Muscles system

Because contracting 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 rip pling 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 intes tines 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. HOW Muscles generate the force required to cause movement by contracting, a process in which proteins inside the muscle fibers overlap more than when they are at rest. Instructors may assign a related “Building Vocabulary” activity using Mastering A&P 6.1 Overview of Muscle Tissues ✓ Learning Objectives ✓ Describe similarities and differences in the structure and function of the three types of muscle tissue, and indicate where they are found in the body. ✓ Define muscular system. ✓ ✓ ✓ Define and explain the role of the following: endomysium, perimysium, epimysium, tendon, and aponeurosis. 6.1a Muscle Types There are three types of muscle tissue—skeletal, smooth, and cardiac (Table 6.1, p. 200). These differ 199 200 Essentials of Human Anatomy and Physiology Table 6.1 Comparison of Skeletal, Cardiac, and Smooth Muscles Characteristic Skeletal Cardiac Body location Attached to bones or, for some facial muscles, to skin Walls of the heart Smooth Walls of hollow organs (other than the heart) Cell shape and appearance Single, very long, cylindrical, multinucleate cells with very obvious striations Branching chains of cells; uninucleate, striations; intercalated discs Single, fusiform, uninucleate; no striations Connective tissue components Epimysium, perimysium, and endomysium Epimysium Perimysium Endomysium Endomysium attached to the fibrous skeleton of the heart Endomysium Cells Endomysium Endomysium Regulation of contraction Voluntary; via nervous system controls Involuntary; internal heart pacemaker; nervous system controls; hormones Involuntary; nervous system controls; hormones, chemicals, stretch Speed of contraction Slow to fast Slow Very slow Rhythmic contraction No Yes Yes, in some Chapter 6: The Muscular System 201 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 elon gated. 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 cytoskeleton (studied in Chapter 3). A third similar ity 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 sar coplasm (sar9ko-plaz0um). 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 mus cle 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 com mand”) 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 (Figure 6.1). Each muscle fiber is enclosed in a delicate connective tissue sheath called endomysium (en0do-mis9e-um). Several sheathed muscle fibers are then wrapped by a coarser fibrous membrane Q What is the meaning of epi? Of mys? How do these word roots relate to the role and position of the epimysium? Bone Epimysium (wraps entire muscle) Blood vessel Tendon Perimysium Fascicle (wrapped in perimysium) Muscle fiber (wrapped in endomysium) Figure 6.1 Connective tissue wrappings of skeletal muscle. called perimysium to form a bundle of fibers called a fascicle (fas9˘ı-kul). 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 sheetlike aponeurosis (ap0o-nu-ro9sis), which indirectly attaches the muscle to bone, cartilage, or another connective tissue covering. In addition to anchoring muscles, tendons per form 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 A Epi = upon, over, above; and mys = muscle. The epimysium is a connective tissue sheath upon or over a muscle. 6 202 Essentials of Human Anatomy and Physiology way their fibers are arranged. Many are spindle shaped as just described, but in others, the fibers are arranged in a fan shape, a circle, or a featherlike pattern (as described on pp. 220–221). 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 (tubelike) organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels sub stances along a pathway. Think of smooth muscle as visceral, nonstriated, and involuntary. Smooth muscle fibers are spindle-shaped, uni nucleate, and surrounded by scant endomysium (see Table 6.1). They are arranged in layers, and most often there are two such layers, one running circu larly and the other longitudinally (Figure 6.2a). 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” activi ties 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. 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 skele tal muscle, cardiac muscle is striated, and like smooth muscle, it is uninucleate and under involun tary control. 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 (Figure 6.2b). When the heart contracts, its internal chambers become smaller, forcing blood on a one-way path through the cham bers and into the large arteries leaving the heart. Cardiac muscle fibers are branching cells joined by special gap junctions called intercalated discs (see Figure 3.20 on p. 119 and Chapter 3, p. 118). These two structural features and the spiral arrangement of the muscle bundles in the heart allow heart activity to be closely coordinated. Mucosa Submucosa (a) Circular layer of smooth muscle (longitudinal view of cells) Longitudinal layer of smooth muscle (cross-sectional view of cells) (b) Cardiac muscle bundles Figure 6.2 Arrangement of smooth and cardiac muscle cells. (a) Diagrammatic view of a cross section of the intestine. (b) Longitudinal view of the heart showing the spiral arrangement of the cardiac muscle cells in its walls. Chapter 6: The Muscular System 203 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. As you can see, each of the three muscle types has a structure and function well suited for its job in the body. But because the term muscular system applies specifically to skeletal muscle, we will con centrate on this muscle type for the rest of this chapter. 6.1b Muscle Functions All muscle types produce movement, but skeletal muscle plays three other important roles in the body as well: it maintains posture and body position, stabilizes joints, and generates heat. Let’s take a look. Produce Movement Skeletal muscles are responsible for our body’s mobility, including all locomotion (walking, swim ming, 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 lan guage of facial expressions. They are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, which work together to circulate blood and main tain 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 func tion almost continuously, making one tiny adjust ment 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 move ments, 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 of contraction. As ATP is used to power muscle con traction, 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. Additional Functions Muscles perform other important functions as well. Smooth muscles form valves that regulate the pas sage 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 pro tect fragile internal organs. Did You Get It? 1. How do cells of the three types of muscle tissues differ from one another anatomically? 2. Which muscle type has the most elaborate connec tive tissue wrappings? 3. What does striated mean relative to muscle cells? 4. How do the movements promoted by skeletal muscle differ from those promoted by smooth or cardiac muscle? For answers, see Appendix A. 6.2 Microscopic Anatomy of Skeletal Muscle 6 ✓ Learning Objective ✓ Describe the microscopic structure of skeletal muscle, and explain the role of actin- and myosin containing myofilaments. As mentioned previously, skeletal muscle fibers (cells) are multinucleate (Figure 6.3a, p. 204). Many oval nuclei can be seen just beneath the plasma membrane, which is called the sarcolemma ( sar0ko-lem9ah; “muscle husk”) in muscle fibers. The nuclei are pushed aside by long ribbonlike organ elles, the myofibrils (mi0o-fi9brilz), which nearly fill 204 Essentials of Human Anatomy and Physiology dark A band has a lighter central area called the H zone (Figure 6.3b). 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 myofi brils. First, we find that the myofibrils are actually 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 remem ber which band is which.) A closer look at the band ing pattern reveals that the light I band has a midline interruption, a darker area called the Z disc, and the Nucleus Light (I) band Dark (A) band I band Z disc Z disc I band A band H zone (b) Myofibril or fibril (complex organelle composed of bundles of myofilaments) Myofibril (a) Segment of a muscle fiber (cell) (c) Sarcomere (segment of a myofibril) Thin (actin) myofilament Thick (myosin) myofilament Thin (actin) myofilament Z disc Z disc M line M line Sarcomere Thick (myosin) myofilament Elastic filament (titin) Endomysium Sarcolemma Figure 6.3 Anatomy of a skeletal muscle fiber (cell). (a) A portion of a muscle fiber. One myofibril has been extended. (b) Enlarged view of a section of a myofibril showing its banding pattern. (c) Enlarged view of one sarcomere (contractile unit) of a myofibril. Chapter 6: The Muscular System 205 chains of tiny contractile units called sarcomeres (sar9ko-mˉerz), which are the structural and func tional 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 (myofila ments) within sarcomeres that produces the stria tions in skeletal muscle fibers. Let’s examine how the arrangement of the myo filaments leads to the banding pattern. There are two types of threadlike protein myofilaments within each sarcomere (Figure 6.3c). The thick filaments are made mostly of bundled molecules of the pro tein myosin. Notice that the thick filaments extend the entire length of the dark A band. Also, notice that the midparts of the thick filaments are smooth but that their ends are studded with small projections (Figure 6.3c). These projections, or myosin heads, contain ATPase enzyme activity and split ATP to release the energy used for muscle contraction. The myosin heads also form cross bridges when they bind to the thin myofilaments during contraction. Myosin filaments are attached to the Z discs by titin, elastic filaments that run through the core of the thick filaments. The thin filaments are composed of the con tractile protein called actin, plus some regulatory proteins that play a role in allowing (or preventing) binding of myosin heads to actin. The thin fila ments are anchored to the Z disc (a disclike mem brane). 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 (not shown in Figure 6.3). 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 cal cium and to release it on demand when the muscle fiber is stimulated to contract. As you will see, cal cium provides the final “go” signal for contraction. Did You Get It? 5. Specifically, what structure(s) is/are responsible for the banding pattern in skeletal muscle cells? For the answer, see Appendix A. 6.3 Skeletal Muscle Activity 6.3a Stimulation and Contraction of Single Skeletal Muscle Fibers Learning Objective ✓ Describe how an action potential is initiated in a ✓ muscle cell. 6 Muscle fibers have several special functional prop erties that enable them to perform their duties. The first of these is irritability, also called respon siveness, which is the ability to receive and respond to a stimulus. The second, contractility, is the abil ity to forcibly shorten when adequately stimu lated. 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 stimu lated by nerve impulses. One motor neuron (nerve cell) may stimulate a few muscle fibers or hun dreds 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 stimu lates (Figure 6.4, p. 206). When the long, thread like extension of a 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 fiber (Figure 6.5, p. 207). These junctions, called neuromuscular (literally, “nerve-muscle”) junctions, contain syn aptic vesicles filled with a chemical referred to as a neurotransmitter. The specific neurotransmit ter that stimulates skeletal muscle fibers is acetylcholine (as0e-til-ko9lˉen), 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. 206 Essentials of Human Anatomy and Physiology 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 (Na1), which rush into the muscle fiber, and to potassium ions (K1), which diffuse out of the muscle fiber. However, more Na1 enters than K1 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 Na1 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 sur face 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 6 to Spinal cord Axon terminals at neuromuscular junctions Motor neuron cell bodies Muscle Branching axon to motor unit Muscle fibers Nerve Motor unit 1 Motor unit 2 Muscle fibers (a) Axon of motor neuron (b) Figure 6.4 Motor units. Each motor unit consists of a motor neuron and all the muscle fibers it activates. (a) Portions of two motor units are shown. The motor neurons reside in the spinal cord, and their axons extend to the muscle. Within the muscle, each axon divides into a number of axon terminals distributed to muscle fibers scattered throughout the muscle. (b) Photo of a portion of a motor unit (11503). Homeostatic Imbalance 6.1 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 neu rons degenerate over time, resulting in paralysis that gradually worsens. The cause of ALS is unknown, though some cases are hereditary. Common character istics include malfunctioning mitochondria, inflam mation, and the generation of free radicals that damage DNA and tissue much like intense UV light. The prog nosis for patients with ALS is generally death within three to five years because the breathing muscles will eventually be affected, resulting in suffocation. Now that we have described the structure of the neuromuscular junction, refer to the numbered steps in Figure 6.5 as we examine what happens there. When a nerve impulse reaches the axon terminals 1, calcium channels open, and calcium (Ca21) enters the terminal 2. Calcium entry causes some of the synaptic vesicles in the axon terminal to fuse with the Chapter 6: The Muscular System 207 Nerve impulse Nucleus Myelinated axon of motor neuron Nerve impulse reaches axon terminal of motor neuron. 6 5 4 3 2 1 Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Synaptic vesicle containing ACh Axon terminal of motor neuron Mitochondrion In response to a nerve impulse, calcium (Ca2+) channels open, and Ca2+ enters the axon terminal. Ca2+ entry causes some synaptic vesicles to release their contents (the neurotransmitter acetylcholine) by exocytosis. Ca2+ Ca2+ ACh receptor Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma. ACh binds and opens channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions leave, producing a local change in the electrical conditions of the membrane (depolarization). This eventually leads to an action potential. ACh The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending the stimulation of the muscle fiber. Synaptic cleft ACh Na+ K+ Degraded ACh Na+ Acetylcholinesterase K+ Figure 6.5 Events at the neuromuscular junction. eText Video Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Folds of sarcolemma Ion channel in sarcolemma opens; ions pass. Ion channel closes; ions cannot pass. Mastering A&P > Study Area > Interactive Physiology (IP) 6 208 Essentials of Human Anatomy and Physiology Neuromuscular junction Small twig Match flame 1 Flame ignites the twig. (a) Figure 6.6 Comparing the action potential to a flame consuming a dry twig. (a) The first event in igniting a dry twig is holding the match flame under one area of the twig. The second event is the twig’s prevent continued contraction of the muscle fiber in the absence of additional nerve impulses. The mus cle fiber relaxes until stimulated by the next round of acetylcholine release, so a single nerve impulse pro duces only one contraction. Let’s compare this series of events to lighting a match under a small dry twig (Figure 6.6). The char ring 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). We explain this series of events more fully in the discussion of nerve physiology (Chapter 7, pp. 251–253). The events that return the cell to its resting state include (1) diffusion of potassium ions (K1) 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. Did You Get It? 6. What two structures are closely associated at a neuromuscular junction? 7. Which ions enter the muscle cell during the generation of an action potential? 8. What is the role of calcium ions in muscle contraction? Nerve fiber Muscle fiber (cell) Striations 2 Flame spreads rapidly along the twig. 1 Na+ diffuses into the cell. (b) bursting into flame when it has been heated enough and spreading of the flame to burn the entire twig. (b) The first event in exciting a muscle fiber is the rapid diffusion of sodium ions (Na1) into the cell when 2 Action potential spreads rapidly along the sarcolemma. the permeability of the sarcolemma changes. The second event is the spreading of the action potential along the sarcolemma when enough sodium ions have entered to upset the electrical conditions in the cell. Mechanism of Muscle Contraction: The Sliding Filament Theory ✓ Learning Objective ✓ Describe the events of muscle cell contraction. For answers, see Appendix A. We have just described the events at the neuromus cular junction that lead to an action potential. Now we’ll describe excitation-contraction coupling (that is, combining what just happened with what’s about to happen), and the cross-bridge cycle, during which myofilaments overlap to a greater degree and sarco meres (and the whole muscle) shorten. 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. The formation of cross bridges—when the myo sin heads attach to actin—requires calcium ions (Ca21) 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, in a process called excitation- contraction coupling, the action potential stimulates the sarcoplasmic reticulum to release cal cium ions into the cytoplasm. The calcium ions trig ger the binding of myosin to actin, initiating filament sliding (Figure 6.7). 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 origi nal length. This whole series of events takes a few thousandths of a second. Regulatory proteins Chapter 6: The Muscular System 209 Myosin myofilament Actin myofilament (a) (b) (c) Ca2+ Myosin-binding site Upper part of thick filament only In a relaxed muscle fiber, the regulatory proteins forming part of the actin myofilaments block and prevent myosin binding (see a). When an action potential (AP) sweeps along its sarcolemma and a muscle fiber is excited, calcium ions (Ca2+) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum). The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach (see b), and the myosin heads immediately begin seeking out binding sites. The binding of myosin to actin constitutes cross bridge formation. Using energy from ATP, free myosin heads are “cocked,” much like an oar ready to be pulled on for rowing. Myosin attachment to actin causes the myosin heads to snap (pivot) toward the center of the sarcomere in a rowing motion. When this happens, the thin filaments are pulled slightly toward the center of the sarcomere (see c). ATP provides the energy needed to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament. Figure 6.7 Schematic representation of contraction mechanism: the sliding filament theory. 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 (actin) filaments toward the center of the sarcomere. This “walking” of the myosin 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 simultane ously in sarcomeres throughout the muscle fiber, it eText Video Mastering A&P > Study Area > Interactive Physiology (IP) shortens (Figure 6.8, p. 210). Notice that the myofil aments themselves do not shorten during contrac tion; they simply slide past each other and overlap more than in the relaxed state. Did You Get It? 9. Which chemical—ATP or Ca21—triggers sliding of the muscle filaments? 10. Which is a cross-bridge attachment more similar to: a synchronized rowing team or a person pulling a bucket on a rope out of a well? For answers, see Appendix A. 6 210 Essentials of Human Anatomy and Physiology Myosin Actin Z I (a) Relaxed sarcomere H A Z I Z I (b) Fully contracted sarcomere Z A I Figure 6.8 Diagrammatic views of a sarcomere. Notice that in the contracted sarcomere (b), as a result of increased myofilament overlap, the light H zone in the center of the A band has disappeared, the Z discs are closer to the thick filaments, and the I bands have nearly disappeared. The A bands of adjacent sarcomeres move closer together but do not change in length. 6.3b Contraction of a Skeletal Muscle as a Whole ✓ Learning Objective ✓ Define graded response, tetanus, isotonic and isometric contractions, and muscle tone as these terms apply to a skeletal muscle. 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 con tract to its fullest extent when it is stimulated ade quately; 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 stimu lation 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. Muscle Response to Increasingly Rapid Stimulation Although muscle twitches (single, brief, jerky con tractions) sometimes result from certain nervous sys tem 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 mus cle exhibits unfused tetanus (tet9ah-nus), or incomplete tetanus. When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sus tained, the muscle is in fused tetanus, or complete tetanus, or in tetanic contraction* (Figure 6.9). Muscle Response to Stronger Stimuli Tetanus pro duces stronger (more forceful) muscle contractions, but its primary role is to produce smooth and pro longed muscle contractions. How forcefully a muscle contracts depends to a large extent on how many of its cells are stimulated. When only a few fibers are stim ulated, 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 ✓ Learning Objective ✓ Describe three pathways for ATP regeneration during muscle activity. As a muscle contracts, the bonds of ATP molecules are hydrolyzed to release the needed energy. *Tetanic contraction is normal and desirable. It is quite differ ent from the pathological condition of tetanus (commonly called lockjaw), which is caused by a toxin made by bacteria. Lockjaw causes muscles to go into uncontrollable spasms, finally causing respiratory arrest. Chapter 6: The Muscular System 211 Fused tetanus Unfused tetanus Tension (g) (Stimuli) Partial relaxation (a) Twitch (b) Summing of contractions Figure 6.9 A whole muscle’s response to different stimulation rates. A single stimulus (a) causes muscle contraction and relaxation (a twitch). As stimuli are delivered more frequently (b), the muscle does not have time to completely relax before the next stimulus; contraction force (c) Unfused (incomplete) tetanus increases because effects of the individual twitches are summed (added). Further fusion (c) of the twitches (unfused tetanus) occurs as stimuli are delivered at a still faster rate. Fused tetanus (d), a smooth continuous contraction without any evidence of relaxation, results from a (d) Fused (complete) tetanus very rapid rate of stimulation. (Points at which stimuli are delivered are indicated by red arrows. Tension [measured in grams] on the vertical axis refers to the relative force of muscle contraction.) CONCEPT LINK Recall that ATP can be compared to a tightly coiled spring that is ready to uncoil with tremendous energy when the “catch” is released (Chapter 2, p. 76). Remember that all bonds store energy and that the “catch” in this example is one of the characteristic high energy phosphate bonds in ATP. 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 regener ate ATP: • Direct phosphorylation of ADP by creatine phosphate (Figure 6.10a, p. 212). 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 regen erating 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 (Figure 6.10b). At rest and dur ing light to moderate exercise, some 95 percent of the ATP used for muscle activity comes from aero bic respiration. Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. These pathways are col lectively 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 (Figure 6.10c). 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. 6 212 Essentials of Human Anatomy and Physiology (a) Direct phosphorylation Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP P Creatine Creatine ADP ATP (b) Aerobic pathway Aerobic cellular respiration Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Glucose (from glycogen breakdown or delivered from blood) Pyruvic acid Fatty acids Amino acids (c) Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol Aerobic respiration in mitochondria CO2 32 H2O O2 ATP net gain per glucose Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Oxygen use: Required Products: 32 ATP per glucose, CO2, H2O Duration of energy provision: Hours Figure 6.10 Methods of regenerating ATP during muscle activity. The fastest mechanism is (a) direct phosphorylation; the slowest is (b) aerobic respiration. 2 ATP net gain Released to blood Pyruvic acid Lactic acid Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 40 seconds, or slightly more eText Video Mastering A&P > Study Area > Animations & Videos > Interactive Physiology (IP) Anaerobic glycolysis produces only about 5 per cent as much ATP from each glucose 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 har vest, and the accumulating lactic acid promotes muscle soreness. Did You Get It? 11. What are the three processes used to generate energy for skeletal muscle contraction? 12. What is the direct source of energy used by muscle fibers for contraction? For answers, see Appendix A. Muscle Fatigue and Oxygen Deficit ✓ Learning Objective ✓ Define oxygen deficit and muscle fatigue, and list possible causes of muscle fatigue. 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 (Ca21, K1) 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 Chapter 6: The Muscular System 213 to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need to continue aerobic ATP production when they are working vig orously. 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 res piration, lactic acid begins to accumulate in the mus cle 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 imbalances tend 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 run ners. 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 indi vidual 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. (I can hear you saying, “What kind of double-talk is that?”—but pay attention.) The event that is common to all muscle contractions is that ten sion (force) develops in the muscle as the myosin cross bridges attempt to slide the thin actin-containing fila ments past the thick myosin filaments. Isotonic contractions (literally, “same tone” or tension) are familiar to most of us. In isotonic con tractions, the myofilaments are successful in their sliding movements, the muscle shortens, and move ment 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 con tractions, the myosin filaments are “spinning their wheels,” and the tension in the muscle keeps increas ing. 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 mus cles are contracting isometrically. Muscle Tone One aspect of skeletal muscle activity cannot be con sciously controlled. Even when a muscle is volun tarily 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 system atic way. Think of these motor units as being “on duty” in case action is required. Homeostatic Imbalance 6.2 If the nerve supply to a muscle is destroyed (as in an accident), the muscle is no longer stimulated in this manner, and it loses tone. Soon after, it becomes flaccid (fla˘9sid), or soft and flabby, and begins to atrophy (waste away). This is called flaccid paralysis. Compare this with a condition that increases muscle tone until the muscle is no longer controllable—for example, the disease tetanus, which is caused by a bacterial toxin. This is called spastic paralysis. Effect of Exercise on Muscles 6 ✓ Learning Objective ✓ Describe the effects of aerobic and resistance exercise on skeletal muscles and other body organs. The amount of work a muscle does changes the mus cle. 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 pro duce these effects—in fact, there are important dif ferences in the benefits of exercise. Aerobic exercise, or endurance exercise, such as participating in an aerobics class, jogging, or bik ing (Figure 6.11a, p. 214), results in stronger, more flexible muscles with greater resistance to fatigue. 214 Essentials of Human Anatomy and Physiology (a) (b) Figure 6.11 The effects of aerobic training versus strength training. (a) A marathon runner. (b) A weight lifter. 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 skeletal muscles. It makes overall body metabolism more efficient, improves digestion (and elimination), enhances neuromuscular coordina tion, 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 vigor ously 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 profes sional bodybuilder result mainly from resistance exercise, or isometric exercise (Figure 6.11b), which pit the muscles against an immovable (or dif ficult 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 mus cles 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 enlarge ment 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 pro duce 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. Did You Get It? 13. Gary is trying with all his might to pull a tree stump out of the ground. It does not budge. Which type of contraction are his muscles performing? 14. What is meant by the term oxygen deficit? 15. To develop big skeletal muscles, you should focus on which type of exercise: aerobic or resistance exercise? For answers, see Appendix A. 6.4 Muscle Movements, Roles, and Names ✓ Learning Objectives ✓ Define origin, insertion, prime mover, antagonist, ✓ synergist, and fixator as they relate to muscles. ✓ Demonstrate or identify the different types of body movements. 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 (Table 6.2). 6.4a 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 (Figure 6.12). 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. Chapter 6: The Muscular System 215 Table 6.2 The Five Golden Rules of Skeletal Muscle Activity 1. With few exceptions, all skeletal muscles cross at least one joint. 2. Typically, the bulk of a skeletal muscle lies proximal to the joint crossed. 3. All skeletal muscles have at least two attachments: the origin and the insertion. 4. Skeletal muscles can only pull; they never push. 5. During contraction, a skeletal muscle 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 ante rior 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 (Figure 6.13, pp. 216–217). Try to act out each movement as you read the following descriptions: • Flexion. Flexion is a movement, generally in the sagittal plane, that decreases the angle of the joint and brings two bones closer together (Figures 6.13a and 6.13b). Flexion is typical of hinge joints (bending the knee or elbow), but it is also common at ball-and-socket joints (for example, bending forward at the hip). • Extension. Extension is the opposite of flex ion, 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 Muscle contracting Origin Brachialis Tendon Insertion Figure 6.12 Muscle attachments (origin and insertion). When a skeletal muscle contracts, its insertion moves toward its origin. so that your chin points toward the ceiling) is called hyperextension (Figures 6.13a and 6.13b). • Rotation. Rotation is movement of a bone around its longitudinal axis (Figure 6.13c). 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 in the frontal plane) from the midline, or median plane, of the body (Fig ure 6.13d). 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 (Figure 6.13d). Think of adduction as “adding” a body part by bringing it closer to the trunk. • Circumduction. Circumduction is a combina tion 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 (Figure 6.13d), as when you do big arm circles. (Text continues on page 218.) 6 216 Essentials of Human Anatomy and Physiology (b) Flexion, extension, and hyperextension Hyperextension Extension Flexion Flexion Flexion Extension Hyperextension Extension (a) Flexion, extension, and hyperextension of the shoulder and knee Lateral rotation Rotation (c) Rotation Medial rotation Figure 6.13 Body movements. Chapter 6: The Muscular System 217 6 Abduction Adduction (d) Abduction, adduction, and circumduction Circumduction Dorsiflexion Plantar flexion Supination (radius and ulna are parallel) Pronation (radius rotates over ulna) Eversion Inversion Opposition Point toes up reduced angle with tibia Reference line Dorsiflexion Plantar flexion Point toes down, reduced angle with imaginary extension of leg P S (f) Inversion and eversion (g) Supination (S) and pronation (P) (h) Opposition (e) Dorsiflexion and plantar flexion Figure 6.13 (continued) 218 Essentials of Human Anatomy and Physiology Special Movements Certain movements do not fit into any of the previ ous categories and occur at only a few joints. • Dorsiflexion and plantar flexion. Up-and down movements of the foot at the ankle are given special names. Lifting the foot so that its superior surface approaches the shin (point ing your toe toward your head) is dorsiflexion, whereas pointing the toes away from your head is plantar flexion (Figure 6.13e). Dorsiflexion of the foot corresponds to extension and hyperex tension of the hand at the wrist, whereas plantar flexion of the foot corresponds to flexion of the hand. • Inversion and eversion. Inversion and ever sion are also special movements of the foot (Figure 6.13f). 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 (soo0p˘ı-na9shun; “turning back ward”) and pronation (pro-na9shun; “turning forward”) refer to movements of the radius around the ulna (Figure 6.13g). 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 posteri orly (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 sad dle joint between metacarpal 1 and the carpals allows opposition of the thumb (Figure 6.13h). 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 manip ulating objects. 6.4b 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 mus cles can reverse. In general, groups of muscles that produce opposite movements lie on opposite sides of a joint (Figure 6.14). Because of this arrangement, muscles are able to bring about an immense variety of movements. 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 (an-tag9o-nists). When a prime mover is active, its antagonist is stretched and relaxed. Antagonists can be prime movers in their own right, but for different actions. 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 (sin9er-jists; 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 con traction 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 mus cles 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 verte bral 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. Did You Get It? 16. What action is being performed by a person who sticks out his thumb to hitch a ride? 17. What actions take place at the neck when you nod your head up and down as if saying “yes”? 18. In what way are fixator and synergist muscles important? For answers, see Appendix A. Chapter 6: The Muscular System 219 6 eText Video Mastering A&P > Study Area > Animations & Videos > A&P Flix (a) A muscle that crosses on the anterior side of a joint produces flexion* (b) A muscle that crosses on the posterior side of a joint produces extension* (c) A muscle that crosses on the lateral side of a joint produces abduction (d) A muscle that crosses on the medial side of a joint produces adduction * These generalities are reversed for the knee and ankle because the lower limb is rotated during development. The muscles that cross these joints posteriorly produce flexion, and those that cross anteriorly produce extension. Example: Pectoralis major (anterior view) Example: Latissimus dorsi (posterior view) The latissimus dorsi is the antagonist of the pectoralis major. Example: Deltoid middle fibers (anterolateral view) The teres major is the antagonist of the deltoid. Example: Teres major (posterolateral view) Figure 6.14 Muscle action. The action of a muscle can be inferred by the muscle’s position as it crosses a joint. 220 Essentials of Human Anatomy and Physiology 6.4c Naming Skeletal Muscles ✓ Learning Objective ✓ List seven criteria used in naming 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 char acteristic. Paying close attention to these cues can greatly simplify your task of learning muscle names and actions: • Direction of the muscle fibers. Some mus cles 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 or whole structure 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 associ ated. For example, the temporalis and frontalis muscles overlie the temporal and frontal bones of the skull, respectively. Note the spellings! • 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 inser tion. Occasionally, muscles are named for their attachment sites. For example, the sternocleido mastoid 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 triangu lar (deltoid means “triangular”), like the Greek letter delta (Δ). • 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. 6.4d Arrangement of Fascicles Skeletal muscles consist of fascicles (refer to Fig ure 6.1), but fascicle arrangements vary, producing muscles with different structures and functional prop erties. 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 (Figure 6.15a). 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. In a convergent muscle, the fascicles converge toward a single insertion tendon. A convergent mus cle is triangular or fan-shaped, such as the pectoralis major muscle of the anterior thorax (Figure 6.15b). In a parallel arrangement, the length of the fas cicles run parallel to the long axis of the muscle, as in the sartorius of the anterior thigh. These muscles are straplike (Figure 6.15d). A modification of the parallel arrangement, called fusiform, results in a spindle-shaped muscle with an expanded belly (midsection) and tapered ends. An example is the biceps brachii muscle of the arm (Figure 6.15c). In a pennate (pen9ˉat; “feather”) pattern, short fas cicles 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 (Figure 6.15g). If the fascicles insert into opposite sides of the tendon, the muscle is bipennate (Figure 6.15f). If the fascicles insert from several differ ent sides, the muscle is multipennate (Figure 6.15e). A muscle’s fascicle arrangement determines its 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 multipen nate muscles, which pack in the most fibers, shorten very little but are very powerful, like the rectus femo ris of the anterior thigh. Chapter 6: The Muscular System 221 (a) (b) (c) (a) Circular (orbicularis oris) (b) Convergent (pectoralis major) (d) (c) Fusiform (biceps brachii) (d) Parallel (sartorius) Figure 6.15 Relationship of fascicle arrangement to muscle structure. Did You Get It? 19. Based on their names, deduce some characteristics of the following muscles: tibialis anterior, erector spinae, rectus abdominis, extensor carpi radialis longus. 20. What is the fascicle arrangement of the orbicularis oris muscle? For answers, see Appendix A. (e) (f) (g) (e) Multipennate (deltoid) (f) Bipennate (rectus femoris) (g) Unipennate (extensor digitorum longus) 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 frown ing, 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. 6.5 Gross Anatomy of Skeletal Muscles ✓ Learning Objective ✓ Name and locate the major muscles of the human body (on a torso model, muscle chart, or diagram), and state the action of each. It is beyond the scope of this book to describe the hundreds of skeletal muscles of the human body. We describe only the most important muscles here. All the superficial muscles we consider are summarized in Tables 6.3 and 6.4 (pp. 234–235) and illustrated in the overall body views shown later in the chapter (Figures 6.22 and 6.23, pp. 232–233). 6.5a