Human Anatomy and Physiology: Muscular System PDF

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material Human Anatomy and Physiology: Muscular System CPHZabariza VI. MUSCULAR SYSTEM Because flexing muscles look like mice scurrying beneath the skin, a scientist long ago dubbed them muscles, from the Latin...

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material Human Anatomy and Physiology: Muscular System CPHZabariza VI. MUSCULAR SYSTEM Because flexing muscles look like mice scurrying beneath the skin, a scientist long ago dubbed them muscles, from the Latin word mus, meaning “little mouse.” Indeed, the rippling muscles of professional athletes often come to mind when we hear the word muscle. But muscle is also the dominant tissue in the heart and in the walls of other hollow organs of the body such as the intestines and blood vessels, and it makes up nearly half the body’s mass. The essential function of muscle is to contract, or shorten—a unique characteristic that sets it apart from other body tissues. As a result of this ability, muscles are responsible for all body movements and can be viewed as the “machines” of the body. A. Overview of Muscle Tissues Muscle Types There are three types of muscle tissue—skeletal, smooth, and cardiac. These differ in their cell structure, body location, and how they are stimulated to contract. But before we explore their differences, let’s look at how they are similar. First, skeletal and smooth muscle cells are elongated. For this reason, these types of muscle cells (but not cardiac muscle cells) are called muscle fibers. Second, the ability of muscle to shorten, or contract, depends on two types of myofilaments, the muscle cell equivalents of the microfilaments of the. A third similarity has to do with terminology. Whenever you see the prefixes myo- or mys- (“muscle”) or sarco- (“flesh”), you will know that muscle is being referred to. For example, in muscle cells, the cytoplasm is called sarcoplasm. 1. Skeletal Muscle Skeletal muscle fibers are packaged into organs called skeletal muscles that attach to the skeleton. As the skeletal muscles cover our bone and cartilage framework, they help form the smooth contours of the body. Skeletal muscle fibers are large, cigar-shaped, multinucleate cells. They are the largest muscle fibers—some ranging up to 30 cm (nearly 1 foot) in length. Indeed, the fibers of large, hardworking muscles, such as the antigravity muscles of the hip, are so big and coarse that they can be seen with the naked eye. Skeletal muscle is also known as striated muscle (because its fibers have obvious stripes) and as voluntary muscle (because it is the only muscle type subject to conscious control). However, it is important to recognize that skeletal muscles can be activated by reflexes (without our “willed command”) as well. Skeletal muscle tissue can contract rapidly and with great force, but it tires easily and must rest after short periods of activity. When you think of skeletal muscle tissue, the key words to remember are skeletal, striated, and voluntary. Skeletal muscle fibers are soft and surprisingly fragile. Yet skeletal muscles can exert tremendous power—indeed, the force they generate while lifting a weight is often much greater than that required to lift the weight. The reason they are not ripped apart as they exert force is that connective tissue bundles thousands of their fibers together, which strengthens and supports the muscle as a whole. Each muscle fiber is enclosed in a delicate connective tissue sheath called endomysium. Several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called perimysium to form a bundle of fibers called a fascicle. Many fascicles are bound together by an even tougher “overcoat” of connective tissue called an epimysium, which covers the entire muscle. The ends of the epimysium that extend beyond the muscle (like the wrapper on a piece of candy) blend either into a strong, cordlike tendon or a sheet- like aponeurosis, which indirectly attaches the muscle to bone, cartilage, or another connective tissue covering. In addition to anchoring muscles, tendons perform several other functions. The most important are providing durability and conserving space. Tendons are mostly tough collagen fibers, so they can cross rough bony projections, which would tear the more delicate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint. Many people think of muscles as always having an enlarged “belly” that tapers down to a tendon at each end. However, muscles vary considerably in the way their fibers are arranged. Many are spindle-shaped as just described, but in others, the fibers are arranged in a fan shape or a circle. Human Anatomy and Physiology: Muscular System CPHZabariza  2. Smooth Muscle Smooth muscle has no striations and is involuntary, which means that we cannot consciously control it. Found mainly in the walls of hollow (tube-like) visceral organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels substances along a pathway. Think of smooth muscle as visceral, non-striated, and involuntary. Smooth muscle fibers are spindle-shaped, uni-nucleate, and surrounded by scant endomysium. They are arranged in layers, and most often there are two such layers, one running circularly and the other longitudinally. As the two layers alternately contract and relax, they change the size and shape of the organ. Moving food through the digestive tract and emptying the bowels and bladder are examples of “housekeeping” activities normally handled by smooth muscles. Smooth muscle contraction is slow and sustained. To use a running analogy, if skeletal muscle is like a sprinter, who runs fast but tires quickly, then smooth muscle is like a marathoner, who runs more slowly but keeps up the pace for many miles. 3. Cardiac Muscle Cardiac muscle is found in only one place in the body—the heart, where it forms the bulk of the heart walls. The heart serves as a pump, propelling blood through blood vessels to all body tissues. Like skeletal muscle, cardiac muscle is striated, and like smooth muscle, it is uni-nucleate and its control is involuntary. Important key words for this muscle type are cardiac, striated, and involuntary. The cardiac cells are cushioned by small amounts of endomysium and are arranged in spiral or figure 8–shaped bundles. When the heart contracts, its internal chambers become smaller, forcing blood into the large arteries leaving the heart. Cardiac muscle fibers are branching cells joined by special gap junctions called intercalated discs. These two structural features and the spiral arrangement of the muscle bundles in the heart allow heart activity to be closely coordinated. Cardiac muscle usually contracts at a fairly steady rate set by the heart’s “in-house” pacemaker. However, the nervous system can also stimulate the heart to shift into “high gear” for short periods, as when you run to catch a bus. Muscle Functions Produce Movement Skeletal muscles are responsible for our body’s mobility, including all locomotion (walking, swimming, and cross-country skiing, for instance) and manipulating things with your agile upper limbs. They enable us to respond quickly to changes in the external environment. For example, their speed and power enable us to jump out of the way of a runaway car and then follow its flight with our eyes. They also allow us to express our emotions with the silent language of smiles and frowns. They are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, which work together to circulate blood and maintain blood pressure, and the smooth muscle of other hollow organs, which forces fluids (urine, bile) and other substances (food, a baby) through internal body channels. Maintain Posture and Body Position We are rarely aware of the workings of the skeletal muscles that maintain body posture. Yet they function almost continuously, making one tiny adjustment after another so that we maintain an erect or seated posture, even when we slouch, despite the never-ending downward pull of gravity. Stabilize Joints As skeletal muscles pull on bones to cause movements, they also stabilize the joints of the skeleton. Muscles and tendons are extremely important in reinforcing and stabilizing joints that have poorly articulating surfaces, such as the shoulder and knee joints. In fact, physical therapy for knee injuries includes exercise to strengthen thigh muscles because they support the knee. Generate Heat Muscle activity generates body heat as a by-product. As ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. This heat is vital in maintaining normal body temperature. Skeletal muscle accounts for at least 40 percent of body mass, so it is the muscle type most responsible for generating heat. Human Anatomy and Physiology: Muscular System CPHZabariza  Additional Functions Muscles perform other important functions as well. Smooth muscles form valves that regulate the passage of substances through internal body openings, dilate and constrict the pupils of our eyes, and make up the arrector pili muscles that cause our hairs to stand on end. Skeletal muscles form valves that are under voluntary control, and they enclose and protect fragile internal organs. B. Microscopic Anatomy of Skeletal Muscles Skeletal muscle fibers (cells) are multinucleate. Many oval nuclei can be seen just beneath the plasma membrane, which is called the sarcolemma (“muscle husk”) in muscle fibers. The nuclei are pushed aside by long ribbon-like organelles, the myofibrils, which nearly fill the cytoplasm. Alternating light (I) bands and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle fiber its striated (banded) appearance. (Think of the second letter of light, I, and the second letter of dark, A, to help you remember which band is which.) A closer look at the banding pattern reveals that the light I band has a midline interruption, a darker area called the Z disc, and the dark A band has a lighter central area called the H zone. The M line in the center of the H zone contains tiny protein rods that hold adjacent thick filaments together. So why are we bothering with all these terms— dark this and light that? Because the banding pattern reveals the working structure of the myofibrils. First, we find that the myofibrils are actually chains of tiny contractile units called sarcomeres, which are the structural and functional units of skeletal muscle. The sarcomeres are aligned end to end like boxcars in a train along the length of the myofibrils. Second, it is the precise arrangement of even smaller structures (myofilaments) within sarcomeres that produces the striations in skeletal muscle fibers. Let’s examine how the arrangement of the myofilaments leads to the banding pattern. There are two types of threadlike protein myofilaments within each sarcomere. The thick filaments are made mostly of bundled molecules of the protein myosin, but they also contain ATPase enzymes, which split ATP to release the energy used for muscle contraction. Notice that the thick filaments extend the entire length of the dark A band. Also, notice that the mid parts of the thick filaments are smooth, but their ends are studded with small projections. These projections, or myosin heads, form cross bridges when they link the thick and thin filaments together during contraction. Myosin filaments are attached to the Z discs by titin, elastic filaments that run through the core of the thick filament. The thin filaments are composed of the contractile protein called actin, plus some regulatory proteins that play a role in allowing (or preventing) binding of myosin heads to actin. The thin filaments are anchored to the Z disc (a disc-like membrane). Notice that the light I band includes parts of two adjacent sarcomeres and contains only the thin filaments. Although they overlap the ends of the thick filaments, the thin filaments do not extend into the middle of a relaxed sarcomere, and thus the central region (the H zone) looks a bit lighter. When the actin-containing thin filaments slide toward each other during contraction, the H zones disappear because the actin and myosin filaments completely overlap. Another very important muscle fiber organelle— the sarcoplasmic reticulum (SR)—is a specialized smooth endoplasmic reticulum. The interconnecting tubules and sacs of the SR surround every myofibril just as the sleeve of a loosely crocheted sweater surrounds your arm. The major role of this elaborate system is to store calcium and to release it on demand when the muscle fiber is stimulated to contract. As you will see, calcium provides the final “go” signal for contraction. Human Anatomy and Physiology: Muscular System CPHZabariza C. Skeletal Muscle Activity Stimulation and Contraction of Single Skeletal Muscle Fibers Muscle fibers have several special functional properties that enable them to perform their duties. The first of these is irritability, also termed responsiveness, which is the ability to receive and respond to a stimulus. The second, contractility, is the ability to forcibly shorten when adequately stimulated. This property sets muscle apart from all other tissue types. Extensibility is the ability of muscle fibers to stretch, whereas elasticity is their ability to recoil and resume their resting length after being stretched. The Nerve Stimulus and the Action Potential To contract, skeletal muscle fibers must be stimulated by nerve impulses. One motor neuron (nerve cell) may stimulate a few muscle fibers or hundreds of them, depending on the particular muscle and the work it does. A motor unit consists of one neuron and all the skeletal muscle fibers it stimulates. When a long, threadlike extension of the neuron, called the axon, reaches the muscle, it branches into a number of axon terminals, each of which forms junctions with the sarcolemma of a different muscle cell. These junctions, called neuromuscular (literally, “nerve-muscle”) junctions, contain synaptic vesicles filled with a chemical referred to as a neurotransmitter. The specific neurotransmitter that stimulates skeletal muscle fibers is acetylcholine, or ACh. Although the nerve endings and the muscle fiber membranes are very close, they never touch. The gap between them, the synaptic cleft, is filled with interstitial fluid. Homeostatic Imbalance: In some cases, a motor nerve impulse is unable to reach the muscle. In ALS, or amyotrophic lateral sclerosis (also called Lou Gehrig’s disease), motor neurons degenerate over time, resulting in paralysis that gradually worsens. The cause of ALS is unknown, but common characteristics include malfunctioning mitochondria, inflammation, and the generation of free radicals that damage DNA and tissue much like intense UV light. The prognosis for patients with ALS is generally death within three to five years because the breathing muscles will eventually be affected, resulting in suffocation. When a nerve impulse reaches the axon terminals- 1. calcium channels open, and calcium (Ca2+) enters the terminal. 2. Calcium entry causes some of the synaptic vesicles in the axon terminal to fuse with the cell membrane and release acetylcholine, 3. which then diffuses across the synaptic cleft and attaches to membrane receptors in highly folded regions of the sarcolemma. 4. If enough acetylcholine is released, the sarcolemma at that point becomes temporarily even more permeable to sodium ions (Na+), which rush into the muscle fiber, and to potassium ions (K+), which diffuse out of the muscle fiber. However, more Na+ enters than K+ leaves. This imbalance gives the cell interior an excess of positive ions, which reverses the resting electrical conditions of the sarcolemma. This event, called depolarization, opens more channels that only allow Na+ entry. 5. This movement of ions generates an electrical current called an action potential. Once begun, the action potential is unstoppable; it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other. The result is contraction of the muscle fiber. Note that while the action potential is occurring, the enzyme acetylcholinesterase (AChE), present on the sarcolemma and in the synaptic cleft, breaks down acetylcholine to acetic acid and choline Human Anatomy and Physiology: Muscular System CPHZabariza  6. For this reason, a single nerve impulse produces only one contraction. This prevents continued contraction of the muscle fiber in the absence of additional nerve impulses. The muscle fiber relaxes until stimulated by the next round of acetylcholine release. Let’s compare this series of events to lighting a match under a small dry twig. The charring of the twig by the flame can be compared to the change in membrane permeability that allows sodium ions into the cell. When that part of the twig becomes hot enough (when enough sodium ions have entered the cell), the twig will suddenly burst into flame, and the flame will move along the twig (the action potential will be conducted along the entire length of the sarcolemma). The events that return the cell to its resting state include (1) diffusion of potassium ions (K+) out of the cell and (2) operation of the sodium-potassium pump, the active transport mechanism that moves the sodium and potassium ions back to their initial positions. Mechanism of Muscle Contraction: The Sliding Filament Theory What causes the filaments to slide? This question brings us back to the myosin heads that protrude all around the ends of the thick filaments. When the nervous system activates muscle fibers as just described, the myosin heads attach to binding sites on the thin filaments, and the sliding begins. Each cross bridge attaches and detaches several times during a contraction, generating tension that helps pull the thin filaments toward the center of the sarcomere. This “walking” of the myosin cross bridges, or heads, along the thin filaments during muscle shortening is much like a centipede’s gait. Some myosin heads (“legs”) are always in contact with actin (“the ground”), so that the thin filaments cannot slide backward, and this cycle repeats again and again during contraction. As this event occurs simultaneously in sarcomeres throughout the muscle fiber, the cell shortens. Notice that the myofilaments themselves do not shorten during contraction; they simply slide past each other. The formation of cross bridges—when the myosin heads attach to actin—requires calcium ions (Ca2+) and ATP (to “energize” the myosin heads). So where does the calcium come from? Action potentials pass deep into the muscle fiber along membranous tubules that fold inward from the sarcolemma. Inside the cell, the action potentials stimulate the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions trigger the binding of myosin to actin, initiating filament sliding. When the action potential ends, calcium ions are immediately returned to the SR storage areas, the regulatory proteins return to their resting shape and block myosin-binding sites, and the muscle fiber relaxes and settles back to its original length. This whole series of events takes a few thousandths of a second. Contraction of a Skeletal Muscle as a Whole Graded Responses In skeletal muscles, the “all-or-none” law of muscle physiology applies to the muscle fiber, not to the whole muscle. It states that a muscle fiber will contract to its fullest extent when it is stimulated adequately; it never partially contracts. However, the whole muscle reacts to stimuli with graded responses, or different degrees of shortening, which generate different amounts of force. In general, graded muscle contractions can be produced two ways: (1) by changing the frequency of muscle stimulation and (2) by changing the number of muscle fibers being stimulated at one time. Next, let’s describe a muscle’s response to each of these. Human Anatomy and Physiology: Muscular System CPHZabariza  Muscle Response to Increasingly Rapid Stimulation Although muscle twitches (single, brief, jerky contractions) sometimes result from certain nervous system problems, this is not the way our muscles normally operate. In most types of muscle activity, nerve impulses are delivered to the muscle at a very rapid rate— so rapid that the muscle does not get a chance to relax completely between stimuli. As a result, the effects of the successive contractions are “summed” (added) together, and the contractions of the muscle get stronger and smoother. The muscle exhibits unfused tetanus, or incomplete tetanus. When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is in fused tetanus, or complete tetanus, or in tetanic contraction. Muscle Response to Stronger Stimuli Tetanus produces stronger (more forceful) muscle contractions, but its primary role is to produce smooth and prolonged muscle contractions. How forcefully a muscle contracts depends to a large extent on how many of its cells are stimulated. When only a few cells are stimulated, the muscle as a whole contracts only slightly. When all the motor units are active and all the muscle fibers are stimulated, the muscle contraction is as strong as it can get. Thus, muscle contractions can range from slight to vigorous depending on the work to be done. The same hand that lifts a single sheet of paper can also lift a heavy backpack full of books! Providing Energy for Muscle Contraction As a muscle contracts, the bonds of ATP molecules are hydrolyzed to release the needed energy. Surprisingly, muscles store very limited supplies of ATP—only a few seconds’ worth, just enough to get you going. Because ATP is the only energy source that can be used directly to power muscle activity, ATP must be regenerated continuously if contraction is to continue. Working muscles use three pathways to regenerate ATP: Direct phosphorylation of ADP by creatine phosphate. The unique high-energy molecule creatine phosphate (CP) is found in muscle fibers but not other cell types. As ATP is depleted, interactions between CP and ADP result in transfers of a high-energy phosphate group from CP to ADP, thus regenerating more ATP in a fraction of a second. Although muscle fibers store perhaps five times as much CP as ATP, the CP supplies are also soon exhausted (in less than 15 seconds). Aerobic pathway. At rest and during light to moderate exercise, some 95 percent of the ATP used for muscle activity comes from aerobic respiration. Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. These pathways are collectively referred to as oxidative phosphorylation. During aerobic respiration, glucose is broken down completely to carbon dioxide and water, and some of the energy released as the bonds are broken is captured in the bonds of ATP molecules. Although aerobic respiration provides a rich ATP harvest (about 32 ATP per 1 glucose), it is fairly slow and requires continuous delivery of oxygen and nutrient fuels to the muscle to keep it going. Anaerobic glycolysis and lactic acid formation. The initial steps of glucose breakdown occur via a pathway called glycolysis, which does not use oxygen and hence is anaerobic (literally “without oxygen”). During glycolysis, which occurs in the cytosol, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds (2 ATP per 1 glucose molecule). As long as enough oxygen is present, the pyruvic acid then enters the oxygen-requiring aerobic pathways that occur within the mitochondria to produce more ATP as described above. However, when muscle activity is intense, or oxygen and glucose delivery is temporarily inadequate to meet the needs of working muscles, the sluggish aerobic pathways cannot keep up with the demands for ATP. Under these conditions, the pyruvic acid generated during glycolysis is converted to lactic acid, and the overall process is referred to as anaerobic glycolysis. Anaerobic glycolysis produces only about 5 percent as much ATP from each glucose Human Anatomy and Physiology: Muscular System CPHZabariza  molecule as aerobic respiration. However, it is some 2½ times faster, and it can provide most of the ATP needed for 30 to 40 seconds of strenuous muscle activity. Anaerobic glycolysis has two main shortcomings: it uses huge amounts of glucose for a small ATP harvest, and the accumulating lactic acid promotes muscle soreness. Muscle Fatigue and Oxygen Deficit If we exercise our muscles strenuously for a long time, muscle fatigue occurs. A muscle is fatigued when it is unable to contract even though it is still being stimulated. Without rest, a working muscle begins to tire and contracts more weakly until it finally ceases reacting and stops contracting. Factors that contribute to muscle fatigue are not fully known. Suspected causes are imbalances in ions (Ca2+, K+) and problems at the neuromuscular junction. However, many agree that the major factor is the oxygen deficit that occurs during prolonged muscle activity. Oxygen deficit is not a total lack of oxygen; rather, it happens when a person is not able to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need when they are working vigorously. Obviously, then, the work that a muscle can do and how long it can work without becoming fatigued depend on how good its blood supply is. When muscles lack sufficient oxygen for aerobic respiration, lactic acid begins to accumulate in the muscle via the anaerobic pathway. We can recognize this event by the burning sensation we experience. In addition, the muscle’s ATP supply starts to run low, and ionic imbalance tends to occur. Together these factors cause the muscle to contract less and less effectively and finally to stop contracting altogether. True muscle fatigue, in which the muscle quits entirely, rarely occurs in most of us because we feel tired long before it happens and we simply slow down or stop our activity. It does happen in marathon runners. Many of them have literally collapsed when their muscles became fatigued and could no longer work. Oxygen deficit, which always occurs to some extent during vigorous muscle activity, is like a loan that must be “paid back” whether fatigue occurs or not. During the recovery period after activity, the individual breathes rapidly and deeply. This continues until the muscles have received the amount of oxygen needed to get rid of the accumulated lactic acid and replenish ATP and creatine phosphate reserves. Types of Muscle Contractions—Isotonic and Isometric Until now, we have been discussing contraction in terms of shortening, but muscles do not always shorten when they contract. The event that is common to all muscle contractions is that tension (force) develops in the muscle as the actin and myosin myofilaments interact and the myosin cross bridges attempt to slide the thin actin-containing filaments past the thick myosin filaments. Isotonic contractions (literally, “same tone” or tension) are familiar to most of us. In isotonic contractions, the myofilaments are successful in their sliding movements, the muscle shortens, and movement occurs. Bending the knee, lifting weights, and smiling are all examples of isotonic contractions. Contractions in which the muscles do not shorten are called isometric contractions (literally, “same measurement” or length). In isometric contractions, the myosin filaments are “spinning their wheels,” and the tension in the muscle keeps increasing. They are trying to slide, but the muscle is pitted against some more or less immovable object. For example, when you push the palms of your hands together in front of you, your arms and chest muscles are contracting isometrically. Muscle Tone One aspect of skeletal muscle activity cannot be consciously controlled. Even when a muscle is voluntarily relaxed, some of its fibers are contracting- first one group and then another. These contractions are not visible, but thanks to them, the muscle remains firm, healthy, and constantly ready for action. This state of continuous partial contractions is called muscle tone. Muscle tone is the result of different motor units, which are scattered through the muscle, being stimulated by the nervous system in a systematic way. Think of these motor units as being “on duty” in case action is required. Effect of Exercise on Muscles The amount of work a muscle does changes the muscle. Muscle inactivity (due to a loss of nerve supply, immobilization, or whatever the cause) always leads to muscle weakness and wasting. Muscles are no exception to the saying “Use it or lose it!” Conversely, regular exercise increases muscle size, strength, and endurance. However, not all types of exercise produce these effects—in fact, there are important differences in the benefits of exercise. Aerobic exercise, or endurance exercise, such as participating in an aerobics class, jogging, or biking, results in stronger, more flexible muscles with greater resistance to fatigue. These changes come about, at least partly, because the blood supply to the muscles increases, and the individual muscle fibers form more mitochondria and store more oxygen. Aerobic exercise helps us reach a steady rate of ATP production and improves the efficiency of aerobic respiration. However, aerobic exercise benefits much more than the Human Anatomy and Physiology: Muscular System CPHZabariza skeletal muscles. It makes overall body metabolism more efficient, improves digestion (and elimination), enhances neuromuscular coordination, and strengthens the skeleton. The heart enlarges (hypertrophies) and pumps out more blood with each beat, helping to clear more fat deposits from the blood vessel walls. The lungs become more efficient in gas exchange. These benefits may be permanent or temporary, depending on how often and how vigorously a person exercises. Aerobic exercise does not cause the muscles to increase much in size, even though the exercise may go on for hours. The bulging muscles of a professional bodybuilder result mainly from resistance exercise, or isometric exercise, which pit the muscles against an immovable (or difficult to move) object. Resistance exercises require very little time and little or no special equipment. A few minutes every other day is usually sufficient. You can push against a wall, and you can strongly contract buttock muscles even while standing in line at the grocery store. The key is forcing your muscles to contract with as much force as possible. The increased muscle size and strength that result are due mainly to enlargement of individual muscle fibers (they make more contractile myofilaments) rather than to an increase in their number. The amount of connective tissue that reinforces the muscle also increases. Because endurance and resistance exercises produce different patterns of muscle response, it is important to know what your exercise goals are. Lifting weights will not improve your endurance for a marathon. By the same token, jogging will not make you stronger for lifting furniture. Obviously, the best exercise program for most people includes both types of exercise. Muscle Movements, Roles, and Names There are five basic guidelines for understanding gross muscle activity. We refer to these as the Five Golden Rules of skeletal muscle activity because they make it easier to understand muscle movements and appreciate muscle interactions. Types of Body Movements Every one of our 600-odd skeletal muscles is attached to bone, or to other connective tissue structures, at no fewer than two points. One of these points, the origin, is attached to the immovable or less movable bone. Think of the origin as the anchor, or leverage, point. Another point, the insertion, is attached to the movable bone. When the muscle contracts, the insertion moves toward the origin. Some muscles have interchangeable origins and insertions, depending on the action being performed. For example, the rectus femoris muscle of the anterior thigh crosses both the hip and knee joints. Its most common action is to extend the knee, in which case the proximal pelvic attachment is the origin. However, when the knee bends (by other muscles), the rectus femoris can flex the hip, and then its distal attachment on the leg is considered the origin. Generally speaking, body movement occurs when muscles contract across joints. The type of movement depends on the mobility of the joint and the location of the muscle in relation to the joint. The most obvious examples of the action of muscles on bones are the movements that occur at the joints of the limbs. However, less freely movable bones are also tugged into motion by the muscles, such as the vertebrae’s movements when we bend to one side. Next we describe the most common types of body movements. Flexion. Flexion is a movement, generally in the sagittal plane, that decreases the angle of the joint and brings two bones closer together. Flexion is typical of hinge joints (bending the knee or elbow), Human Anatomy and Physiology: Muscular System CPHZabariza but it is also common at ball-and-socket joints (for example, bending forward at the hip). Extension. Extension is the opposite of flexion, so it is a movement that increases the angle, or distance, between two bones or parts of the body (straightening the knee or elbow). Extension that is greater than 180° (as when you move your arm posteriorly beyond its normal anatomical position, or tip your head so that your chin points toward the ceiling) is called hyperextension. Rotation. Rotation is movement of a bone around its longitudinal axis. Rotation is a common movement of ball-and socket joints and describes the movement of the atlas around the dens of the axis (as in shaking your head “no”). Abduction. Abduction is moving a limb away (generally on the frontal plane) from the midline, or median plane, of the body. The terminology also applies to the fanning movement of your fingers or toes when they are spread apart. Adduction. Adduction is the opposite of abduction, so it is the movement of a limb toward the body midline. Think of adduction as “adding” a body part by bringing it closer to the trunk. Circumduction. Circumduction is a combination of flexion, extension, abduction, and adduction commonly seen in ball-and- socket joints, such as the shoulder. The proximal end of the limb is stationary, and its distal end moves in a circle. The limb as a whole outlines a cone, as when you do big arm circles. Special Movements Certain movements do not fit into any of the previous categories and occur at only a few joints. Dorsiflexion and plantar flexion. Up-and down movements of the foot at the ankle that its superior surface approaches the shin (pointing your toe toward your head) is dorsiflexion, whereas pointing the toes away from your head is plantar flexion. Dorsiflexion of the foot corresponds to extension and hyperextension of the hand at the wrist, whereas plantar flexion of the foot corresponds to flexion of the hand. Inversion and eversion. Inversion and eversion are also special movements of the foot. To invert the foot, turn the sole medially, as if you were looking at the bottom of your foot. To evert the foot, turn the sole laterally. Supination and pronation. The terms supination (“turning backward”) and pronation (“turning forward”) refer to movements of the radius around the ulna. Supination occurs when the forearm rotates laterally so that the palm faces anteriorly (or up) and the radius and ulna are parallel, as in anatomical position. Pronation occurs when the forearm rotates medially so that the palm faces posteriorly (or down). Pronation brings the radius across the ulna so that the two bones form an X. A helpful memory trick: If you lift a cup of soup up to your mouth on your palm, you are supinating (“soup”-inating). Opposition. In the palm of the hand, the saddle joint between metacarpal 1 and the carpals allows opposition of the thumb. This is the action by which you move your thumb to touch the tips of the other fingers on the same hand. This unique action makes the human hand a fine tool for grasping and manipulating objects. Interactions of Skeletal Muscles in the Body Muscles can’t push—they can only pull as they contract—so most often body movements result from two or more muscles acting together or against each other. Muscles are arranged so that whatever one muscle (or group of muscles) can do, other muscles can reverse. In general, groups of muscles that produce opposite movements lie on opposite sides of a joint. Because of this arrangement, muscles are able to bring about an immense variety of movements. Human Anatomy and Physiology: Muscular System CPHZabariza The muscle that has the major responsibility for causing a particular movement is called the prime mover. Muscles that oppose or reverse a movement are antagonists. When a prime mover is active, its antagonist is stretched and relaxed. Antagonists can be prime movers in their own right. For example, the biceps brachii and brachialis muscles of the arm (prime movers of elbow flexion) are antagonized by the triceps brachii (a prime mover of elbow extension). Synergists (syn = together, erg = work) help prime movers by producing the same movement or by reducing undesirable movements. When a muscle crosses two or more joints, its contraction will cause movement in all the joints crossed unless synergists are there to stabilize them. For example, the flexor muscles of the fingers cross both the wrist and the finger joints. You can make a fist without bending your wrist because synergist muscles stabilize the wrist joints and allow the prime mover to act on your finger joints. Fixators are specialized synergists. They hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the insertion bone. The postural muscles that stabilize the vertebral column are fixators, as are the muscles that anchor the scapulae to the thorax. In summary, although prime movers seem to get all the credit for causing certain movements, the actions of antagonistic and synergistic muscles are also important in producing smooth, coordinated, and precise movements. Naming Skeletal Muscles Like bones, muscles come in many shapes and sizes to suit their particular tasks in the body. Muscles are named on the basis of several criteria, each of which focuses on a particular structural or functional characteristic. Paying close attention to these cues can greatly simplify your task of learning muscle names and actions: Direction of the muscle fibers. Some muscles are named in reference to some imaginary line, usually the midline of the body or the long axis of a limb bone. When a muscle’s name includes the term rectus (straight), its fibers run parallel to that imaginary line. For example, the rectus femoris is the straight muscle of the thigh. Similarly, the term oblique in a muscle’s name tells you that the muscle fibers run obliquely (at a slant) to the imaginary line. Relative size of the muscle. Such terms as maximus (largest), minimus (smallest), and longus (long) are sometimes used in the names of muscles—for example, the gluteus maximus is the largest muscle of the gluteus muscle group. Location of the muscle. Some muscles are named for the bone with which they are associated. For example, the temporalis and frontalis muscles overlie the temporal and frontal bones of the skull, respectively. Number of origins. When the term biceps, triceps, or quadriceps forms part of a muscle name, you can assume that the muscle has two, three, or four origins, respectively. For example, the biceps muscle of the arm has two heads, or origins, and the triceps muscle has three. Location of the muscle’s origin and insertion. Occasionally, muscles are named for their attachment sites. For example, the sternocleidomastoid muscle has its origin on the sternum (sterno) and clavicle (cleido) and inserts on the mastoid process of the temporal bone. Shape of the muscle. Some muscles have a distinctive shape that helps to identify them. For example, the deltoid muscle is roughly triangular (deltoid means “triangular”). Action of the muscle. When muscles are named for their actions, terms such as flexor, extensor, and adductor appear in their names. For example, the adductor muscles of the thigh all bring about its adduction, and the extensor muscles of the wrist all extend the wrist. Arrangement of Fascicles Skeletal muscles consist of fascicles, but fascicle arrangements vary, producing muscles with different structures and functional properties. Next, let’s look at the most common patterns of fascicle arrangement: circular, convergent, fusiform, parallel, multipennate, bipennate, and unipennate. In a circular pattern, the fascicles are arranged in concentric rings. Circular muscles are typically found surrounding external body openings which they close by contracting, creating a valve. A general term for such muscles is sphincters (“squeezers”). Examples are the orbicularis muscles surrounding the eyes and mouth. Human Anatomy and Physiology: Muscular System CPHZabariza In a convergent muscle, the fascicles converge toward a single insertion tendon. A convergent muscle is triangular or fan-shaped, such as the pectoralis major muscle of the anterior thorax. In a parallel arrangement, the length of the fascicles run parallel to the long axis of the muscle, as in the sartorius of the anterior thigh. These muscles are strap-like. A modification of the parallel arrangement, called fusiform, results in a spindle- shaped muscle with an expanded belly (midsection); an example is the biceps brachii muscle of the arm. In a pennate (“feather”) pattern, short fascicles attach obliquely to a central tendon. In the extensor digitorum muscle of the leg, the fascicles insert into only one side of the tendon, and the muscle is unipennate. If the fascicles insert into opposite sides of the tendon, the muscle is bipennate. If the fascicles insert from several different sides, the muscle is multipennate. A muscle’s fascicle arrangement determines it range of motion and power. The longer and the more nearly parallel the fascicles are to a muscle’s long axis, the more the muscle can shorten, but such muscles are not usually very powerful. Muscle power depends more on the total number of muscle fibers in the muscle. The stocky bipennate and multipennate muscles, which pack in the most fibers, shorten very little but are very powerful. Gross Anatomy of Skeletal Muscles Head and Neck Muscles The head muscles are an interesting group. They have many specific functions but are usually grouped into two large categories— facial muscles and chewing muscles. Facial muscles are unique because they insert into soft tissues, such as other muscles or skin. When they pull on the skin of the face, they permit us to express ourselves by frowning, smiling, and so forth. The chewing muscles begin to break down food for the body. All head and neck muscles we describe are paired except for the platysma, orbicularis oris, frontalis, and occipitalis. Facial Muscles Frontalis - The frontalis, which covers the frontal bone, runs from the cranial aponeurosis to the skin of the eyebrows, where it inserts. This muscle allows you to raise your eyebrows, as in surprise, and to wrinkle your forehead. At the posterior end of the cranial aponeurosis is the small occipitalis muscle, which covers the posterior aspect of the skull and pulls the scalp posteriorly. Orbicularis Oculi - The fibers of the orbicularis oculi run in circles around the eyes. It allows you to close your eyes, squint, blink, and wink. Orbicularis Oris - The orbicularis oris is the circular muscle of the lips. Often called the “kissing” muscle, it closes the mouth and protrudes the lips. Buccinator - The fleshy buccinators muscle runs horizontally across the cheek and inserts into the orbicularis oris. It flattens the cheek (as in whistling or blowing a trumpet). It is also listed as a chewing muscle because it compresses the cheek to hold food between the teeth during chewing. Human Anatomy and Physiology: Muscular System CPHZabariza  Zygomaticus - The zygomaticus extends from the corner of the mouth to the cheekbone. It is often referred to as the “smiling” muscle because it raises the corners of the mouth. Masseter - As it runs from the zygomatic process of the temporal bone to the mandible, the masseter covers the angle of the lower jaw. This muscle closes the jaw by elevating the mandible. Temporalis - The temporalis is a fan-shaped muscle overlying the temporal bone. It inserts into the mandible and acts as a synergist of the masseter in closing the jaw. Neck Muscles For the most part, the neck muscles, which move the head and shoulder girdle, are small and strap-like. We consider only two neck muscles here. Platysma - The platysma is a single sheet-like muscle that covers the anterolateral neck. It originates from the connective tissue covering of the chest muscles and inserts into the area around the mouth. Its action is to pull the corners of the mouth inferiorly, producing a downward sag of the mouth (the “sad clown” face). Sternocleidomastoid - The paired sternocleidomastoid muscles are two-headed muscles, one found on each side of the neck. Of the two heads of each muscle, one arises from the sternum, and the other arises from the clavicle. The heads fuse before inserting into the mastoid process of the temporal bone. When both sternocleidomastoid muscles contract together, they flex your neck. (It is this action of bowing the head that has led some people to call these muscles the “prayer” muscles.) If just one muscle contracts, the face is rotated toward the shoulder on the opposite side and tilts the head to its own side. Trunk Muscles The trunk muscles include (1) muscles that move the vertebral column (most of which are posterior antigravity muscles); (2) anterior thorax muscles, which move the ribs, head, and arms; and (3) muscles of the abdominal wall, which “hold your guts in” by forming a natural girdle and help to move the vertebral column. Anterior Muscles Pectoralis Major - The pectoralis major is a large fan-shaped muscle covering the upper part of the chest. Its origin is from the sternum, shoulder girdle, and the first six ribs. It inserts on the proximal end of the humerus. This muscle forms the anterior wall of the axilla (armpit) and acts to adduct and flex the arm. Intercostal Muscles - The intercostal muscles are deep muscles found between the ribs. The external intercostals are important in breathing because they help to raise the rib cage when you inhale. The internal intercostals, which lie deep to the external intercostals, depress the rib cage, helping to move air out of the lungs when you exhale forcibly. Muscles of the Abdominal Girdle - The anterior abdominal muscles (rectus abdominis, external and internal obliques, and transversus abdominis) form a natural “girdle” that reinforces the body trunk. Taken together, they resemble the structure of plywood because the fibers of each muscle or muscle pair run in a different direction. Just as plywood is exceptionally strong for its thickness, the abdominal muscles form a muscular wall that is well suited for its job of containing and protecting the abdominal contents. Rectus abdominis. The paired strap-like rectus abdominis muscles are the most superficial muscles of the abdomen. They run from the pubis to the rib cage, enclosed in an aponeurosis. Their main function is to flex the vertebral column. They also compress the abdominal contents during defecation and childbirth (they help you “push”) and are involved in forced breathing. External oblique. The external oblique muscles are paired superficial muscles that make up the lateral walls of the abdomen. Their fibers run downward and medially from the last eight ribs and Human Anatomy and Physiology: Muscular System CPHZabariza  insert into the ilium. Like the rectus abdominis, they flex the vertebral column, but they also rotate the trunk and bend it laterally. Internal oblique. The internal oblique muscles are paired muscles deep to the external obliques. Their fibers run at right angles to those of the external obliques. They arise from the iliac crest and insert into the last three ribs. Their functions are the same as those of the external obliques. Transversus abdominis. The deepest muscle of the abdominal wall, the transversus abdominis has fibers that run horizontally across the abdomen. It arises from the lower ribs and iliac crest and inserts into the pubis. This muscle compresses the abdominal contents. Posterior Muscles Trapezius - The trapezius muscles are the most superficial muscles of the posterior neck and upper trunk. When seen together, they form a diamond- or kite-shaped muscle mass. Their origin is very broad. Each muscle runs from the occipital bone of the skull down the vertebral column to the end of the thoracic vertebrae. They then flare laterally to insert on the scapular spine and clavicle. The trapezius muscles extend the head (thus they are antagonists of the sternocleidomastoids). They also can elevate, depress, adduct, and stabilize the scapula. Latissimus Dorsi - The latissimus dorsi muscles are the two large, flat muscles that cover the lower back. They originate on the lower spine and ilium and then sweep superiorly to insert into the proximal end of the humerus. Each latissimus dorsi extends and adducts the humerus. These are very important muscles when the arm must be brought down in a power stroke, as when swimming or striking a blow. Erector Spinae - The erector spinae group is the prime mover of back extension. These paired muscles are deep muscles of the back. Each erector spinae is a composite muscle consisting of three muscle columns (longissimus, iliocostalis, and spinalis) that collectively span the entire length of the vertebral column. These muscles not only act as powerful back extensors (“erectors”) action of bending over at the waist. Following injury to back structures, these muscles go into spasms, a common source of lower back pain. Quadratus Lumborum - The fleshy quadratus lumborum muscles form part of the posterior abdominal wall. Acting separately, each muscle of the pair flexes the spine laterally. Acting together, they extend the lumbar spine. These muscles arise from the iliac crests and insert into the upper lumbar vertebrae. Deltoid - The deltoids are fleshy, triangle-shaped muscles that form the rounded shape of your shoulders. Because they are so bulky, they are a favorite injection site when relatively small amounts of medication (less than 5 ml) must be given intramuscularly (into muscle). The origin of each deltoid winds across the shoulder girdle from the spine of the scapula to the clavicle. It inserts into the proximal humerus. The deltoids are the prime movers of arm abduction. Muscles of the Upper Limb The upper limb muscles fall into three groups. The first group includes muscles that arise from the shoulder girdle and cross the shoulder joint to insert into the humerus. We have already considered these muscles, which move the arm—they are the pectoralis major, latissimus dorsi, and deltoid. The second group causes movement at the elbow joint. These muscles enclose the humerus and insert on the forearm bones. In this section we will focus on the muscles of this second group. Human Anatomy and Physiology: Muscular System CPHZabariza The third group of upper limb muscles includes the muscles of the forearm, which insert on the hand bones and cause their movement. The muscles of this last group are thin and spindle-shaped, and there are many of them. We will not consider them here except to mention their general naming and function. As a rule, the forearm muscles have names that reflect their activities. For example, the flexor carpi and flexor digitorum muscles, found on the anterior aspect of the forearm, flex the wrist and fingers, respectively. The extensor carpi and extensor digitorum muscles, found on the lateral and posterior aspect of the forearm, extend the same structures. Muscles Causing Movement at the Elbow Joint All anterior muscles of the humerus cause elbow flexion. In order of decreasing strength these are the brachialis, biceps brachii, and brachioradialis. Biceps Brachii - The biceps brachii is the most familiar muscle of the arm because it bulges when you flex your elbow. It originates by two heads from the shoulder girdle and inserts into the radial tuberosity. This muscle is the powerful prime mover for flexion of the forearm and acts to supinate the forearm. The best way to remember its actions is to think of opening a bottle of wine. The biceps supinates the forearm to turn the corkscrew and then flexes the elbow to pull the cork. Brachialis - The brachialis lies deep to the biceps brachii and, like the biceps, is a prime mover in elbow flexion. The brachialis lifts the ulna as the biceps lifts the radius. Brachioradialis - The brachioradialis is a fairly weak muscle that arises on the humerus and inserts into the distal forearm. Hence, it resides mainly in the forearm. Triceps Brachii - The triceps brachii is the only muscle fleshing out the posterior humerus. Its three heads arise from the shoulder girdle and proximal humerus, and it inserts into the olecranon process of the ulna. Being the powerful prime mover of elbow extension, it is the antagonist of the biceps brachii and brachialis. This muscle straightens the arm—for instance, to deliver a strong jab in boxing. Muscles of the Lower Limb Muscles that act on the lower limb cause movement at the hip, knee, and foot joints. They are among the largest, strongest muscles in the body and are specialized for walking and balancing the body. Because the pelvic girdle is composed of heavy, fused bones that allow little movement, no special group of muscles is necessary to stabilize it. This is very different from the shoulder girdle, which requires several fixator muscles. Many muscles of the lower limb span two joints and can cause movement at both of them. Therefore, in reference to these muscles, the terms origin and insertion are often interchangeable depending on the action being performed. Muscles acting on the thigh are massive muscles that help hold the body upright against the pull of gravity and cause various movements at the hip joint. Muscles acting on the leg form the flesh of the thigh. (In common usage, the term leg refers to the whole lower limb, but anatomically the term refers only to that part between the knee and the ankle.) The thigh muscles cross the knee and cause its flexion or extension. Because many of the thigh muscles also have attachments on the pelvic girdle, they can cause movement at the hip joint as well. Muscles originating on the leg cause assorted movements of the ankle and foot. We will consider only three muscles of this group, but there are many others that extend and flex the ankle and toe joints. Muscles Causing Movement at the Hip Joint Gluteus Maximus - The gluteus maximus is a superficial muscle of the hip that forms most of the flesh of the buttock. It is a powerful hip extensor that acts to bring the thigh in a straight line with the pelvis. Although it is not very important in walking, it is probably the most important muscle for extending the hip when power is needed, as when climbing stairs or jumping. It originates from the sacrum and iliac bones and inserts on the gluteal tuberosity of the femur and into the large tendinous iliotibial tract. Gluteus Medius The gluteus medius runs from the ilium to the femur, beneath the gluteus maximus for most of its length. The gluteus medius is a hip abductor and is important in steadying the pelvis during walking. It is also an important site for giving intramuscular injections, particularly when administering more than 5 ml. Although it might appear that the large, fleshy gluteus maximus that forms the bulk of the buttock mass would be a better choice, notice that the medial part of each buttock overlies the large sciatic nerve; hence this area must be carefully avoided to prevent nerve Human Anatomy and Physiology: Muscular System CPHZabariza  damage. This can be accomplished by imagining the buttock is divided into four equal quadrants. The superolateral quadrant then overlies the gluteus medius muscle, which is usually a very safe site for an intramuscular injection. Injections at or near the sciatic nerve can result in physical trauma from the needle or degeneration of the nerve itself. Iliopsoas - The iliopsoas is a fused muscle composed of two muscles, the iliacus and the psoas major. It runs from the iliac bone and lower vertebrae deep inside the pelvis to insert on the lesser trochanter of the femur. It is a prime mover of hip flexion. It also acts to keep the upper body from falling backward when we are standing erect. Adductor Muscles - The muscles of the adductor group form the muscle mass at the medial side of each thigh. As their name indicates, they adduct, or press, the thighs together. However, because gravity does most of the work for them, they tend to become flabby very easily. Special exercises are usually needed to keep them toned. The adductors have their origin on the pelvis and insert on the proximal aspect of the femur. Muscles Causing Movement at the Knee Joint Hamstring Group - The muscles forming the muscle mass of the posterior thigh are the hamstrings. The group consists of three muscles— the biceps femoris, semimembranosus, and semitendinosus—which originate on the ischial tuberosity and run down the thigh to insert on both sides of the proximal tibia. They are prime movers of thigh extension and knee flexion. Their name comes from the fact that butchers use their tendons to hang hams (consisting of thigh and hip muscles) for smoking. You can feel these tendons at the back of your knee. Sartorius - Compared with other thigh muscles described here, the thin, strap- like sartorius muscle is not too important. However, it is the most superficial muscle of the thigh and so is rather hard to miss. It runs obliquely across the thigh from the anterior iliac crest to the medial side of the tibia. It is a weak thigh flexor. The sartorius is commonly referred to as the “tailor’s” muscle because it acts as a synergist to help tailors sit with both legs crossed in front of them. Quadriceps Group - The quadriceps group consists of four muscles—the rectus femoris and three vastus muscles—that flesh out the anterior thigh. The vastus muscles originate from the femur; the rectus femoris originates on the pelvis. All four muscles insert into the tibial tuberosity via the patellar ligament. The group as a whole acts to extend the knee powerfully, as when kicking a soccer ball. Because the rectus femoris crosses two joints, the hip and knee, it can also help to flex the hip. The vastus lateralis and rectus femoris are sometimes used as intramuscular injection sites, particularly in infants, who have poorly developed gluteus muscles. Muscles Causing Movement at the Ankle and Foot Tibialis Anterior - The tibialis anterior is a superficial muscle on the anterior leg. It arises from the upper tibia and then parallels the anterior crest as it runs to the tarsal bones, where it inserts by a long tendon. It acts to dorsiflex and invert the foot. Extensor Digitorum Longus - Lateral to the tibialis anterior, the extensor digitorum longus muscle arises from the lateral tibial condyle and proximal three-quarters of the fibula and inserts into the phalanges of toes 2 to 5. It is a prime mover of toe extension. Fibularis Muscles - The three fibularis muscles— longus, brevis, and tertius—are found on the lateral part of the leg. They arise from the fibula and insert into the metatarsal bones of the foot. The group as a whole plantar flexes and everts the foot, which is antagonistic to the tibialis anterior. Gastrocnemius - The gastrocnemius muscle is a two-bellied muscle that forms the curved calf of the posterior leg. It arises by two heads, one from each side of the distal femur, and inserts through the large calcaneal (Achilles) tendon into the heel of the foot. It is a prime Human Anatomy and Physiology: Muscular System CPHZabariza  mover for plantar flexion of the foot; for this reason it is often called the “toe dancer’s” muscle. If the calcaneal tendon is severely damaged or cut, walking is very difficult. The foot drags because it is not able to “push off” the toe (raise the heel). Soleus - Deep to the gastrocnemius is the fleshy soleus muscle. Because it arises on the tibia and fibula (rather than the femur), it does not affect knee movement, but like the gastrocnemius, it inserts into the calcaneal tendon and is a strong plantar flexor of the foot. D. Developmental Aspects of the Muscular System In the developing embryo, the muscular system is laid down in segments, and then each segment is invaded by nerves. The muscles of the thoracic and lumbar regions become very extensive because they must cover and move the bones of the limbs. The muscles and their control by the nervous system develop rather early in pregnancy. The expectant mother is often astonished by the first movements (called the quickening) of the fetus, which usually occur by the 16th week of pregnancy. Homeostatic Imbalance Very few congenital muscular problems occur. The exception to this is muscular dystrophy—a group of inherited muscle-destroying diseases that affect specific muscle groups. The muscles appear to enlarge because of fat and connective tissue deposits, but the muscle fibers degenerate and atrophy. The most common and serious form is Duchenne’s muscular dystrophy, which is expressed almost exclusively in boys. This tragic disease is usually diagnosed between the ages of 2 and 7 years. Active, normal-appearing children become clumsy and fall frequently as their muscles weaken. The disease progresses from the extremities upward, finally affecting the head and chest muscles. Children with this disease rarely live beyond their early twenties and generally die of respiratory failure. Although researchers have identified the cause of muscular dystrophy— the diseased muscle fibers lack a protein (called dystrophin) that helps maintain the sarcolemma— a cure is still elusive. After birth, a baby’s movements are all gross reflex types of movements. Because the nervous system must mature before the baby can control muscles, we can trace the increasing efficiency of the nervous system by observing a baby’s development of muscle control. This development proceeds in a superior/inferior direction, and gross muscular movements precede fine ones. Babies can raise their heads before they can sit up and can sit up before they can walk. Muscular control also proceeds in a proximal/ distal direction; that is, babies can perform the gross movements like waving “bye-bye” and pulling objects to themselves before they can use the pincer grasp to pick up a pin. All through childhood, the nervous system’s control of the skeletal muscles becomes more and more precise. By mid-adolescence, we have reached the peak level of development of this natural control and can simply accept it or bring it to a fine edge by athletic training. Because of its rich blood supply, skeletal muscle is amazingly resistant to infection throughout life, and given good nutrition, relatively few problems afflict skeletal muscles. We repeat, however, that muscles, like bones, will atrophy, even with normal tone, if they are not used continually. A lifelong program of regular exercise keeps the whole body operating at its best possible level. Homeostatic Imbalance One rare autoimmune disease that can affect muscles during adulthood is myasthenia gravis (asthen = weakness; gravi = heavy), a disease characterized by drooping upper eyelids, difficulty in swallowing and talking, and generalized muscle weakness and fatigability. The disease involves a shortage of acetylcholine receptors at neuromuscular junctions caused by antibodies specific for acetylcholine receptors. Antibodies are immune molecules that will mark the receptors for destruction. Because the muscle fibers are not stimulated properly, they get progressively weaker. Death usually occurs when the respiratory muscles can no longer function, which leads to respiratory failure. As we age, the amount of connective tissue in muscles increases, and the amount of muscle tissue decreases; thus the muscles become stringier, or more sinewy. Because skeletal muscles represent so much of our body mass, body weight begins to decline in older adults as this natural loss in muscle mass occurs. Muscle strength also decreases by about 50 percent by age 80. Regular exercise can help offset the effects of aging on the muscular system, and frail older people who begin to “pump iron” (use leg and hand weights) can rebuild muscle mass and dramatically increase their strength. Human Anatomy and Physiology: Muscular System CPHZabariza MAJOR SUPERFICIAL MUSCLES OF THE BODY: Human Anatomy and Physiology: Muscular System CPHZabariza Human Anatomy and Physiology: Muscular System CPHZabariza Reference: Marieb, Elaine Nicpon; Keller, Suzanne M. (2018). Essentials of Human Anatomy and Physiology 12th Edition Pearson Education Inc., 300 Hudson Street, NY NY 10013 Human Anatomy and Physiology: Muscular System CPHZabariza