Muscles And Muscle Tissue - Anatomy
9Muscles and Muscle TissueOverview of Muscle Tissues (pp. 276–278)Types of Muscle Tissue (p. 277)Special Characteristics of Muscle Tissue(p. 277)Muscle Functions (pp. 277–278)Skeletal Muscle (pp. 278–305)Gross Anatomy of a Skeletal Muscle(pp. 278–279)Microscopic Anatomy of a Skeletal MuscleFiber (pp. 279–285)Sliding Filament Model of Contraction(p. 285)Physiology of Skeletal Muscle Fibers(pp. 285–293)Contraction of a Skeletal Muscle(pp. 293–298)Muscle Metabolism (pp. 298–301)Force of Muscle Contraction (pp. 301–302)Velocity and Duration of Contraction(pp. 302–304)Adaptations to Exercise (pp. 304–305)Smooth Muscle (pp. 305–311)Microscopic Structure of Smooth MuscleFibers (pp. 305–307)Contraction of Smooth Muscle(pp. 307–309)Types of Smooth Muscle (p. 309)Developmental Aspects of Muscles(pp. 312–313, 315)Because flexing muscles look like mice scurrying beneath the skin,some scientist long ago dubbed them muscles, from the Latin mus meaning “littlemouse.” Indeed, we tend to think of the rippling muscles of professional boxers orweight lifters when we hear the word muscle. But muscle is also the dominant tissue inthe heart and in the walls of other hollow organs. In all its forms, muscle tissue makes upnearly half the body’s mass.Muscles are distinguished by their ability to transform chemical energy (ATP) intodirected mechanical energy. In so doing, they become capable of exerting force.Overview of Muscle TissuesCompare and contrast the three basic types of muscle tissue.List four important functions of muscle tissue.276
Chapter 9 Muscles and Muscle TissueTypes of Muscle TissueChapter 4 introduced the three types of muscle tissue—skeletal,cardiac, and smooth. Now we are ready to describe each type indetail, but before we do, let’s introduce some terminology. Skeletal and smooth muscle cells (but not cardiac musclecells) are elongated, and for this reason, they are called muscle fibers.Whenever you see the prefixes myo or mys (both are word rootsmeaning “muscle”) or sarco (flesh), the reference is to muscle.For example, the plasma membrane of muscle cells is called thesarcolemma (sar!ko-lem"ah), literally, “muscle” (sarco) “husk”(lemma), and muscle cell cytoplasm is called sarcoplasm.Okay, let’s get to it.Skeletal MuscleSkeletal muscle tissue is packaged into the skeletal muscles, organs that attach to and cover the bony skeleton. Skeletal muscle fibers are the longest muscle cells and have obvious stripescalled striations (see Figure 4.9a, p. 138). Although it is oftenactivated by reflexes, skeletal muscle is called voluntary musclebecause it is the only type subject to conscious control. When you think of skeletal muscle tissue, the key words tokeep in mind are skeletal, striated, and voluntary.Skeletal muscle is responsible for overall body mobility. Itcan contract rapidly, but it tires easily and must rest after shortperiods of activity. Nevertheless, it can exert tremendous power,a fact revealed by reports of people lifting cars to save their lovedones. Skeletal muscle is also remarkably adaptable. For example,your forearm muscles can exert a force of a fraction of an ounceto pick up a paper clip—or a force of about 6 pounds to pick upthis book!Cardiac MuscleCardiac muscle tissue occurs only in the heart (the body’sblood pump), where it constitutes the bulk of the heart walls.Like skeletal muscle cells, cardiac muscle cells are striated (seeFigure 4.9b, p. 139), but cardiac muscle is not voluntary. Indeed,it can and does contract without being stimulated by the nervous system. Most of us have no conscious control over how fastour heart beats. Key words to remember for cardiac muscle are cardiac, striated, and involuntary.Cardiac muscle usually contracts at a fairly steady rate setby the heart’s pacemaker, but neural controls allow the heart tospeed up for brief periods, as when you race across the tenniscourt to make that overhead smash.Smooth MuscleSmooth muscle tissue is found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages.Its role is to force fluids and other substances through internal bodychannels. Like skeletal muscle, smooth muscle consists of elongatedcells, but smooth muscle has no striations (see Figure 4.9c, p. 139).277Like cardiac muscle, smooth muscle is not subject to voluntary control. Its contractions are slow and sustained. We can describe smooth muscle tissue as visceral, nonstriated, and involuntary.Special Characteristics of Muscle TissueWhat enables muscle tissue to perform its duties? Four specialcharacteristics are key.Excitability, also termed responsiveness, is the ability toreceive and respond to a stimulus, that is, any change in theenvironment either inside or outside the body. In the case ofmuscle, the stimulus is usually a chemical—for example, a neurotransmitter released by a nerve cell, or a local change in pH.The response (sometimes separated out as an additional characteristic called conductivity) is to generate an electrical impulsethat travels along the plasma membrane of the muscle cell andcauses the cell to contract.Contractility is the ability to shorten forcibly when adequately stimulated. This ability sets muscle apart from all othertissue types.Extensibility is the ability to extend or stretch. Muscle cellsshorten when contracting, but they can stretch, even beyondtheir resting length, when relaxed.Elasticity is the ability of a muscle cell to recoil and resumeits resting length after stretching.Muscle FunctionsMuscle performs at least four important functions for the body.It produces movement, maintains posture, stabilizes joints, generates heat, and more.Producing MovementJust about all movements of the human body and its parts resultfrom muscle contraction. Skeletal muscles are responsible forall locomotion (moving from place to place) and manipulation.They enable you to respond quickly to changes in the externalenvironment—for example, jump out of the way of a car, directyour eyes, and smile or frown.Blood courses through your body because of the rhythmicallybeating cardiac muscle of your heart and the smooth musclein the walls of your blood vessels, which helps maintain bloodpressure. Smooth muscle in organs of the digestive, urinary, andreproductive tracts propels, or squeezes, substances (foodstuffs,urine, semen) through the organs and along the tract.Maintaining Posture and Body PositionWe are rarely aware of the skeletal muscles that maintain bodyposture. Yet these muscles function almost continuously, making one tiny adjustment after another to counteract the neverending downward pull of gravity.Stabilizing JointsEven as muscles pull on bones to cause movement, they stabilizeand strengthen the joints of the skeleton (Chapter 8).9
278UNIT 2 Covering, Support, and Movement of the BodyGenerating HeatMuscles generate heat as they contract. This heat is vitally important in maintaining normal body temperature. Becauseskeletal muscle accounts for at least 40% of body mass, it is themuscle type most responsible for generating heat.Additional Functions of MuscleWhat else do muscles do? Skeletal muscles protect the more fragile internal organs (the viscera) by enclosing them. Smooth muscle forms valves to regulate the passage of substances throughinternal body openings, dilates and constricts the pupils of youreyes, and forms the arrector pili muscles attached to hair follicles.Connective Tissue Sheaths 9stimulation, every skeletal muscle fiber is supplied with a nerveending that controls its activity.Skeletal muscle has a rich blood supply. This is understandable because contracting muscle fibers use huge amounts ofenergy and require almost continuous delivery of oxygen andnutrients via the arteries. Muscle cells also give off large amountsof metabolic wastes that must be removed through veins if contraction is to remain efficient. Muscle capillaries, the smallest ofthe body’s blood vessels, are long and winding and have numerous cross-links, features that accommodate changes in musclelength. They straighten when the muscle stretches and contortwhen the muscle contracts.In this chapter, we examine the structure and function ofskeletal muscle. Then we consider smooth muscle more briefly,largely by comparing it with skeletal muscle. We describe cardiac muscle in detail in Chapter 18, but for easy comparison,Table 9.3 on p. 310 summarizes the characteristics of all threemuscle types.In an intact muscle, several different connective tissue sheathswrap individual muscle fibers. Together these connective tissuesheaths support each cell and reinforce and hold together themuscle as a whole, preventing the bulging muscles from bursting during exceptionally strong contractions.Let’s consider these connective tissue sheaths from externalto internal (see Figure 9.1 and the top three rows of Table 9.1). Check Your Understanding1. When describing muscle, what does “striated” mean?2. Harry is pondering an exam question that asks, “Whichmuscle type has elongated cells and is found in the walls ofthe urinary bladder?” How should he respond? For answers, see Appendix H.Skeletal MuscleDescribe the gross structure of a skeletal muscle.Describe the microscopic structure and functional roles ofthe myofibrils, sarcoplasmic reticulum, and T tubules ofskeletal muscle fibers.Describe the sliding filament model of muscle contraction.For easy reference, Table 9.1 on p. 283 summarizes the levelsof skeletal muscle organization, gross to microscopic, that wedescribe in the following sections.Gross Anatomy of a Skeletal MuscleEach skeletal muscle is a discrete organ, made up of severalkinds of tissues. Skeletal muscle fibers predominate, but bloodvessels, nerve fibers, and substantial amounts of connective tissue are also present. We can easily examine a skeletal muscle’sshape and its attachments in the body without a microscope.Nerve and Blood SupplyIn general, one nerve, one artery, and one or more veins serveeach muscle. These structures all enter or exit near the centralpart of the muscle and branch profusely through its connective tissue sheaths (described below). Unlike cells of cardiacand smooth muscle tissues, which can contract without nerve Epimysium.The epimysium (ep!ĭ-mis"e-um; meaning “outside the muscle”) is an “overcoat” of dense irregular connective tissue that surrounds the whole muscle. Sometimes itblends with the deep fascia that lies between neighboringmuscles or the superficial fascia deep to the skin.Perimysium and fascicles. Within each skeletal muscle, themuscle fibers are grouped into fascicles (fas"ĭ-klz; “bundles”)that resemble bundles of sticks. Surrounding each fascicle isa layer of fibrous connective tissue called perimysium (per!ĭmis"e-um; meaning “around the muscle [fascicles]”).Endomysium. The endomysium (en!do-mis"e-um; meaning“within the muscle”) is a wispy sheath of connective tissuethat surrounds each individual muscle fiber. It consists offine areolar connective tissue.As shown in Figure 9.1, all of these connective tissue sheathsare continuous with one another as well as with the tendons thatjoin muscles to bones. When muscle fibers contract, they pullon these sheaths, which transmit the pulling force to the boneto be moved. The sheaths contribute somewhat to the naturalelasticity of muscle tissue, and also provide entry and exit routesfor the blood vessels and nerve fibers that serve the muscle.AttachmentsRecall from Chapter 8 that most skeletal muscles span jointsand attach to bones (or other structures) in at least two places.When a muscle contracts, the movable bone, the muscle’s insertion, moves toward the immovable or less movable bone, themuscle’s origin. In the muscles of the limbs, the origin typicallylies proximal to the insertion.Muscle attachments, whether origin or insertion, may be direct or indirect. In direct, or fleshy, attachments, the epimysium of the muscle is fused to the periosteum of a bone or perichondrium ofa cartilage.
279Chapter 9 Muscles and Muscle siumMuscle fiberin middle ofa fascicle(b)Blood vesselPerimysium wrapping a fascicleEndomysium(between individual muscle fibers)9Muscle fiberFasciclePerimysium(a)Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium,and endomysium. (b) Photomicrograph of a cross section of part of a skeletal muscle (30#).(For a related image, see A Brief Atlas of the Human Body, Plate 29.) In indirect attachments, the muscle’s connective tissuewrappings extend beyond the muscle either as a ropeliketendon (Figure 9.1a) or as a sheetlike aponeurosis (ap!onu-ro"sis). The tendon or aponeurosis anchors the muscle tothe connective tissue covering of a skeletal element (bone orcartilage) or to the fascia of other muscles.Indirect attachments are much more common because oftheir durability and small size. Tendons are mostly tough collagen fibers which can withstand the abrasion of rough bony projections that would tear apart the more delicate muscle tissues.Because of their relatively small size, more tendons than fleshymuscles can pass over a joint—so tendons also conserve space.Before moving on to microscopic anatomy, you may want toreview the top three rows of Table 9.1.Microscopic Anatomy of a SkeletalMuscle FiberEach skeletal muscle fiber is a long cylindrical cell with multipleoval nuclei just beneath its sarcolemma or plasma membrane(Figure 9.2b). Skeletal muscle fibers are huge cells. Their diameter typically ranges from 10 to 100 μm—up to ten times thatof an average body cell—and their length is phenomenal, someup to 30 cm long. Their large size and multiple nuclei are notsurprising once you learn that hundreds of embryonic cells fuseto produce each fiber.Sarcoplasm, the cytoplasm of a muscle cell, is similar to thecytoplasm of other cells, but it contains unusually large amountsof glycosomes (granules of stored glycogen that provide glucoseduring muscle cell activity) and myoglobin, a red pigment thatstores oxygen. Myoglobin is similar to hemoglobin, the pigmentthat transports oxygen in blood.In addition to the usual organelles, a muscle cell containsthree structures that are highly modified: myofibrils, sarcoplasmic reticulum, and T tubules. Let’s look at these structures moreclosely because they play important roles in muscle contraction.MyofibrilsA single muscle fiber contains hundreds to thousands of rodlike myofibrils that run parallel to its length (Figure 9.2b). Themyofibrils, each 1–2 μm in diameter, are so densely packed inthe fiber that mitochondria and other organelles appear to besqueezed between them. They account for about 80% of cellularvolume.
280UNIT 2 Covering, Support, and Movement of the Body(a) Photomicrograph of portionsof two isolated musclefibers (700 ). Notice theobvious striations(alternating dark and lightbands).NucleiDark A bandLight I bandFiber(b) Diagram of part of amuscle fiber showingthe myofibrils. Onemyofibril extends fromthe cut end of the fiber.SarcolemmaMitochondrion9MyofibrilDark A bandThin (actin)filamentLight I bandNucleusH zoneZ discZ disc(c) Small part of onemyofibril enlarged toshow the myofilamentsresponsible for thebanding pattern. Eachsarcomere extends fromone Z disc to the next.Thick (myosin)filamentI bandA bandSarcomereI bandM lineZ discM lineZ discThin (actin)filament(d) Enlargement of onesarcomere (sectionedlengthwise). Notice themyosin heads on the thickfilaments.Elastic (titin)filamentsThick(myosin)filament(e) Cross-sectional view of asarcomere cut through indifferent locations.MyosinfilamentActinfilamentI bandthin filamentsonlyH zonethick filamentsonlyM linethick filaments linkedby accessory proteinsFigure 9.2 Microscopic anatomy of a skeletal muscle fiber. (For a related image, seeA Brief Atlas of the Human Body, Plate 28.)Outer edge of A bandthick and thinfilaments overlap
Chapter 9 Muscles and Muscle TissueMyofibrils contain the contractile elements of skeletal musclecells, the sarcomeres, which contain even smaller rodlike structures called myofilaments. Table 9.1 (bottom three rows; p. 283)summarizes these structures, which we discuss next.Striations, a repeating series of dark and light bands, are evident along thelength of each myofibril. In an intact muscle fiber, the dark Abands and light I bands are nearly perfectly aligned, giving thecell its striated appearance.As illustrated in Figure 9.2c:Striations, Sarcomeres, and Myofilaments Each dark A band has a lighter region in its midsection calledthe H zone (H for helle; “bright”).Each H zone is bisected vertically by a dark line called theM line (M for middle) formed by molecules of the proteinmyomesin.Each light I band also has a midline interruption, a darkerarea called the Z disc (or Z line).The region of a myofibril between two successive Z discs isa sarcomere (sar"ko-mĕr; literally, “muscle segment”). Averaging 2 μm long, a sarcomere is the smallest contractile unitof a muscle fiber—the functional unit of skeletal muscle. Itcontains an A band flanked by half an I band at each end.Within each myofibril, the sarcomeres align end to end likeboxcars in a train.If we examine the banding pattern of a myofibril at the molecular level, we see that it arises from orderly arrangement ofeven smaller structures within the sarcomeres. These smallerstructures, the myofilaments or filaments, are the muscleequivalents of the actin- or myosin-containing microfilamentsdescribed in Chapter 3. As you will recall, the proteins actinand myosin play a role in motility and shape change in virtuallyevery cell in the body. This property reaches its highest development in the contractile muscle fibers.The central thick filaments containing myosin (red) extendthe entire length of the A band (Figure 9.2c and d). They are connected in the middle of the sarcomere at the M line. The morelateral thin filaments containing actin (blue) extend across theI band and partway into the A band. The Z disc, a coin-shapedsheet composed largely of the protein alpha-actinin, anchorsthe thin filaments. We describe the third type of myofilamentillustrated in Figure 9.2d, the elastic filament, in the next section.Intermediate (desmin) filaments (not illustrated) extend fromthe Z disc and connect each myofibril to the next throughoutthe width of the muscle cell.Looking at the banding pattern more closely, we see that theH zone of the A band appears less dense because the thin filaments do not extend into this region. The M line in the center ofthe H zone is slightly darker because of the fine protein strandsthere that hold adjacent thick filaments together. The myofilaments are connected to the sarcolemma and held in alignmentat the Z discs and the M lines.A longitudinal view of the myofilaments (Figure 9.2d) is abit misleading because it looks as if each thick (red) filamentinterdigitates with only four thin (blue) filaments. The cross section of a sarcomere on the far right in Figure 9.2e shows an281area where thick and thin filaments overlap. Notice that a hexagonal arrangement of six thin filaments surrounds each thickfilament, and three thick filaments enclose each thin filament.Muscle contractiondepends on the myosin- and actin-containing myofilaments.As noted earlier, thick filaments (about 16 nm in diameter) arecomposed primarily of the protein myosin. Each myosin molecule consists of two heavy and four light polypeptide chains,and has a rodlike tail attached by a flexible hinge to two globularheads (Figure 9.3). The tail consists of two intertwined helicalpolypeptide heavy chains.The globular heads, each associated with two light chains,are the “business end” of myosin. During contraction, they linkthe thick and thin filaments together, forming cross bridges(Figure 9.4 on p. 284), and swivel around their point of attachment. As we will explain shortly, these cross bridges act asmotors to generate force.Each thick filament contains about 300 myosin moleculesbundled together, with their tails forming the central part ofthe thick filament and their heads facing outward at the endof each thick filament (Figure 9.3). As a result, the centralportion of a thick filament (in the H zone) is smooth, but itsends are studded with a staggered array of myosin heads. Theheads bear actin and ATP-binding sites and also have intrinsicATPase activity that splits ATP to generate energy for musclecontraction.The thin filaments (7–8 nm thick) are composed chieflyof the protein actin (blue in Figure 9.3). Actin has kidneyshaped polypeptide subunits, called globular actin or G actin,which bear the active sites to which the myosin heads attachduring contraction. In the thin filaments, G actin subunits arepolymerized into long actin filaments called filamentous, or F,actin. Two intertwined actin filaments, resembling a twisteddouble strand of pearls, form the backbone of each thin filament (Figure 9.3).Thin filaments also contain several regulatory proteins.Molecular Composition of Myofilaments Polypeptide strands of tropomyosin (tro!po-mi"o-sin),a rod-shaped protein, spiral about the actin core and helpstiffen and stabilize it. Successive tropomyosin molecules arearranged end to end along the actin filaments, and in a relaxed muscle fiber, they block myosin-binding sites on actinso that myosin heads on the thick filaments cannot bind tothe thin filaments.Troponin (tro"po-nin), the other major protein in thin filaments, is a globular three-polypeptide complex (Figure 9.3).One of its polypeptides (TnI) is an inhibitory subunit thatbinds to actin. Another (TnT) binds to tropomyosin and helpsposition it on actin. The third (TnC) binds calcium ions.Both troponin and tropomyosin help control the myosin-actininteractions involved in contraction. Several other proteins helpform the structure of the myofibril. The elastic filament we referred to earlier is composed ofthe giant protein titin (Figure 9.2d). Titin extends from theZ disc to the thick filament, and then runs within the thickfilament (forming its core) to attach to the M line. It holds9
282UNIT 2 Covering, Support, and Movement of the BodyLongitudinal section of filaments within onesarcomere of a myofibrilThick filamentThin filamentIn the center of the sarcomere, the thick filamentslack myosin heads. Myosin heads are present onlyin areas of myosin-actin overlap.9Thick filamentThin filamentEach thick filament consists of many myosin moleculeswhose heads protrude at opposite ends of the filament.A thin filament consists of two strands of actin subunitstwisted into a helix plus two types of regulatory proteins(troponin and tropomyosin).Portion of a thick filamentPortion of a thin filamentMyosin headTropomyosinTroponinActinActin-binding sitesHeadsATPbindingsiteTailFlexible hinge regionActive sitesfor myosinattachmentActin subunitsMyosin moleculeActin subunitsFigure 9.3 Composition of thick and thin filaments. the thick filaments in place, thus maintaining the organization of the A band, and helps the muscle cell spring back intoshape after stretching. (The part of the titin that spans the Ibands is extensible, unfolding when the muscle stretches andrecoiling when the tension is released.) Titin does not resiststretching in the ordinary range of extension, but it stiffensas it uncoils, helping the muscle resist excessive stretching,which might pull the sarcomeres apart.Another important structural protein is dystrophin, whichlinks the thin filaments to the integral proteins of the sarcolemma (which in turn are anchored to the extracellularmatrix).Other proteins that bind filaments or sarcomeres togetherand maintain their alignment include nebulin, myomesin,and C proteins.Sarcoplasmic Reticulum and T TubulesSkeletal muscle fibers contain two sets of intracellular tubulesthat help regulate muscle contraction: (1) the sarcoplasmic reticulum and (2) T tubules.
Chapter 9 Muscles and Muscle TissueTable 9.1283Structure and Organizational Levels of Skeletal MuscleCONNECTIVETISSUE WRAPPINGSSTRUCTURE AND ORGANIZATIONAL LEVELDESCRIPTIONMuscle (organ)A muscle consists of hundreds to thousands ofmuscle cells, plus connective tissue wrappings,blood vessels, and nerve fibers.Covered externally bythe epimysiumA fascicle is a discrete bundle of muscle cells,segregated from the rest of the muscle by aconnective tissue sheath.Surrounded byperimysiumEpimysiumMuscleTendonFascicleFascicle (a portion of the muscle)PerimysiumPart of fascicle9Muscle fiberMuscle Fiber (cell)EndomysiumSarcolemmaNucleusPart of muscle fiberA muscle fiber is an elongated multinucleate cell;it has a banded (striated) appearance.Surrounded byendomysiumMyofibrils are rodlike contractile elements thatoccupy most of the muscle cell volume. Composedof sarcomeres arranged end to end, they appearbanded, and bands of adjacent myofibrils arealigned.—A sarcomere is the contractile unit, composed ofmyofilaments made up of contractile proteins.—Contractile myofilaments are of two types—thick and thin. Thick filaments contain bundledmyosin molecules; thin filaments contain actinmolecules (plus other proteins). The sliding of thethin filaments past the thick filaments producesmuscle shortening. Elastic filaments (not shownhere) maintain the organization of the A bandand provide elastic recoil when musclecontraction ends.—MyofibrilMyofibril or Fibril (complex organelle composed of bundles ofmyofilaments)SarcomereSarcomere (a segment of a myofibril)SarcomereThin (actin) filamentThick (myosin) filamentMyofilament or Filament (extended macromolecularstructure)Thick filamentThin filamentHead of myosin moleculeActin molecules
284UNIT 2 Covering, Support, and Movement of the BodyThin filament (actin)Myosin headsThick filament (myosin)Figure 9.4 Myosin heads forming cross bridges that generate muscular contractile force. Part of a sarcomere is seen in atransmission electron micrograph (277,000#).9Sarcoplasmic Reticulum Shown in blue in Figure 9.5, thesarcoplasmic reticulum (SR) is an elaborate smooth endoplasmic reticulum. Its interconnecting tubules surround eachmyofibril the way the sleeve of a loosely crocheted sweatersurrounds your arm.Most SR tubules run longitudinally along the myofibril,communicating with each other at the H zone. Others calledterminal cisterns (“end sacs”) form larger, perpendicular crosschannels at the A band–I band junctions and they always occurin pairs. Closely associated with the SR are large numbers ofI bandPart of a skeletalmuscle fiber (cell)Z discmitochondria and glycogen granules, both involved in producing the energy used during contraction.The SR regulates intracellular levels of ionic calcium. It storescalcium and releases it on demand when the muscle fiber isstimulated to contract. As you will see, calcium provides thefinal “go” signal for contraction.T Tubules At each A band–I band junction, the sarcolemmaof the muscle cell protrudes deep into the cell interior, forming an elongated tube called the T tubule (T for “transverse”).The T tubules, shown in gray in Figure 9.5, tremendouslyincrease the muscle fiber’s surface area. Possibly the result offusing tubelike caveolae (inpocketings of the sarcolemma),the lumen (cavity) of the T tubule is continuous with theextracellular space.Along its length, each T tubule runs between the paired terminal cisterns of the SR, forming triads, successive groupingsof the three membranous structures (terminal cistern, T tubule,and terminal cistern). As they pass from one myofibril to thenext, the T tubules also encircle each sarcomere.Muscle contraction is ultimately controlled by nerveinitiated electrical impulses that travel along the sarcolemma.Because T tubules are continuations of the sarcolemma, theyconduct impulses to the deepest regions of the muscle cell andevery sarcomere. These impulses signal for the release of calcium from the adjacent terminal cisterns. Think of the T tubulesas a rapid telegraph system that ensures that every myofibril inthe muscle fiber contracts at virtually the same time.A bandI bandH zoneZ discMlineSarcolemmaMyofibrilTriad: T tubule Terminalcisternsof the SR (2)SarcolemmaTubules ofthe SRMyofibrilsMitochondriaFigure 9.5 Relationship of thesarcoplasmic reticulum and T tubulesto myofibrils of skeletal muscle. Thetubules of the SR (blue) fuse to form a net ofcommunicating channels at the levelof the H zone and the saclike terminalcisterns abutting the A-I junctions. TheT tubules (gray) are inward invaginations ofthe sarcolemma that run deep into the cellbetween the terminal cisterns. (See detailedview in Figure 9.11, pp. 290–291.) Sitesof close contact of these three elements(terminal cistern, T tubule, and terminalcistern) are called triads.
285Chapter 9 Muscles and Muscle TissueThe roles of the T tubules and SR in providing signals for contraction are tightly linked. At the triads,where these organelles come into closest contact, integral proteins (some from the T tubule and others from the SR) protrude into the intermembrane spaces. The protruding integralproteins of the T tubule act as voltage sensors. Those of the SRform gated channels through which the terminal cisterns release Ca2 . We will return to their interaction shortly.Triad Relationships1 Fully relaxed sarcomere of a muscle fiberSliding Filament Model of ContractionWe almost always think “shortening” when we hear theword contraction, but to physiologists the term refers onlyto the activation of myosin’s cross bridges, which are theforce-generating sites. Shortening occurs if and when thecross bridges generate enough tension on the thin filamentsto exceed the forces that oppose shortening. Contractionends when the cross bridges become inactive, the tensiondeclines, and then the muscle fiber relaxes.In a relaxed muscle fiber, the thin and thick filaments overlaponly at the ends of the A band (Figure 9.6 1 ). The slidingfilament model of contraction states that during contractionthe thin filaments slide past the thick ones so that the
of skeletal muscle organization, gross to microscopic, that we describe in the following sections. Gross Anatomy of a Skeletal Muscle Each skeletal muscle is a discrete organ, made up of several kinds of tissues. Skeletal muscle "bers predominate, but blood vessels, nerve "bers, and su
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