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Chapter 5: The Skeletal System 177 is pathologic fractures (breaks that occur sponta- neo...

Chapter 5: The Skeletal System 177 is pathologic fractures (breaks that occur sponta- neously without apparent injury), which increase dramatically with age and are the single most common skeletal problem for this age group. Advancing years also take their toll on joints. Weight-bearing joints in particular begin to degen- erate, and osteoarthritis is common. Such degenera- tive joint changes lead to the complaint often heard from an aging person: “My joints are getting so stiff.” 5 Did You Get It? 37. Ninety-year-old Mrs. Pelky is groaning in pain. Her grandson has just given her a bear hug. What do you think might have happened to her spine, and what Age 40 Age 60 Age 70 bone condition may she be suffering from? Figure 5.35 Vertebral collapse due to 38. Which two regions of the skeleton grow most rapidly osteoporosis. Women with postmenopausal during childhood? osteoporosis are at risk for vertebral fractures. For answers, see Appendix A. Eventually these vertebrae collapse, producing spinal curvature that causes loss of height, a tilted rib cage, a dowager’s hump, and a protruding abdomen. Summary Bones: An Overview (pp. 134–146) 6. Bones change in shape throughout life. This remod- eling occurs in response to hormones (for example, 1. Bones support and protect body organs; serve as PTH, which regulates blood calcium levels) and levers for the muscles to pull on to cause move- mechanical stresses acting on the skeleton. ment at joints; and store calcium, fats, and other substances for the body. Some contain red marrow, 7. A fracture is a break in a bone. Common types of the site of blood cell production. fractures include simple, compound, compression, comminuted, and greenstick. Bone fractures must 2. Bones are classified into four groups—long, short, be reduced to heal properly. flat, and irregular—on the basis of their shape and the amount of compact or spongy bone they con- Axial Skeleton (pp. 146–158) tain. Bone markings are important anatomical land- marks that reveal where muscles attach and where 1. The skull is formed by cranial and facial bones. blood vessels and nerves pass. Eight cranial bones protect the brain: frontal, occipi- tal, ethmoid, and sphenoid bones, and the paired 3. A long bone is composed of a shaft (diaphysis) parietal and temporal bones. The 14 facial bones are with two ends (epiphyses). The shaft is compact all paired (maxillae, zygomatics, palatines, nasals, bone; its cavity contains yellow marrow. The lacrimals, and inferior nasal conchae), except for the epiphyses are covered with hyaline cartilage; they vomer and mandible. The hyoid bone, not really a contain spongy bone (where red marrow is skull bone, is supported in the neck by ligaments. found). 2. The vertebral column is formed from 24 vertebrae, 4. The organic parts of the matrix make bone flexible; the sacrum, and the coccyx. There are 7 cervical calcium salts deposited in the matrix make bone hard. vertebrae, 12 thoracic vertebrae, and 5 lumbar verte- 5. Bones form in the fetus on hyaline cartilage or brae, which have common and unique features. The fibrous membrane “models.” Epiphyseal plates per- vertebrae are separated by fibrocartilage discs that sist to provide for longitudinal growth of long allow the vertebral column to be flexible. The verte- bones during childhood and become inactive (by bral column is C-shaped at birth (thoracic and sacral calcifying) when adolescence ends. curvatures are present); the secondary curvatures 178 Essentials of Human Anatomy and Physiology (cervical and lumbar) form when the baby begins to 3. Joints also can be classified structurally as fibrous, lift its head and walk. After infancy, the vertebral cartilaginous, or synovial joints, depending on the column is S-shaped to allow for upright posture. substance separating the articulating bones. 3. The bony thorax is formed from the sternum and 12 4. Most fibrous joints are synarthrotic, and most carti- pairs of ribs. All ribs attach posteriorly to thoracic ver- laginous joints are amphiarthrotic. Fibrous and car- tebrae. Anteriorly, the first 7 pairs attach directly to the tilaginous joints occur mainly in the axial skeleton. sternum (true ribs); the last 5 pairs attach indirectly or 5. All joints of the limbs are synovial joints. In syno- not at all (false ribs). The bony thorax encloses the vial joints, the articulating bone surfaces are cov- lungs, heart, and other organs of the thoracic cavity. ered with articular cartilage and enclosed within the joint cavity by a fibrous capsule lined with a Appendicular Skeleton (pp. 158–166) synovial membrane that secretes lubricating syno- 1. The shoulder girdles are each composed of two vial fluid. All synovial joints are diarthroses. bones—a scapula and a clavicle—and attach the 6. The most common joint problem is arthritis, or upper limbs to the axial skeleton. They are light, inflammation of the joints. Osteoarthritis, or de- poorly reinforced girdles that allow the upper limbs generative arthritis, is a result of the “wear and a great deal of freedom of movement. tear” on joints over many years and is a common 2. The bones of the upper limb include the humerus affliction of the aged. Rheumatoid arthritis occurs of the arm, the radius and ulna of the forearm, and in both young and older adults; it is believed to be the carpals, metacarpals, and phalanges of the hand. an autoimmune disease. Gouty arthritis, caused by 3. The pelvic girdle is formed by the two coxal bones, or the deposit of uric acid crystals in joints, typically hip bones, and the sacrum (part of the axial skeleton). affects a single joint. Each hip bone is the result of fusion of the ilium, ischium, and pubis bones. The pelvic girdle is securely Developmental Aspects of the Skeleton attached to the vertebral column, and the socket for (pp. 173–177) the thigh bone is deep and heavily reinforced. This 1. Fontanels, which allow brain growth and ease birth girdle receives the weight of the upper body and passage, are present in the skull at birth. Growth of transfers it to the lower limbs. The female pelvis is the cranium after birth is related to brain growth; lighter and broader than the male’s; its inlet and outlet the increase in size of the facial skeleton follows are larger, reflecting the childbearing function. tooth development and enlargement of the respira- 4. The bones of the lower limb include the femur of tory passageways. the thigh; the tibia and fibula of the leg; and the 2. Long bones continue to grow in length until late tarsals, metatarsals, and phalanges of the foot. adolescence. By the age of 10, the head and trunk are approximately the same height as the lower Joints (pp. 166–173) limbs and change little thereafter. 1. Joints hold bones together and allow movement of 3. Fractures are the most common bone problem in the skeleton. elderly people. Osteoporosis, a condition of bone 2. Joints fall into three functional categories: synar- wasting that results mainly from hormone deficit or throses (immovable), amphiarthroses (slightly mov- inactivity, is also common in older individuals. able), and diarthroses (freely movable). Access additional practice questions using your smartphone, tablet, or computer: Review Questions > Study Area > Practice Tests & Quizzes Multiple Choice 2. A passageway connecting neighboring osteocytes in an osteon is a More than one choice may apply. a. central canal. 1. Which of the following are correctly matched? b. lamella. a. Short bone—wrist c. lacuna. b. Long bone—leg d. canaliculus. c. Irregular bone—sternum e. perforating canal. d. Flat bone—cranium  Chapter 5: The Skeletal System 179 3. Which of the following would you expect to be 11. Match the types of joints to the descriptions that prominent in osteoclasts? apply to them. (More than one description might a. Golgi apparatus apply.) b. Lysosomes a. Fibrous joints c. Microfilaments b. Cartilaginous joints d. Exocytosis c. Synovial joints ____ 1. Have no joint cavity 4. Bone pain behind the external acoustic meatus ____ 2. Types are sutures and syndesmoses probably involves the ____ 3. Dense connective tissue fills the space a. maxilla. d. temporal bone. between the bones 5 b. ethmoid bone. e. lacrimal bone. ____ 4. Almost all joints of the skull ____ 5. Types are synchondroses and symphyses c. sphenoid bone. ____ 6. All are diarthroses 5. Bones that articulate with the sphenoid include ____ 7. The most common type of joint in the body which of the following? ____ 8. Nearly all are synarthrotic a. Parietal d. Zygomatic ____ 9. Shoulder, hip, knee, and elbow joints b. Frontal e. Ethmoid 12. Match the bone markings listed on the right with c. Occipital their function listed on the left. 1. Attachment site for ____ a. Trochanter 6. Which humeral process articulates with the radius? muscle or ligament ____ b. Condyle a. Trochlea d. Capitulum 2. Forms a joint ____ c. Foramen b. Greater tubercle e. Olecranon fossa surface ____ d. Process 3. Passageway for ves- ____ e. Facet c. Lesser tubercle sels or nerves ____ f. Tuberosity 7. Which parts of the thoracic vertebrae articulate with the ribs? Short Answer Essay a. Spinous process d. Body 13. Name three functions of the skeletal system. b. Transverse process e. Pedicles 14. What is yellow marrow? How do spongy and c. Superior articular compact bone look different? processes 15. Why do bone injuries heal much more rapidly than 8. Which of the following bones or bone parts injuries to cartilage? articulate with the femur? 16. Compare and contrast the role of PTH (hormone) a. Ischial tuberosity d. Fibula and mechanical forces acting on the skeleton in b. Pubis e. Tibia bone remodeling. c. Patella 17. Which fracture types are most common in older 9. Which bone of the arm corresponds to the femur of people? Why are greenstick fractures more common the leg? in children? a. Ulna 18. Name the eight bones of the cranium. b. Humerus 19. With one exception, all skull bones are joined by c. Radius sutures. What is the exception? d. Tibia 20. What facial bone forms the chin? The cheekbone? e. Fibula The upper jaw? The bony eyebrow ridges? 10. At what stage of life do the lower limbs attain the 21. Name two ways in which the fetal skull differs from same height as the head and trunk? the adult skull. a. At birth 22. How many vertebrae are there in each of the three b. By 10 years of age superior regions of the vertebral column? c. At puberty 23. Diagram the normal spinal curvatures and then the d. When the epiphyseal plates fuse curvatures seen in scoliosis and lordosis. e. Never 180 Essentials of Human Anatomy and Physiology 24. What is the function of the intervertebral discs? 32. Describe the structure of a synovial joint. Use this What is a herniated (slipped) disc? description to briefly explain what occurs in a joint with osteoarthritis. 25. Name the major components of the thorax. 33. Professor Rogers pointed to the foramen magnum 26. Is a floating rib a true or a false rib? Why are of the skull and said, “The food passes through this floating ribs easily broken? hole when you swallow.” Some students believed 27. Name the bones of the shoulder girdle. him, but others said that this was a big mistake. What do you think? Support your answer. 28. Name all the bones with which the ulna articulates. 34. Yolanda is asked to review a bone slide that has 29. What bones make up each hip bone (coxal bone)? been set up under a microscope. She sees Which of these is the largest? Which has tuberosities concentric layers surrounding a central cavity or that we sit on? Which is the most anterior? canal. Is this bone section taken from the diaphysis 30. Name the bones of the lower limb from superior to or the epiphyseal plate of the bone specimen? inferior. 35. List two factors that keep bones healthy. List two 31. Compare the amount of movement possible in syn- factors that can cause bones to become soft or to arthrotic, amphiarthrotic, and diarthrotic joints. atrophy. Relate these terms to the structural classification of 36. Contrast the location and characteristics of foramen joints—that is, to fibrous, cartilaginous, and syno- magnum and obturator foramen. vial joints. Critical Thinking and Clinical Application Questions 37. A 75-year-old woman and her 9-year-old grand- 41. At work, a box fell from a shelf onto Ella’s acromial daughter were in a car accident in which both sus- region. In the emergency room, the physician felt tained trauma to the chest while seated next to that the head of her humerus had moved into the each other. X-ray images showed that the grand- axilla. What had happened to Ella? mother had several fractured ribs, but her grand- 42. An X-ray image of the arm of an accident victim daughter had none. Explain these different findings. reveals a faint line curving around and down the 38. The pediatrician explains to parents of a newborn shaft. What kind of fracture might this indicate? that their son suffers from cleft palate. She tells 43. Bone X-ray studies are sometimes used to them that the normal palate fuses in an anterior-to- determine whether a person has reached his or her posterior pattern. The child’s palatine processes of final height. What are the clinicians checking out? the maxilla have not fused. Have his palatine bones fused normally? 44. A patient complains of pain starting in the jaw and radiating down the neck. When he is questioned 39. After having a severe cold accompanied by nasal further, he states that when he is under stress he congestion, Nicole complained that she had a grinds his teeth. What joint is causing his pain? frontal headache and that the right side of her face ached. What bony structures probably became 45. Dr. Davis is palpating Lauren’s vertebral column to infected by the bacteria or viruses causing the cold? determine whether she is beginning to exhibit scoliosis. What part or region of her vertebrae was 40. Deborah, a 75-year-old woman, stumbled slightly he feeling as he ran his fingers along her spine? while walking, then felt a terrible pain in her left hip. At the hospital, X-ray images revealed that the 46. Mike’s big toe is swollen and extremely painful. hip was broken. Also, the compact bone and Mike is 42 years old and remembers his father and spongy bone throughout her spine were very thin. grandfather experiencing the same symptoms. Tests What was her probable condition? reveal Mike has arthritis in his toe. Which kind of arthritis is most likely, and what is the cause? 6 The Muscular System WHAT Muscles are responsible for body movements, stabilizing joints, HOW and generating heat. 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. WHY In addition to whole body movements, muscles are needed to INSTRUCTORS move substances inside our bodies: for New Building example, air into and out of our lungs, Vocabulary Coaching food through our digestive tracts, and Activities for this blood through our heart and chapter are assignable blood vessels. in B ecause flexing muscles look like mice scur- rying 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 Overview of Muscle Tissues ➔ Learning Objectives □□ Describe similarities and differences in the structure and function of the three types of muscle tissue, mind when we hear the word muscle. But muscle and indicate where they are found in the body. is also the dominant tissue in the heart and in the □□ Define muscular system. walls of other hollow organs of the body such as □□ Define and explain the role of the following: the intestines and blood vessels, and it makes up endomysium, perimysium, epimysium, tendon, and nearly half the body’s mass. aponeurosis. The essential function of muscle is to contract, or shorten—a unique characteristic that sets it apart Muscle Types from other body tissues. As a result of this ability, There are three types of muscle tissue—skeletal, muscles are responsible for all body movements smooth, and cardiac (Table 6.1, p. 182). These differ and can be viewed as the “machines” of the body. 181 182 Essentials of Human Anatomy and Physiology Table 6.1 Comparison of Skeletal, Cardiac, and Smooth Muscles Characteristic Skeletal Cardiac Smooth Body location Attached to bones or, for some Walls of the heart Mostly in walls of hollow visceral facial muscles, to skin organs (other than the heart) Cell shape and Single, very long, cylindrical, Branching chains of cells; Single, fusiform, uninucleate; appearance multinucleate cells with very uninucleate, striations; no striations obvious striations intercalated discs Connective Epimysium, perimysium, and Endomysium attached to the Endomysium tissue endomysium fibrous skeleton of the heart components Endomysium Epimysium Endomysium Endomysium Perimysium Cells Regulation of Voluntary; via nervous system Involuntary; the heart has a Involuntary; nervous system contraction controls pacemaker; also nervous system controls; hormones, chemicals, controls; hormones stretch Speed of Slow to fast Slow Very slow contraction Rhythmic No Yes Yes, in some contraction  Chapter 6: The Muscular System 183 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. Q: What is the meaning of epi? Of mys? How do these word roots relate to the role and position of the epimysium? First, skeletal and smooth muscle cells are Muscle elongated. For this reason, these types of muscle fiber cells (but not cardiac muscle cells) are called mus- Blood vessel (cell) cle fibers. Second, the ability of muscle to shorten, or contract, depends on two types of myofila- Perimysium ments, the muscle cell equivalents of the microfila- ments of the cytoskeleton (studied in Chapter 3). Epimysium 6 A third similarity has to do with terminology. (wraps entire Whenever you see the prefixes myo- or mys- muscle) (“muscle”) or sarco- (“flesh”), you will know that Fascicle muscle is being referred to. For example, in mus- (wrapped by cle cells, the cytoplasm is called sarcoplasm perimysium) (sar′ko-plaz″um). Skeletal Muscle Endomysium Skeletal muscle fibers are packaged into organs (between fibers) called skeletal muscles that attach to the skeleton. Tendon As the skeletal muscles cover our bone and carti- lage framework, they help form the smooth con- tours of the body. Skeletal muscle fibers are large, Bone 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 Figure 6.1 Connective tissue wrappings large, hardworking muscles, such as the antigrav- of skeletal muscle. ity 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) (en″do-mis′e-um). Several sheathed muscle fibers and as voluntary muscle (because it is the only are then wrapped by a coarser fibrous membrane muscle type subject to conscious control). called perimysium to form a bundle of fibers However, it is important to recognize that skeletal called a fascicle (fas′ı̆-kul). Many fascicles are muscles can be activated by reflexes (without our bound together by an even tougher “overcoat” of “willed command”) as well. Skeletal muscle tissue connective tissue called an epimysium, which can contract rapidly and with great force, but it covers the entire muscle. The ends of the epimy- tires easily and must rest after short periods of sium that extend beyond the muscle (like the activity. When you think of skeletal muscle tissue, wrapper on a piece of candy) blend either into a the key words to remember are skeletal, striated, strong, cordlike tendon or a sheetlike aponeuro- and voluntary. sis (ap″o-nu-ro′sis), which indirectly attaches the Skeletal muscle fibers are soft and surprisingly muscle to bone, cartilage, or another connective fragile. Yet skeletal muscles can exert tremendous tissue covering. power—indeed, the force they generate while lifting In addition to anchoring muscles, tendons per- a weight is often much greater than that required to form several other functions. The most important lift the weight. The reason they are not ripped apart are providing durability and conserving space. as they exert force is that connective tissue bundles Tendons are mostly tough collagen fibers, so they 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 A: epimysium is a sheath upon or over a muscle. Epi = upon, over, above; and mys = muscle. The connective tissue sheath called endomysium 184 Essentials of Human Anatomy and Physiology Circular layer can cross rough bony projections, which would of smooth muscle tear the more delicate muscle tissues. Because of (longitudinal view their relatively small size, more tendons than fleshy Mucosa of cells) muscles can pass over a joint. Many people think of muscles as always hav- ing an enlarged “belly” that tapers down to a ten- don 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 (as described on pp. 202–203). Smooth Muscle Smooth muscle has no striations and is invol- untary, which means that we cannot consciously control it. Found mainly in the walls of hollow (tubelike) visceral organs such as the stomach, Longitudinal layer urinary bladder, and respiratory passages, smooth Submucosa of smooth muscle muscle propels substances along a pathway. Think (cross-sectional of smooth muscle as visceral, nonstriated, and view of cells) involuntary. (a) 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 run- ning circularly 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” activities normally handled by Cardiac smooth muscles. Smooth muscle contraction is muscle bundles 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 mara- thoner, 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 (b) heart walls. The heart serves as a pump, propelling Figure 6.2 Arrangement of smooth and cardiac blood through blood vessels to all body tissues. muscle cells. (a) Diagrammatic view of a cross section Like skeletal muscle, cardiac muscle is striated, and of the intestine. (b) Longitudinal view of the heart like smooth muscle, it is uninucleate and its control showing the spiral arrangement of the cardiac muscle is involuntary. Important key words for this muscle cells in its walls. type are cardiac, striated, and involuntary. The cardiac cells are cushioned by small amounts of endomysium and are arranged in spi- ral or figure 8–shaped bundles (Figure 6.2b). When the heart contracts, its internal chambers  Chapter 6: The Muscular System 185 become smaller, forcing blood into the large arter- Stabilize Joints ies leaving the heart. Cardiac muscle fibers are As skeletal muscles pull on bones to cause move- branching cells joined by special gap junctions ments, they also stabilize the joints of the skeleton. called intercalated discs (see Figure 3.20 on p. 99 Muscles and tendons are extremely important in and Chapter 3, p. 98). These two structural fea- reinforcing and stabilizing joints that have poorly tures and the spiral arrangement of the muscle articulating surfaces, such as the shoulder and bundles in the heart allow heart activity to be knee joints. In fact, physical therapy for knee inju- closely coordinated. ries includes exercise to strengthen thigh muscles Cardiac muscle usually contracts at a fairly because they support the knee. steady rate set by the heart’s “in-house” pace- 6 maker. However, the nervous system can also Generate Heat stimulate the heart to shift into “high gear” for Muscle activity generates body heat as a by-prod- short periods, as when you run to catch a bus. uct. As ATP is used to power muscle contraction, As you can see, each of the three muscle types nearly three-quarters of its energy escapes as heat. has a structure and function well suited for its job This heat is vital in maintaining normal body tem- in the body. But because the term muscular sys- perature. Skeletal muscle accounts for at least 40 tem applies specifically to skeletal muscle, we will percent of body mass, so it is the muscle type concentrate on this muscle type in this chapter. most responsible for generating heat. Muscle Functions Additional Functions All muscle types produce movement, but skeletal Muscles perform other important functions as well. muscle plays three other important roles in the body Smooth muscles form valves that regulate the pas- as well: it maintains posture and body position, sta- sage of substances through internal body open- bilizes joints, and generates heat. Let’s take a look. ings, dilate and constrict the pupils of our eyes, and make up the arrector pili muscles that cause Produce Movement our hairs to stand on end. Skeletal muscles form Skeletal muscles are responsible for our body’s valves that are under voluntary control, and they mobility, including all locomotion (walking, swim- enclose and protect fragile internal organs. ming, and cross-country skiing, for instance) and manipulating things with your agile upper limbs. Did You Get It? They enable us to respond quickly to changes in 1. How do cells of the three types of muscle tissues differ the external environment. For example, their speed from one another anatomically? and power enable us to jump out of the way of 2. Which muscle type has the most elaborate connective a runaway car and then follow its flight with our tissue wrappings? eyes. They also allow us to express our emotions 3. What does striated mean relative to muscle cells? 4. How do the movements promoted by skeletal muscle with the silent language of smiles and frowns. differ from those promoted by smooth or cardiac muscle? They are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, For answers, see Appendix A. which work together to circulate blood and main- tain blood pressure, and the smooth muscle of other hollow organs, which forces fluids (urine, Microscopic Anatomy bile) and other substances (food, a baby) through of Skeletal Muscle internal body channels. ➔ Learning Objective Maintain Posture and Body Position □□ Describe the microscopic structure of skeletal muscle, and explain the role of actin- and myosin- We are rarely aware of the workings of the skeletal containing myofilaments. muscles that maintain body posture. Yet they func- tion almost continuously, making one tiny adjust- As mentioned previously, skeletal muscle fibers (cells) ment after another so that we maintain an erect or are multinucleate (Figure 6.3a, p. 186). Many oval seated posture, even when we slouch, despite the nuclei can be seen just beneath the plasma membrane, never-ending downward pull of gravity. which is called the sarcolemma (sar″ko-lem′ah; 186 Essentials of Human Anatomy and Physiology Sarcolemma Myofibril Dark Light Nucleus (A) band (I) band (a) Segment of a muscle fiber (cell) Z disc H zone Z disc Thin (actin) myofilament Thick (myosin) myofilament (b) Myofibril or fibril I band A band I band M line (complex organelle composed of bundles Sarcomere of myofilaments) M line Z disc Z disc Thin (actin) myofilament Thick (myosin) myofilament (c) Sarcomere (segment of a myofibril) 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. “muscle husk”) in muscle fibers. The nuclei are a midline interruption, a darker area called the Z pushed aside by long ribbonlike organelles, the disc, and the dark A band has a lighter central area myofibrils (mi″o-fi′brilz), which nearly fill the called the H zone (Figure 6.3b). The M line in the cytoplasm. Alternating light (I) bands and dark center of the H zone contains tiny protein rods that (A) bands along the length of the perfectly aligned hold adjacent thick filaments together. myofibrils give the muscle fiber its striated (banded) So why are we bothering with all these terms— appearance. (Think of the second letter of light, dark this and light that? Because the banding I, and the second letter of dark, A, to help you pattern reveals the working structure of the myofi- remember which band is which.) A closer look at brils. First, we find that the myofibrils are actually the banding pattern reveals that the light I band has chains of tiny contractile units called sarcomeres  Chapter 6: The Muscular System 187 (sar′ko-mˉerz), which are the structural and func- Did You Get It? tional units of skeletal muscle. The sarcomeres are 5. Specifically, what structure(s) is/are responsible for the aligned end to end like boxcars in a train along banding pattern in skeletal muscle cells? the length of the myofibrils. Second, it is the pre- For the answer, see Appendix A. cise arrangement of even smaller structures (myo- filaments) within sarcomeres that produces the striations in skeletal muscle fibers. Skeletal Muscle Activity Let’s examine how the arrangement of the myofilaments leads to the banding pattern. There Stimulation and Contraction are two types of threadlike protein myofilaments of Single Skeletal Muscle Fibers 6 within each sarcomere (Figure 6.3c). The thick ➔ Learning Objective filaments are made mostly of bundled molecules □□ Describe how an action potential is initiated of the protein myosin, but they also contain in a muscle cell. ATPase enzymes, which split ATP to release the energy used for muscle contraction. Notice that Muscle fibers have several special functional proper- the thick filaments extend the entire length of the ties that enable them to perform their duties. The dark A band. Also, notice that the midparts of the first of these is irritability, also termed responsive- thick filaments are smooth, but their ends are ness, which is the ability to receive and respond studded with small projections (Figure 6.3c). to a stimulus. The second, contractility, is the abil- These projections, or myosin heads, form cross ity to forcibly shorten when adequately stimulated. bridges when they link the thick and thin fila- This property sets muscle apart from all other tissue ments together during contraction. Myosin fila- types. Extensibility is the ability of muscle fibers to ments are attached to the Z discs by titin, elastic stretch, whereas elasticity is their ability to recoil and filaments that run through the core of the thick resume their resting length after being stretched. filament. The thin filaments are composed of the con- The Nerve Stimulus and the Action Potential tractile protein called actin, plus some regulatory To contract, skeletal muscle fibers must be stimu- proteins that play a role in allowing (or prevent- lated by nerve impulses. One motor neuron (nerve ing) binding of myosin heads to actin. The thin cell) may stimulate a few muscle fibers or hundreds filaments are anchored to the Z disc (a disclike of them, depending on the particular muscle and membrane). Notice that the light I band includes the work it does. A motor unit consists of one parts of two adjacent sarcomeres and contains neuron and all the skeletal muscle fibers it stimu- only the thin filaments. Although they overlap the lates (Figure 6.4, p. 188). When a long, threadlike ends of the thick filaments, the thin filaments do extension of the neuron, called the axon, reaches not extend into the middle of a relaxed sarcomere, the muscle, it branches into a number of axon and thus the central region (the H zone) looks a terminals, each of which forms junctions with the bit lighter. When the actin-containing thin fila- sarcolemma of a different muscle cell (Figure 6.5, ments slide toward each other during contraction, p. 189). These junctions, called neuromuscular the H zones disappear because the actin and myo- (literally, “nerve-muscle”) junctions, contain syn- sin filaments completely overlap. aptic vesicles filled with a chemical referred to as Another very important muscle fiber organ- a neurotransmitter. The specific neurotransmitter elle—the sarcoplasmic reticulum (SR)—is a that stimulates skeletal muscle fibers is acetylcho- specialized smooth endoplasmic reticulum (not line (as″e-til-ko′lˉen), or ACh. Although the nerve shown in Figure 6.3). The interconnecting tubules endings and the muscle fiber membranes are very and sacs of the SR surround every myofibril just as close, they never touch. The gap between them, the the sleeve of a loosely crocheted sweater sur- synaptic cleft, is filled with interstitial fluid. rounds your arm. The major role of this elaborate system is to store calcium and to release it on Homeostatic Imbalance 6.1 demand when the muscle fiber is stimulated to contract. As you will see, calcium provides the In some cases, a motor nerve impulse is unable to final “go” signal for contraction. reach the muscle. In ALS, or amyotrophic lateral 188 Essentials of Human Anatomy and Physiology Axon terminals at neuromuscular junctions Muscle fibers Spinal cord Motor Motor unit 1 unit 2 Nerve Axon of motor Motor neuron neuron cell bodies Muscle Muscle fibers Branching axon to motor unit (b) (a) Figure 6.4 Motor units. Each units are shown. The motor neurons number of axon terminals distributed motor unit consists of a motor reside in the spinal cord, and their to muscle fibers scattered throughout neuron and all the muscle fibers it axons extend to the muscle. Within the muscle. (b) Photo of a portion of activates. (a) Portions of two motor the muscle, each axon divides into a a motor unit (1150×). 1 sclerosis (also called Lou Gehrig’s disease), motor (Ca2+) enters the terminal 1 2. Calcium entry neurons degenerate over time, resulting in paralysis causes some of1 the synaptic vesicles in the axon 2 3 that gradually worsens. The cause of ALS is terminal to fuse with the cell membrane and unknown, but common characteristics include mal- 2 release acetylcholine 3 , which 4 then diffuses functioning mitochondria, inflammation, and the across the synaptic cleft and attaches to mem- 3 4 5 generation of free radicals that damage DNA and brane receptors in highly folded regions of the tissue much like intense UV light. The prognosis for sarcolemma 4. If enough 5 6 acetylcholine is patients with ALS is generally death within three to released, the sarcolemma at that point becomes 5 6 five years because the breathing muscles will even- temporarily even more permeable to sodium ions +), which rush into the muscle fiber, and to tually be affected, resulting in suffocation. ______ ✚ (Na 6 potassium ions (K+), which 1 diffuse out of the Did You Get It? muscle fiber. However, more + 2 Na enters than K + 6. What two structures are closely associated at a leaves. This imbalance gives the cell interior an neuromuscular junction? excess of positive ions, which 3 reverses the resting For the answer, see Appendix A. electrical conditions of the sarcolemma. This 4 event, called depolarization, opens more channels Now that we have described the structure of the that only allow Na+ entry 5. This movement of neuromuscular junction, refer to the numbered ions generates an electrical current called an steps in Figure 6.5 as we examine what happens 6 action potential. Once begun, the action poten- there. When a nerve impulse reaches the axon ter- tial is unstoppable; it travels over the entire ­surface minals 1 , calcium channels open, and calcium of the sarcolemma, conducting the electrical  Chapter 6: The Muscular System 189 Myelinated axon Nerve of motor neuron impulse Axon terminal of Nucleus neuromuscular junction Sarcolemma of the muscle fiber 6 Synaptic vesicle containing ACh 1 Nerve impulse reaches axon terminal of motor neuron. Axon terminal of motor neuron Mitochondrion 2 Calcium (Ca2+) channels Ca2+ Ca2+ open, and Ca2+ enters the axon Synaptic terminal. cleft Sarcolemma Fusing synaptic vesicle Sarcoplasm 3 Ca2+ entry causes some ACh of muscle fiber synaptic vesicles to release their Folds of contents (the neurotransmitter ACh receptor sarcolemma acetylcholine) by exocytosis. 4 Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma. Ion channel in 5 ACh binds and opens channels Na+ K+ sarcolemma opens; that allow simultaneous passage ions pass. 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 Degraded ACh Ion channel closes; Na+ ions cannot pass. 6 The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending the process. Acetylcholinesterase K+ Figure 6.5 Events at the neuromuscular junction. 190 Essentials of Human Anatomy and Physiology Neuromuscular junction Muscle fiber Nerve (cell) Small twig Striations fiber Match flame 1 Na+ diffuses into the cell. 1 Flame ignites 2 Flame spreads 2 Action potential spreads the twig. rapidly along the twig. rapidly along the sarcolemma. (a) (b) Figure 6.6 Comparing the when it has been heated enough second event is the spreading of the action potential to a flame and spreading of the flame to burn action potential along the consuming a dry twig. (a) The the entire twig. (b) The first event in sarcolemma when enough sodium first event in igniting a dry twig is exciting a muscle fiber is the rapid ions have entered to upset the holding the match flame under one diffusion of sodium ions (Na+) into electrical conditions in the cell. area of the twig. The second event the cell when the permeability of is the twig’s bursting into flame the sarcolemma changes. The 1 2 impulse from one end of the cell to the other. The the sodium and potassium ions back to their initial result is contraction 3 of the muscle fiber. positions. Note that while the action potential is occur- ring, the4 enzyme acetylcholinesterase (AChE), Did You Get It? present on the sarcolemma and in the synaptic 7. Which ions enter the muscle cell during the 5 cleft, breaks down acetylcholine to acetic acid and generation of an action potential? choline 6. For this reason, a single nerve impulse 8. What is the role of calcium ions in muscle contraction? produces only one contraction. This prevents con- For answers, see Appendix A. tinued contraction of the muscle fiber in the absence of additional nerve impulses. The muscle Mechanism of Muscle Contraction: fiber relaxes until stimulated by the next round of The Sliding Filament Theory acetylcholine release. ➔ Learning Objective Let’s compare this series of events to lighting a □□ Describe the events of muscle cell contraction. match under a small dry twig (Figure 6.6). The charring of the twig by the flame can be compared What causes the filaments to slide? This question to the change in membrane permeability that brings us back to the myosin heads that protrude allows sodium ions into the cell. When that part of all around the ends of the thick filaments. the twig becomes hot enough (when enough When the nervous system activates muscle sodium ions have entered the cell), the twig will fibers as just described, the myosin heads attach to suddenly burst into flame, and the flame will move binding sites on the thin filaments, and the sliding along the twig (the action potential will be con- begins. Each cross bridge attaches and detaches ducted along the entire length of the sarcolemma). several times during a contraction, generating ten- We explain this series of events more fully in sion that helps pull the thin filaments toward the the discussion of nerve physiology (Chapter 7, center of the sarcomere. This “walking” of the pp. 234–239). myosin cross bridges, or heads, along the thin fila- The events that return the cell to its resting state ments during muscle shortening is much like a include (1) diffusion of potassium ions (K+) out of centipede’s gait. Some myosin heads (“legs”) are the cell and (2) operation of the sodium-potassium always in contact with actin (“the ground”), so that pump, the active transport mechanism that moves the thin filaments cannot slide backward, and this  Chapter 6: The Muscular System 191 cycle repeats again and again during contraction. Myosin Actin As this event occurs simultaneously in sarcomeres throughout the muscle fiber, the cell shortens (Figure 6.7). Notice that the myofilaments them- selves do not shorten during contraction; they sim- ply 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 Z Z H heads). So where does the calcium come from? 6 Action potentials pass deep into the muscle fiber I A I along membranous tubules that fold inward from the sarcolemma. Inside the cell, the action poten- (a) Relaxed sarcomere tials stimulate the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions trigger the binding of myosin to actin, initiating fila- ment sliding (Figure 6.8, p. 192). When the action potential ends, calcium ions are immediately returned to the SR storage areas, the regulatory pro- teins 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 Z Z of events takes a few thousandths of a second. I A I Did You Get It? (b) Fully contracted sarcomere 9. Which chemical—ATP or Ca2+—triggers sliding of the Figure 6.7 Diagrammatic views of a sarcomere. muscle filaments? Notice that in the contracted sarcomere (b), the light H 10. Which is a cross-bridge attachment more similar to: a zone in the center of the A band has disappeared, the Z synchronized rowing team or a person pulling a discs are closer to the thick filaments, and the I bands have bucket on a rope out of a well? nearly disappeared. The A bands of adjacent sarcomeres For answers, see Appendix A. move closer together but do not change in length. Contraction of a Skeletal Muscle muscle fibers being stimulated at one time. Next, as a Whole let’s describe a muscle’s response to each of these. ➔ Learning Objective □□ Define graded response, tetanus, isotonic and Muscle Response to Increasingly Rapid Stimulation isometric contractions, and muscle tone as these Although muscle twitches (single, brief, jerky terms apply to a skeletal muscle. contractions) sometimes result from certain ner- vous system problems, this is not the way our Graded Responses muscles normally operate. In most types of muscle In skeletal muscles, the “all-or-none” law of mus- activity, nerve impulses are delivered to the mus- cle physiology applies to the muscle fiber, not to cle at a very rapid rate—so rapid that the muscle the whole muscle. It states that a muscle fiber will does not get a chance to relax completely between contract to its fullest extent when it is stimulated stimuli. As a result, the effects of the successive adequately; it never partially contracts. However, contractions are “summed” (added) together, and the whole muscle reacts to stimuli with graded the contractions of the muscle get stronger and responses, or different degrees of shortening, smoother. The muscle exhibits unfused tetanus which generate different amounts of force. In gen- (tet′ah-nus), or incomplete tetanus. When the eral, graded muscle contractions can be produced muscle is stimulated so rapidly that no evidence of two ways: (1) by changing the frequency of mus- relaxation is seen and the contractions are com- cle stimulation and (2) by changing the number of pletely smooth and sustained, the muscle is in 192 Essentials of Human Anatomy and Physiology Regulatory proteins In a relaxed muscle fiber, the regulatory proteins forming part of the actin myofilaments 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). Myosin myofilament Actin myofilament (a) Myosin-binding site The flood of calcium acts as the final trigger for Ca2+ 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. Upper part of thick filament only (b) The 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 (c) slightly pulled 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 6.8 Schematic representation of contraction mechanism: the sliding filament theory. Play A&P Flix Animation fused tetanus, or complete tetanus, or in tetanic smooth and prolonged muscle contractions. How contraction* (Figure 6.9). forcefully a muscle contracts depends to a large extent on how many of its cells are stimulated. Muscle Response to Stronger Stimuli Tetanus When only a few cells are stimulated, the muscle produces stronger (more forceful) muscle con- as a whole contracts only slightly. When all the tractions, but its primary role is to produce 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 * Tetanic contraction is normal and desirable. It is quite dif- ferent from the pathological condition of tetanus (commonly from slight to vigorous depending on the work to called lockjaw), which is caused by a toxin made by bacteria. be done. The same hand that lifts a single sheet Lockjaw causes muscles to go into uncontrollable spasms, of paper can also lift a heavy backpack full of finally causing respiratory arrest. books!  Chapter 6: The Muscular System 193 Tension (g) (Stimuli) (a) Twitch (b) Summing of (c) Unfused (d) Fused (complete) contractions (incomplete) tetanus tetanus 6 Figure 6.9 A whole muscle’s the next stimulus; contraction force evidence of relaxation, results from a response to different increases because effects of the very rapid rate of stimulation. (Points stimulation rates. A single individual twitches are summed at which stimuli are delivered are stimulus (a) causes muscle (added). Further fusion (c) of the indicated by red arrows. Tension contraction and relaxation (a twitch). twitches (unfused tetanus) occurs as [measured in grams] on the vertical As stimuli are delivered more stimuli are delivered at a still faster axis refers to the relative force of frequently (b), the muscle does not rate. Fused tetanus (d), a smooth muscle contraction.) have time to completely relax before continuous contraction without any Providing Energy for Muscle Contraction to ADP, thus regenerating more ATP in a frac- ➔ Learning Objective tion of a second. Although muscle fibers store perhaps five times as much CP as ATP, the CP □□ Describe three pathways for ATP regeneration during muscle activity. supplies are also soon exhausted (in less than 15 seconds). As a muscle contracts, the bonds of ATP molecules Aerobic pathway (Figure 6.10b). At rest and are hydrolyzed to release the needed energy. during light to moderate exercise, some 95 per- ➔ cent of the ATP used for muscle activity comes CONCEPTLINK from aerobic respiration. Aerobic respiration Recall that ATP can be compared to a tightly coiled occurs in the mitochondria and involves a series spring that is ready to uncoil with tremendous energy of metabolic pathways that use oxygen. These when the “catch” is released (Chapter 2, p. 55). pathways are collectively referred to as oxida- Remember that all bonds store energy and that the tive phosphorylation. During aerobic respiration, “catch” in this example is one of the characteristic glucose is broken down completely to carbon high-energy phosphate bonds in ATP. ➔ dioxide and water, and some of the energy re- leased as the bonds are broken is captured in Surprisingly, muscles store very limited sup- the bonds of ATP molecules. Although aerobic plies of ATP—only a few seconds’ worth, just respiration provides a rich ATP harvest (about enough to get you going. Because ATP is the only 32 ATP per 1 glucose), it is fairly slow and re- energy source that can be used directly to power quires continuous delivery of oxygen and nutri- muscle activity, ATP must be regenerated continu- ent fuels to the muscle to keep it going. ously if contraction is to continue. Anaerobic glycolysis and lactic acid forma- Working muscles use three pathways to regen- tion (Figure 6.10c). The initial steps of glu- erate ATP: cose breakdown occur via a pathway called Direct phosphorylation of ADP by cre- glycolysis, which does not use oxygen and atine phosphate (Figure 6.10a, p. 194). The hence is anaerobic (literally “without oxygen”). unique high-energy molecule creatine phos- During glycolysis, which occurs in the cyto- phate (CP) is found in muscle fibers but not sol, glucose is broken down to pyruvic acid, other cell types. As ATP is depleted, interac- and small amounts of energy are captured in tions between CP and ADP result in transfers ATP bonds (2 ATP per 1 glucose molecule). As of a high-energy phosphate group from CP long as enough oxygen is present, the pyruvic 194 Essentials of Human Anatomy and Physiology (a) Direct phosphorylation (b) Aerobic pathway (c) Anaerobic pathway Coupled reaction of creatine Aerobic cellular respiration Glycolysis and lactic acid phosphate (CP) and ADP formation Energy source: CP Energy source: glucose; pyruvic Energy source: glucose acid; free fatty acids from adipose tissue; amino acids from protein catabolism P Creatine ADP Glucose (from Glucose (from glycogen breakdown or glycogen breakdown or delivered from blood) delivered from blood) Pyruvic acid Glycolysis Creatine ATP Fatty in cytosol acids O2 Aerobic respiration Amino in mitochondria 2 ATP acids Pyruvic acid net gain 32 ATP CO2 Released Lactic acid H2O net gain to blood per glucose Oxygen use: None Oxygen use: Required Oxygen use: None Products: 1 ATP per CP, Products: 32 ATP per glucose, Products: 2 ATP per glucose, creatine CO2, H2O lactic acid Duration of energy provision: Duration of energy provision: Duration of energy provision: 15 seconds Hours 40 seconds, or slightly more Figure 6.10 Methods of regenerating ATP during muscle activity. The fastest mechanism is (a) direct phosphorylation; the slowest is (b) aerobic respiration. acid then enters the oxygen-requiring aerobic Did You Get It? pathways that occur within the mitochondria 11. What are the three processes used to generate energy to produce more ATP as described above. for skeletal muscle contraction? However, when muscle activity is intense, 12. What is the direct source of energy used by muscle or oxygen and glucose delivery is temporar- fibers for contraction? ily inadequate to meet the needs of working For answers, see Appendix A. muscles, the sluggish aerobic pathways can- not keep up with the demands for ATP. Under Muscle Fatigue and Oxygen Deficit these conditions, the pyruvic acid generated ➔ Learning Objective during glycolysis is converted to lactic acid, □□ Define oxygen deficit and muscle fatigue, and list and the overall process is referred to as an- possible causes of muscle fatigue. aerobic glycolysis.   Anaerobic glycolysis produces only about 5 If we exercise our muscles strenuously for a long percent as much ATP from each glucose mole- time, muscle fatigue occurs. A muscle is fatigued cule as aerobic respiration. However, it is some when it is unable to contract even though it is still 2½ times faster, and it can provide most of the being stimulated. Without rest, a working muscle ATP needed for 30 to 40 seconds of strenuous begins to tire and contracts more weakly until it muscle activity. Anaerobic glycolysis has two finally ceases reacting and stops contracting. main shortcomings: it uses huge amounts of Factors that contribute to muscle fatigue are not glucose for a small ATP harvest, and the accu- fully known. Suspected causes are imbalances in mulating lactic acid promotes muscle soreness. ions (Ca2+, K+) and problems at the neuromuscular  Chapter 6: The Muscular System 195 junction. However, many agree that the major factor movement occurs. Bending the knee, lifting is the oxygen deficit that occurs during prolonged weights, and smiling are all examples of isotonic muscle activity. Oxygen deficit is not a total lack of contractions. oxygen; rather, it happens when a person is not Contractions in which the muscles do not able to take in oxygen fast enough to keep the shorten are called isometric contractions (liter- muscles supplied with all the oxygen they need ally, “same measurement” or length). In isometric when they are working vigorously. Obviously, then, contractions, the myosin filaments are “spinning the work that a muscle can do and how long it can their wheels,” and the tension in the muscle keeps work without becoming fatigued depend on how increasing. They are trying to slide, but the muscle good its blood supply is. When muscles lack suffi- is pitted against some more or less immovable 6 cient oxygen for aerobic respiration, lactic acid object. For example, when you push the palms of begins to accumulate in the muscle via the anaero- your hands together in front of you, your arms and bic pathway. We can recognize this event by the chest muscles are contracting isometrically. burning sensation we experience. In addition, the muscle’s ATP supply starts to run low, and ionic Muscle Tone imbalance tends to occur. Together these factors One aspect of skeletal muscle activity cannot be cause the muscle to contract less and less effec- consciously controlled. Even when a muscle is tively and finally to stop contracting altogether. voluntarily relaxed, some of its fibers are con- True muscle fatigue, in which the muscle quits tracting—first one group and then another. These entirely, rarely occurs in most of us because we contractions are not visible, but thanks to them, feel tired long before it happens and we simply the muscle remains firm, healthy, and constantly slow down or stop our activity. It does happen in ready for action. This state of continuous partial marathon runners. Many of them have literally col- contractions is called muscle tone. Muscle tone lapsed when their muscles became fatigued and is the result of different motor units, which are could no longer work. scattered through the muscle, being stimulated by Oxygen deficit, which always occurs to some the nervous system in a systematic way. Think of extent during vigorous muscle activity, is like a these motor units as being “on duty” in case action loan that must be “paid back” whether fatigue is required. occurs or not. During the recovery period after activity, the individual breathes rapidly and deeply. Homeostatic Imbalance 6.2 This continues until the muscles have received the amount of oxygen needed to get rid of the accu- If the nerve supply to a muscle is destroyed (as in mulated lactic acid and replenish ATP and creatine an accident), the muscle is no longer stimulated in phosphate reserves. this manner, and it loses tone. Soon after, it becomes flaccid (flă′sid), or soft and flabby, and Types of Muscle Contractions—Isotonic begins to atrophy (waste away). This is called and Isometric flaccid paralysis. Compare this with a condition Until now, we have been discussing contraction that increases muscle tone until the muscle is no in terms of shortening, but muscles do not always longer controllable—for example, the disease teta- shorten when they contract. (I can hear you say- nus, which is caused by a bacterial toxin. This is ing, “What kind of double-talk is that?”—but pay called spastic paralysis. ______________________ ✚ attention.) The event that is common to all muscle contractions is that tension (force) develops in Effect of Exercise on Muscles the muscle as the actin and myosin myofilaments ➔ Learning Objective interact and the myosin cross bridges attempt to □□ Describe the effects of aerobic and resistance slide the thin actin-containing filaments past the exercise on skeletal muscles and other body organs. thick myosin filaments. Isotonic contractions (literally, “same tone” The amount of work a muscle does changes the or tension) are familiar to most of us. In isotonic muscle. Muscle inactivity (due to a loss of nerve sup- contractions, the myofilaments are successful in ply, immobilization, or whatever the cause) always their sliding movements, the muscle shortens, and leads to muscle weakness and wasting. Muscles 196 Essentials of Human Anatomy and Physiology 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 (a) (b) produce different patterns of muscle response, it is Figure 6.11 The effects of aerobic training important to know what your exercise goals are. versus strength training. (a) A marathon runner. Lifting weights will not improve your endurance (b) A weight lifter. for a marathon. By the same token, jogging will not make you stronger for lifting furniture. Obviously, are no exception to the saying “Use it or lose it!” the best exercise program for most people includes Conversely, regular exercise increases muscle size, both types of exercise. strength, and endurance. However, not all types of exercise produce these effects—in fact, there are Did You Get It? important differences in the benefits of exercise. 13. Gary is trying with all his might to pull a tree stump Aerobic exercise, or endurance exercise, out of the ground. It does not budge. Which type of contraction are his muscles performing? such as participating in an aerobics class, jogging, 14. What is meant by the term oxygen deficit? or biking (Figure 6.11a), results in stronger, more 15. To develop big, beautiful skeletal muscles, you should flexible muscles with greater resistance to fatigue. focus on which type of exercise: aerobic or resistance These changes come about, at least partly, because exercise? the blood supply to the muscles increases, and the For answers, see Appendix A. individual muscle fibers form more mitochondria and store more oxygen. Aerobic exercise helps us reach a steady rate of ATP production and Muscle Movements, Roles, improves the efficiency of aerobic respiration. However, aerobic exercise benefits much more and Names than the skeletal muscles. It makes overall body ➔ Learning Objectives metabolism more efficient, improves digestion (and □□ Define origin, insertion, prime mover, antagonist, elimination), enhances neuromuscular coordina- synergist, and fixator as they relate to muscles. tion, and strengthens the skeleton. The heart □□ Demonstrate or identify the different types of body enlarges (hypertrophies) and pumps out more blood movements. with each beat, helping to clear more fat deposits There are five basic guidelines for understanding from the blood vessel walls. The lungs become gross muscle activity. We refer to these as the Five more efficient in gas exchange. These benefits may Golden Rules of skeletal muscle activity because they be permanent or temporary, depending on how make it easier to understand muscle movements and often and how vigorously a person exercises. appreciate muscle interactions (Table 6.2). Aerobic exercise does not cause the muscles to increase much in size, even though the exercise may Types of Body Movements go on for hours. The bulging muscles of a profes- Every one of our 600-odd skeletal muscles is sional bodybuilder result mainly from resistance attached to bone, or to other connective tissue  Chapter 6: The Muscular System 197 Table 6.2  he Five Golden Rules of Skeletal T Muscle Activity Muscle 1. With a few exceptions, all skeletal muscles cross at contracting 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 origin and the insertion. 4. Skeletal muscles can only pull; they never push. Brachialis 6 5. During contraction, a skeletal muscle insertion moves toward the origin. structures, at no fewer than two points. One of Tendon these points, the origin, is attached to the immov- Insertion able or less movable bone (Figure 6.12). Think of the origin as the anchor, or leverage, point. Figure 6.12 Muscle attachments (origin and Another point, the insertion, is attached to the insertion). When a skeletal muscle contracts, its movable bone. When the muscle contracts, the insertion moves toward its origin. insertion moves toward the origin. Some muscles have interchangeable origins and insertions, depending on the action being performed. Extension. Extension is the opposite of flex- For example, the rectus femoris muscle of the ante- ion, so it is a movement that increases the rior thigh crosses both the hip and knee joints. Its angle, or distance, between two bones or parts most common action is to extend the knee, in which of the body (straightening the knee or elbow). case the proximal pelvic attachment is the origin. Extension that is greater than 180° (as when However, when the knee bends (by other muscles), you move your arm posteriorly beyond its the rectus femoris can flex the hip, and then its distal normal anatomical position, or tip your head attachment on the leg is considered the origin. so that your chin points toward the ceiling) Generally speaking, body movement occurs is called hyperextension (Figures 6.13a and when muscles contract across joints. The type of 6.13b). movement depends on the mobility of the joint and Rotation. Rotation is movement of a bone the location of the muscle in relation to the joint. around its longitudinal axis (Figure 6.13c). The most obvious examples of the action of muscles Rotation is a common movement of ball-and- on bones are the movements that occur at the joints socket joints and describes the movement of of the limbs. However, less freely movable bones are the atlas around the dens of the axis (as in also tugged into motion by the muscles, such as the shaking your head “no”). vertebrae’s movements when we bend to one side. Abduction. Abduction is moving a limb away Next we describe the most common types of (generally on the frontal plane) from the mid- body movements (Figure 6.13, pp. 198–199). Try to line, or median plane, of the body (Figure act out each movement as you read the following 6.13d). The terminology also applies to the descriptions: fanning movement of your fingers or toes Flexion. Flexion is a movement, generally in when they are spread apart. the sagittal plane, that decreases the angle of Adduction. Adduction is the opposite of ab- the joint and brings two bones closer together duction, so it is the movement of a limb to- (Figures 6.13a and 6.13b). Flexion is typical of ward the body midline (Figure 6.13d). Think of hinge joints (bending the knee or elbow), but adduction as “adding” a body part by bringing it is also common at ball-and-socket joints (for it closer to the trunk. example, bending forward at the hip). (Text continues on page 200.) 198 Essentials of Human Anatomy and Physiology Flexion Hyperextension Extension Flexion Extension (a) Flexion, extension, and hyperextension of the shoulder and knee Hyperextension Extension Rotation Flexion Lateral rotation Medial rotation (b) Flexion, extension, and hyperextension Figure 6.13 Body movements. (c) Rotation  Chapter 6: The Muscular System 199 Inversion