the axilla (armpit) on the lateral chest. It runs between the upper ribs and the scapula. It holds the scapula near the thorax, pulling it forward (as when we're pushing on something in front of us). Because this muscle causes a fast-forward jab of the arm, it is often called the boxer's muscle. It also helps to elevate the arm above the horizontal level. Page 151 Muscles That Move the Arm Deltoid is a large, fleshy, triangular muscle (deltoid in Greek means triangular) that covers the shoulder and causes a bulge in the arm where it meets the shoulder. It runs from both the clavicle and the scapula of the pectoral girdle to the humerus. This muscle abducts the arm to the horizontal position. Pectoralis major (Fig. 7.11) is a large anterior muscle of the upper chest. It originates from a clavicle, but also from the sternum and ribs. It inserts on the humerus. The pectoralis major flexes the arm (raises it anteriorly). It also medially rotates and adducts the arm, pulling it toward the chest. Latissimus dorsi (Fig. 7.12) is a large, wide, triangular muscle of the back. This muscle originates from the lower spine and sweeps upward to insert on the humerus. The latissimus dorsi extends, medially rotates, and adducts the arm (brings it down from a raised position). This muscle is very important for swimming, rowing, and climbing a rope. Rotator cuff (Figs. 7.12 and 7.13). This group of muscles is so named because their tendons help form a cuff over the proximal humerus. There are four rotator cuff muscles. Three are located on the posterior scapula: supraspinatus, infraspinatus, and teres minor. The last rotator cuff muscle is the subscapularis muscle located on the anterior surface of the scapula. These muscles lie deep to those already mentioned, and they are synergists to them.

muscles that move thigh

Muscles That Move the Thigh The muscles that move the thigh have at least one origin on the pelvic girdle and insert on the femur. Notice that the iliopsoas is an anterior muscle that moves the thigh, while the gluteal muscles ("glutes") are posterior muscles that move the thigh. The adductor muscles are medial muscles (Figs. 7.14 and 7.15). Before studying the action of these muscles, review the movement of the hip joint when the thigh flexes, extends, abducts, and adducts (see Chapter 6, pp. 124-126). Iliopsoas (includes psoas major and iliacus) originates at the ilium and the bodies of the lumbar vertebrae, and inserts on the femur medially (Fig. 7.14). This muscle is the prime mover for flexing the thigh and also the trunk, as when we bow. As the major flexor of the thigh, the iliopsoas is important to the process of walking. It also helps prevent the trunk from falling backward when a person is standing erect. The gluteal muscles form the buttocks. We will consider only the gluteus maximus and the gluteus medius, both of which are illustrated in Figure 7.15. (The third gluteal muscle, gluteus minimus, is deep to both and therefore not shown in the figure.) Gluteus maximus is the largest muscle in the body and covers a large part of the buttock (gluteus means buttocks in Greek). It originates at the ilium and sacrum, and inserts on the femur. The gluteus maximus is a prime mover of thigh extension, as when a person is walking, climbing stairs, or jumping from a crouched position. Notice that the iliopsoas and the gluteus maximus are antagonistic muscles. Gluteus medius lies partly behind the gluteus maximus (Fig. 7.15). It runs between the ilium and the femur, and functions to abduct the thigh. The gluteus maximus assists the gluteus medius in this function. Therefore, they are synergistic muscles. Adductor group muscles (pectineus, adductor longus, adductor magnus, gracilis) are located on the medial thigh (Fig. 7.14). All of these muscles originate from the pubis and ischium, and insert on the femur; the deep adductor magnus is shown in Figure 7.14. Adductor muscles adduct the thigh—that is, they lower the thigh sideways from a horizontal position. Because they squeeze the thighs together, these are the muscles that keep a rider on a horse. Notice that the glutes and the adductor group are antagonistic muscles.

connective tissue coverings

Muscles are organs, and as such they contain other types of tissues, such as nervous tissue, blood vessels, and connective tissue. Connective tissue is essential to the organization of the fibers within a muscle (Fig. 7.2). First, each fiber is surrounded by a thin layer of areolar (loose) connective tissue called the endomysium. Blood capillaries and nerve fibers reach each muscle fiber by way of the endomysium. Second, the muscle fibers are grouped into bundles called fascicles. The fascicles have a sheath of connective tissue called the perimysium. Finally, the muscle itself is covered by a connective tissue layer called the epimysium. The epimysium blends with the deep fascia, a layer of fibrous tissue that surrounds a set of muscles. Deep fascia separates muscles from each other and from the superficial fascia, also called the hypodermis. Remember (from Chapter 5) that the hypodermis is the layer that lies just deep to the dermis of the skin. Collagen fibers of the epimysium continue as a strong, fibrous tendon that attaches the muscle to a bone. The epimysium merges with the periosteum of the bone.

muscles

Muscles of the Arm The muscles of the arm move the forearm. They are illustrated in Figure 7.13 and listed in Table 7.4. Biceps brachii is a muscle of the proximal anterior arm (Fig. 7.13a) that is familiar because it bulges when the forearm is flexed. It also assists in flexing the arm at the shoulder, and supinates the hand when a doorknob is turned or the cap of a jar is unscrewed. The name of the muscle refers to its two heads that attach to the scapula, where it originates. The biceps brachii inserts on the radius. Brachialis originates on the humerus and inserts on the ulna. It is a muscle of the distal anterior humerus and lies deep to the biceps brachii. It is the strongest forearm flexor muscle, and the biceps brachii is its synergist. Triceps brachii is the only muscle of the posterior arm (Fig. 7.13b). As its name implies, it has three heads. The long head originates from the scapula and humerus, while the medial and lateral heads only originate from the humerus. All three heads join in a common tendon that inserts on the ulna. The triceps extends the arm and forearm. The triceps is also used in tennis to do a backhand volley. Muscles of the Forearm The muscles of the forearm move the hand and fingers. They are illustrated in Figure 7.13c,d and listed in Table 7.4. Note that in anatomical position, extensors of the wrists and fingers are on the posterior and lateral forearm and flexors are on the anterior and medial forearm. Flexor carpi and extensor carpi muscles primarily originate on the humerus and insert on the bones of the hand. The flexor carpi flex the wrists and hands, and the extensor carpi extend the wrists and hands. Flexor digitorum and extensor digitorum muscles also primarily originate on the humerus and insert on the bones of the hand. The flexor digitorum (not shown) flexes the wrist and fingers, and the extensor digitorum extends the wrist and fingers (i.e., the digits). Muscles of the Hip and Lower Limb The muscles of the hip and lower limb are listed in Table 7.5 and shown in Figures 7.14 to 7.17. These muscles, particularly those Page 152of the hips and thigh, tend to be large and heavy because they are used to move the entire weight of the body and to resist the force of gravity. Therefore, they are important for movement and balance.

muscles of walls

Muscles of the Thoracic Wall External intercostal muscles occur between the ribs; they originate on a superior rib and insert on an inferior rib. These muscles elevate the rib cage during the inspiration phase of breathing. The diaphragm is a dome-shaped muscle that, as you know, separates the thoracic cavity from the abdominal cavity (see Fig. 1.5). The diaphragm is the primary muscle for respiration, and it is the only muscle used during normal, quiet breathing. Internal intercostal muscles originate on an inferior rib and insert on a superior rib. These muscles depress the rib cage and contract only during a forced expiration. Normal expiration does not require muscular action. Muscles of the Abdominal Wall The abdominal wall has no bony reinforcement (Fig. 7.11). The wall is strengthened by four pairs of muscles that run at angles to one another. The external and internal obliques and the transversus abdominis occur laterally, but the fasciae of these muscle pairs meet at the midline of the body, forming a tendinous area called the linea alba. The rectus abdominis is a superficial medial pair of muscles. All of the muscle pairs of the abdominal wall compress the abdominal cavity and support and protect the organs within the abdominal cavity. External and internal obliques occur on a slant and are at right angles to one another. They are located between the lower ribs and the pelvic girdle. The internal obliques are deep to the external obliques. These muscles also aid trunk rotation and lateral flexion. Transversus abdominis, deep to the obliques, extends horizontally across the abdomen. The obliques and the transversus abdominis are synergistic muscles. Rectus abdominis has a straplike appearance but takes its name from the fact that it runs straight (rectus means straight) up from the pubic bones to the ribs and sternum. These muscles also help flex and rotate the lumbar portion of the vertebral column.

muscles of shoulder

Muscles of the shoulder are shown in Figures 7.11 and 7.12. They are also listed in Table 7.4. The muscles of the shoulder attach the scapula to the thorax and move the scapula; they also attach the humerus to the scapula and move the arm.

Myofibrils and Sarcomeres

Myofibrils are cylindrical and run the length of the muscle fiber. Each myofibril is composed of numerous sarcomeres, which are microscopic repeating units (Fig. 7.3). Each sarcomere extends between two dark, vertical lines called Z lines. The horizontal stripes, or striations, of skeletal muscle fibers are formed by the placement of myofilaments within the sarcomeres. A sarcomere contains two types of protein myofilaments: The thick filaments are made up of a single protein called myosin. Thin filaments are made up of three proteins: a globular protein called actin, plus Page 138tropomyosin and troponin. At both ends of each sarcomere, the I band is light-colored because it contains only thin filaments attached to a Z line. Note that I bands overlap adjacent sarcomeres. The dark regions of the A band, found between the I bands of the sarcomere, contain overlapping thick and thin filaments. A good way to recall the difference between the I band and A band is to remember that the letter I is found in the word light. I bands are light because they contain only thin filaments. Likewise, the letter A is part of the word dark. A bands are dark because they contain both thick and thin filaments. Directly in the center of the dark A band is the lighter H zone, which contains only myosin filaments (Fig. 7.3b and c).

cont

The contraction of sphincters composed of smooth muscle fibers temporarily prevents the flow of blood into a capillary. This is an important homeostatic mechanism because in times of emergency it is more important, for example, for blood to be directed to the skeletal muscles than to the tissues of the digestive tract. Skeletal muscle in the abdominopelvic region protects all the internal organs it covers. In the digestive system, smooth muscle contraction accounts for peristalsis, the process that moves food along the digestive tract. Without this action, food would never reach all the organs of the digestive tract where digestion releases nutrients that enter the bloodstream. As part of the urinary system, smooth muscle contracts to assist in the voiding of urine, which is necessary for ridding the body of metabolic wastes and for regulating the blood volume, salt concentration, and pH of internal fluids. Contraction of the skeletal muscles that are part of the respiratory system raises and lowers the rib cage and diaphragm during the active phases of breathing. As we breathe, oxygen enters the blood and is delivered to the tissues, including the muscles, where ATP is produced in mitochondria with heat as a by-product. The heat produced by skeletal muscle contraction allows the body temperature to remain within the normal range for human beings. The muscular system maintains the integrity of the skeletal system. Repetitive skeletal muscle contraction helps build bone, Page 157and it strengthens joints by stabilizing their movements. Finally, body movements allow us to perform those daily activities necessary to our health and benefit. Although it may seem as if movement of our limbs does not affect homeostasis, it does so by allowing us to relocate our bodies to keep the external environment within favorable limits for our existence.

homestatis intro

The illustration in Human Systems Work Together on page 158 tells how the muscular system works with other systems of the body to maintain homeostasis. Cardiac muscle contraction accounts for the heartbeat, which creates blood pressure, the force that propels blood in the arteries and arterioles. The walls of the arteries and arterioles contain smooth muscle. Constriction of arteriole walls is regulated to help maintain blood pressure. Arterioles branch into the capillaries, where exchange takes place that creates and cleanses tissue fluid. Blood and tissue fluid are the internal environment of the body, and Page 156without cardiac and smooth muscle contraction, blood would never reach the capillaries for exchange to take place. Blood is returned to the heart in cardiovascular veins, and excess tissue fluid is returned to the cardiovascular system within lymphatic vessels. In turn, skeletal muscle contraction presses on the cardiovascular veins and lymphatic vessels, and this creates the pressure that moves fluids in both types of vessels. Without the return of blood to the heart, circulation would stop, and without the return of lymph to the blood vessels, normal blood pressure could not be maintained.

motor neuron

The mechanism behind skeletal muscle contraction is a phenomenon called the sliding filament theory, and it begins with nervous stimulation. Muscle fibers are innervated—that is, they are stimulated to contract by nerve cells called motor neurons. Multiple motor neurons are organized into motor nerves, which are controlled by a specific motor control area of the brain (Fig. 7.4). You'll remember (from Chapter 4) that an axon is the single, long extension of a neuron. The axon of one motor neuron has several branches and can stimulate from a few to several hundred muscle fibers of a particular muscle. This entire region is called a motor unit, and it consists of the single motor nerve axon and the entire collection of muscle fibers it innervates. Each branch of the axon ends in an axon terminal, an expanded area that lies in close proximity to the sarcolemma of the muscle fiber. This region is called the neuromuscular junction. Because a small gap, called a synaptic cleft, separates the axon terminal from the sarcolemma, the two do not physically touch (Fig. 7.4). Axon terminals contain synaptic vesicles that are filled with acetylcholine (ACh), one of a large category of biochemicals called neurotransmitters. When signals from the brain's motor control area travel down the motor neuron and arrive at the axon terminal, the synaptic vesicles release neurotransmitter into the synaptic cleft. The ACh quickly diffuses across the cleft and binds to receptors on the sarcolemma. Now the sarcolemma generates an electrical signal, called an action potential, that spreads over the surface of the sarcolemma. (You will learn much more about action potentials in Chapter 8.) The action potential signal next spreads down T tubules to the sarcoplasmic reticulum, triggering the release of calcium from the sarcoplasmic reticulum. Calcium enables the next phase of muscle fiber contraction, involving interaction between myosin and actin myofilaments. It's interesting to note that one of the world's deadliest poisons, a protein produced by the bacterium called Clostridium botulinum, works at the neuromuscular synapse. The botulism toxin paralyzes muscle, including respiratory muscles, by blocking the release of acetylcholine from the motor neuron. Without acetylcholine, skeletal muscles cannot generate their action potential signals, and poisoning victims die from suffocation. However, the diluted toxin, called Botox, is routinely used for therapeutic and cosmetic reasons. Painful muscle spasms called contractures can be relaxed with Botox, and it has also been used to treat migraine headaches. Cosmetically, the appearance of facial wrinkles can be reduced by paralyzing the muscles of facial expression.

muscles of facial expression

The muscles of facial expression are located on the scalp and face. These muscles are unusual in that they insert into and move the skin. Therefore, we expect them to move the skin and not a bone. As you know from the chapter introduction, these muscles communicate whether we are surprised, angry, fearful, happy, and so forth. Frontalis lies over the frontal bone; it raises the eyebrows and wrinkles the brow. Frequent use results in furrowing of the forehead. Orbicularis oculi is a ringlike band of muscle that encircles (forms an orbit about) the eye. It causes the eye to close or blink, and is responsible for "crow's feet" at the eye corners. Orbicularis oris encircles the mouth and is used to pucker the lips, as in forming a kiss. Frequent use results in lines about the mouth. Buccinator muscles are located in the cheek areas. When a buccinator contracts, the cheek is compressed, as when a person whistles or blows out air. Therefore, this muscle is called the "trumpeter's muscle." Important to everyday life, the buccinator helps hold food in contact with the teeth during chewing. Babies use this muscle for suckling. It is also used in swallowing, as discussed next. Zygomaticus extends from each zygomatic arch (cheekbone) to the corners of the mouth. It raises the corners of the mouth when a person smiles. Levator anguli oris and levator labii superioris muscles lift the upper edge and corners of the lip. Simultaneously contracting these muscles on both sides of the mouth helps produce a smile. However, a person will sneer if he uses only the set on one side of his mouth. Depressor anguli oris and depressor labii inferioris pull the lower edge and corners of the lip down, as when a person is frowning or a child is pouting.

muscles of trunk

The muscles of the trunk are listed in Table 7.3 and illustrated in Figure 7.11. The muscles of the thoracic wall are primarily Page 149involved in breathing. The muscles of the abdominal wall protect and support the organs within the abdominal cavity.

functions

This chapter details skeletal muscles, so it's important to independently consider their functions (cardiac and smooth muscle function will be detailed in later chapters). The role of skeletal muscles in homeostasis can be summarized: Skeletal muscles support the body. Skeletal muscle contraction opposes the force of gravity, allowing us to remain upright. Some skeletal muscles are serving this purpose even when you think you're relaxed. Skeletal muscles make bones and other body parts move. Muscle contraction accounts not only for the movement of limbs but also for eye movements, facial expressions, and breathing. Skeletal muscles help maintain a constant body temperature. Skeletal muscle is thermogenic; that is, its contractions generate heat. Muscle breaks down ATP as it contracts, and heat energy is distributed around the body. Involuntary muscle contractions that cause shivering help to maintain body temperature in the cold. Skeletal muscle contraction assists fluid movement in cardiovascular and lymphatic vessels. The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels. Skeletal muscles help protect bones and internal organs and stabilize joints. Muscles pad the bones, and the muscular wall in the abdominal region protects the internal organs. Muscle tendons help hold bones together at the joints.

muscles of mastication

We use the muscles of mastication when we chew food or bite something. Although there are four pairs of muscles for chewing, only two pairs are superficial and shown in Figure 7.10. As you might expect, both pairs insert on the mandible. Each masseter has its origin on the zygomatic arch and its insertion on the mandible. The masseter is a muscle of mastication (chewing) because it is a prime mover for elevating the mandible. Each temporalis is a fan-shaped muscle that overlies the temporal bone. It is also a prime mover for elevating the mandible. The masseter and temporalis are synergists.

excersise

What's at the top of many New Year's resolution lists every January 1? You guessed it: get more exercise! Have you ever made that resolution, only to find yourself floundering and failing? Keep reading to learn why it's best to stick with your New Year's exercise plan, instead of admitting defeat when February 1 rolls around. Exercise builds muscles and saves your skeleton. Exercise programs improve muscular strength, endurance, and flexibility. Muscular strength is the force a muscle (or muscle group) can exert against resistance. Muscle endurance is the muscle's ability to contract repeatedly or to sustain a contraction for an extended time. Muscle flexibility is tested by measuring a joint's range of motion. As muscular strength improves, the muscle's overall size increases. Muscle fibers synthesize more thick and thin myofilaments, and myofibrils increase in size. Though muscle fibers don't reproduce by mitosis, they do enlarge by creating these new myofibrils. Simultaneously, the total protein, numbers of capillaries, and the amounts of connective tissue, including tissue found in tendons and ligaments, also increase. Physical training with weights can improve muscular strength and endurance in all adults, regardless of age. Over time, increased muscle strength creates stronger bones and increases joint stability. Exercise also helps prevent osteoporosis, the condition described in Chapter 6 in which the bones are weak and easily broken. Exercise stimulates the activity of osteoblasts (the bone building cells) in young and old alike, and even those with joint diseases such as osteoarthritis can benefit. Patients who regularly exercise report much less pain, swelling, fatigue, and depression. Exercise helps control weight and keeps blood sugar concentration in the normal range. Increased activity can help to take off unwanted pounds and to keep them off (and that's no surprise, right?). Moreover, glucose moves into muscle cells during contraction, which reduces the amount of glucose in the blood. As a result, blood glucose homeostasis is maintained. This is extremely important for your overall health, not just your waistline. Excess body weight and elevated blood glucose levels contribute to metabolic syndrome and type II diabetes, two disorders of glucose metabolism (see pages 230-231). Long-term complications of diabetes include kidney failure, blindness, and cardiovascular disease. However, type II diabetics who exercise regularly can often reduce or even eliminate the need for insulin and other medications. Exercise protects your heart, blood vessels, and your brain. The life-long benefits of exercise are most apparent with regard to cardiovascular health. Regular exercise raises the blood levels of high-density lipoprotein (HDL, the so-called "good cholesterol"; see Chapter 12), and lowers blood levels of low-density lipoprotein (LDL, or "bad cholesterol"). These effects protect both the heart and all blood vessels—including those supplying the brain—from long-term damage. Thus, regular exercise reduces the risk of heart attack and stroke. Further, it's important to get moving to protect your brain. Exercise can help to moderate the effects of depression because it leads to an increase in the level of brain neurotransmitters that naturally elevate mood. Both chronic depression and poor cardiovascular health are strongly linked to the development of dementia, including Alzheimer disease. Exercise helps prevent cancer. You know (from Chapter 4) that cancer prevention requires eating properly, not smoking, and avoiding exposure to radiation and cancer-causing chemicals. To detect cancer early, when it's most curable, you'll need to undergo appropriate medical screening tests and know the early warning signs of cancer. But did you know that exercise also leads to a reduced risk of certain kinds of cancer? Evidence shows that people who exercise are less likely to develop colon, breast, cervical, uterine, and ovarian cancer. So how do you keep that promise you made to yourself to get more exercise? Remember to use the acronym SMART as you plan your exercise routine. Studies show that successful people choose specific and measurable goals, for starters. So instead of "exercise more," maybe it's "attend three workout classes at the gym each week" or "take a 30-minute walk around the block five times a week." Activity plans must be attainable and realistic as well. If you can't fit those three workout classes into your frantically busy life, why not schedule two instead, and walk in between? Realistic exercisers who stick with their programs also choose activities they already enjoy and will be more likely to keep doing. Finally, any exercise goal must be time-limited—have a definite end time—or you just might get burned out. Check out Table 7A for ways to include exercise in your lifestyle.

basic principles

When a muscle contracts at a joint, one bone remains fairly stationary, and the other one moves. The origin of a muscle is on the stationary bone, and the insertion of a muscle is on the bone that moves. Frequently, a body part is moved by a group of muscles working together. Even so, one muscle does most of the work, and this muscle is called the prime mover or the agonist. For example, in flexing the forearm at the elbow, the prime mover is the brachialis muscle (Fig. 7.8). The assisting muscles are called the synergists. The biceps brachii (see Fig. 7.13) is a synergist that helps the brachialis flex the elbow. A prime mover can have several synergists. When muscles contract, they shorten. Therefore, muscles can only pull; they cannot push. However, agonist muscles are paired with antagonists, and antagonistic pairs work opposite one another to bring about movement in opposite directions. For example, the brachialis and biceps brachii and the triceps brachii are antagonists; the first pair flexes the forearm, and the other extends the forearm (Fig. 7.8). Later on in our discussion, we'll encounter other antagonistic pairs.

oxygen dept

When a muscle uses fermentation to supply its energy needs, it incurs an oxygen debt. Oxygen debt is obvious when a person continues to breathe heavily after exercising. The ability to run up an oxygen debt is one of muscle tissue's greatest assets. Brain tissue cannot last nearly as long without oxygen as muscles can. Repaying an oxygen debt requires replenishing creatine phosphate supplies and disposing of lactic acid. Lactic acid can be changed back to a compound called pyruvic acid (or pyruvate) and metabolized completely in mitochondria, or it can be sent to the liver to reconstruct glycogen. A marathon runner who has just crossed the finish line is not exhausted due to oxygen debt. Instead, the runner has used up all the muscles' glycogen supply, and probably the liver's glycogen as well. It takes about two days to replace glycogen stores on a high-carbohydrate diet. People who train rely more heavily on cellular respiration than do people who do not train. In people who train, the number of muscle mitochondria increases, and so fermentation is not needed to produce ATP. Their mitochondria can start consuming glucose and oxygen as soon as the ADP concentration starts rising during muscle contraction. Because mitochondria can break down fatty acids, instead of glucose, blood glucose is spared for the activity of the brain. (The brain, unlike other organs, ordinarily utilizes only glucose to produce ATP.) Because less lactate is produced in people who train, the pH of the blood remains steady, and there is less oxygen debt.

rigor mortis

When a person dies, the physiologic events that accompany death occur in an orderly progression. Respiration ceases, the heart ultimately stops beating, and tissue cells begin to die. The first tissues to die are those with the highest oxygen requirement. Brain and nervous tissues have an extremely high requirement for oxygen. Deprived of oxygen, these cells typically die after only six minutes, as the ATP energy reserve within the cell is used up. However, tissues that can produce ATP by fermentation (which does not require oxygen) can "live" for an hour or more before ATP is completely depleted. Muscle is capable of generating ATP by both fermentation and creatine phosphate breakdown; thus, muscle cells can survive for a time after clinical death occurs. Muscle death is signaled by a process termed rigor mortis—the "stiffness of death." Rigor mortis develops and reverses itself within a known time period of hours to days after death occurs. Forensic pathologists use this information to study the condition of a body to approximate the time of death. Rigor mortis occurs as dying muscle cells deplete the last of their ATP energy reserve. Relaxing the muscle becomes impossible because ATP energy is needed to break the bond between an actin binding site and the myosin cross-bridge. In addition, ATP energy is required for muscle relaxation to return calcium ions from the sarcoplasm to the sarcoplasmic reticulum. Thus, without ATP energy, the muscle remains fixed in the same state of contraction that preceded that person's death. If, for example, a murder victim dies while sitting at a desk, the body in rigor mortis will be frozen in the sitting position. Forensic pathologists use body temperature and the presence or absence of rigor mortis to estimate time of death. For example, the body of someone who has been dead for 3 hours or less will still be warm (close to normal body temperature, 98.6°F or 37°C) and rigor mortis will be absent. After approximately 3 hours, the body will be significantly cooler than normal, and rigor mortis will begin to develop. The corpse of an individual who has been dead at least 8 hours will be in full rigor mortis, and the temperature of the body will be the same as the surroundings. Rigor mortis resolves approximately 24-36 hours after death. Muscles lose their stiffness because lysosomes inside the cell eventually rupture, releasing enzymes that break the myosin-actin bonds. Forensic pathologists know that a person has been dead for more than 24 hours if the body temperature is the same as the environment and there is no longer a trace of rigor mortis.