Quizlet the Transverse Tubules Also Known as the Ttubules Are Continuous With the

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Skeletal (or striated) muscle consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm. During embryonic muscle development, mesenchymal myoblasts (L. myo, muscle) fuse, forming myotubes with many nuclei. Myotubes then further differentiate to form striated muscle fibers (Figure 10–2). Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to skeletal muscle fibers/cells. A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of differentiated skeletal muscle.

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FIGURE 10–2

Development of skeletal muscle.

Skeletal muscle begins to differentiate when mesenchymal cells, called myoblasts, align and fuse together to make longer, multinucleated tubes called myotubes. Myotubes synthesize the proteins to make up myofilaments and gradually begin to show cross-striations by light microscopy. Myotubes continue differentiating to form functional myofilaments, and the nuclei are displaced against the sarcolemma.

Part of the myoblast population does not fuse and differentiate but remains as a group of mesenchymal cells called muscle satellite cells located on the external surface of muscle fibers inside the developing external lamina. Satellite cells proliferate and produce new muscle fibers following muscle injury. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008).

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Organization of a Skeletal Muscle

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Thin layers of connective tissue surround and organize the contractile fibers in all three types of muscle, and these layers are seen particularly well in skeletal muscle (Figures 10–3 and 10–4). The concentric organization given by these supportive layers resembles that in large peripheral nerves:

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• The epimysium, an external sheath of dense irregular connective tissue, surrounds the entire muscle. Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle.

• The perimysium is a thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle (Figure 10–3). Each fascicle of muscle fibers makes up a functional unit in which the fibers work together. Nerves, blood vessels, and lymphatics penetrate the perimysium to supply each fascicle.

• Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts, the endomysium, surrounds the external lamina of individual muscle fibers. In addition to nerve fibers, capillaries form a rich network in the endomysium bringing O₂ to the muscle fibers (Figure 10–5).

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FIGURE 10–3

Organization of skeletal muscle.

An entire skeletal muscle is enclosed within a thick layer of dense connective tissue called the epimysium that is continuous with fascia and the tendon binding muscle to bone. Large muscles contain several fascicles of muscle tissue, each wrapped in a thin but dense connective tissue layer called the perimysium. Within fascicles individual muscle fibers (elongated multinuclear cells) are surrounded by a delicate connective tissue layer, the endomysium. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).

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FIGURE 10–4

Skeletal muscle.

• (a) A cross section of striated muscle demonstrating all three layers of connective tissue and cell nuclei. The endomysium (En) surrounds individual muscle, and perimysium (P) encloses a group of muscle fibers comprising a fascicle. A thick epimysium (E) surrounds the entire muscle. All three of these tissues contain collagen types I and III (reticulin). (X200; H&E)

• (b) An adjacent section immunohistochemically stained for laminin, which specifically stains the external laminae of the muscle fibers, surrounded by endomysium. (X400; Immunoperoxidase)

• (c) Longitudinal section of a myotendinous junction. Tendons develop together with skeletal muscles and join muscles to the periosteum of bones. The dense collagen fibers of a tendon (T) are continuous with those in the three connective tissue layers around muscle fibers (M), forming a strong unit that allows muscle contraction to move other structures. (X400; H&E)

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FIGURE 10–5

Capillaries of skeletal muscle.

The blood vessels were injected with a dark plastic polymer before the muscle was collected and sectioned longitudinally. A rich network of capillaries in endomysium surrounding muscle fibers is revealed by this method. (X200; Giemsa with polarized light)

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Collagens in these connective tissue layers of muscle serve to transmit the mechanical forces generated by the contracting muscle cells/fibers; individual muscle fibers seldom extend from one end of a muscle to the other.

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All three layers plus the dense irregular connective tissue of the deep fascia, which overlies the epimysium, are continuous with the tough connective tissue of a tendon at myotendinous junctions which join the muscle to bone, skin, or another muscle (Figures 10–3 and 10–4c). Ultrastructural studies show that in these transitional regions, collagen fibers from the tendon insert themselves among muscle fibers and associate directly with complex infoldings of sarcolemma.

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Organization Within Muscle Fibers

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Longitudinally sectioned skeletal muscle fibers show striations of alternating light and dark bands (Figure 10–6a). The sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called myofibrils that run parallel to the long axis of the fiber (Figure 10–6b). The dark bands on the myofibrils are called A bands (anisotropic or birefringent in polarized light microscopy); the light bands are called I bands (isotropic, do not alter polarized light). In the TEM (Figure 10–6c), each I band is seen to be bisected by a dark transverse line, the Z disc (Ger. zwischen, between). The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc (Figure 10–6c) and is about 2.5-μm long in resting muscle.

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FIGURE 10–6

Striated skeletal muscle in longitudinal section.

Longitudinal sections reveal the striations characteristic of skeletal muscle.

• (a) Parts of three muscle fibers are separated by very thin endomysium that includes one fibroblast nucleus (F). Muscle nuclei (N) are found against the sarcolemma. Along each fiber thousands of dark-staining A bands alternate with lighter I bands. (X200; H&E) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).

• (b) At higher magnification, each fiber can be seen to have three or four myofibrils, here with their striations slightly out of alignment with one another. Myofibrils are cylindrical bundles of thick and thin myofilaments which fill most of each muscle fiber. (X500; Giemsa)

• (c) TEM showing one contractile unit (sarcomere) in the long series that comprises a myofibril. In its middle is an electron-dense A band bisected by a narrow, less dense region called the H zone. On each side of the A band are the lighter-stained I bands, each bisected by a dense Z disc which marks one end of the sarcomere. Mitochondria (M), glycogen granules, and small cisternae of SER occur around the Z disc. (X24,000) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).

(Figure 10–6c, used with permission from Mikel H. Snow, Department of Cell and Neurobiology, Keck School of Medicine at the University of Southern California, Los Angeles.)

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Mitochondria and sarcoplasmic reticulum are found between the myofibrils, which typically have diameters of 1-2 μm. Myofibrils consist of an end-to-end repetitive arrangement of sarcomeres (Figure 10–7); the lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations.

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FIGURE 10–7

Molecules composing thin and thick filaments.

Myofilaments, which include both thick and thin filaments, consist of contractile protein arrays bundled within myofibrils. (a) A thick myofilament contains 200-500 molecules of myosin. (b) A thin filament contains F-actin, tropomyosin, and troponin.

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The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively, organized within each myofibril in a symmetric pattern containing thousands of each filament type (Figure 10–7).

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The thick myosin filaments are 1.6-μm long and 15-nm wide; they occupy the A band at the middle region of the sarcomere. Myosin is a large complex (~500 kDa) with two identical heavy chains and two pairs of light chains. Myosin heavy chains are thin, rodlike motor proteins (150-nm long and 2-3 nm thick) twisted together as myosin tails (Figure 10–7). Globular projections containing the four myosin light chains form a head at one end of each heavy chain. The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity). Several hundred myosin molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end (Figure 10–7a).

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The thin, helical actin filaments are each 1.0-μm long and 8-nm wide and run between the thick filaments. Each G-actin monomer contains a binding site for myosin (Figure 10–7b). The thin filaments have two tightly associated regulatory proteins (Figure 10–7b):

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• Tropomyosin, a 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands

• Troponin, a complex of three subunits: TnT, which attaches to tropomyosin; TnC, which binds Ca²⁺; and TnI, which regulates the actin-myosin interaction

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Troponin complexes attach at specific sites regularly spaced along each tropomyosin molecule.

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The organization of important myofibril components is shown in Figure 10–8. I bands consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands, which is why I bands stain more lightly than A bands. Actin filaments are anchored perpendicularly on the Z disc by the actin-binding protein α-actinin and exhibit opposite polarity on each side of this disc (Figure 10–8c). An important accessory protein in I bands is titin (3700 kDa), the largest protein in the body, with scaffolding and elastic properties, which supports the thick myofilaments and connects them to the Z disc (Figure 10–8c). Another large accessory protein, nebulin, binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis.

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FIGURE 10–8

Structure of a myofibril: A series of sarcomeres.

• (a) The diagram shows that each muscle fiber contains several parallel bundles called myofibrils.

• (b) Each myofibril consists of a long series of sarcomeres, separated by Z discs and containing thick and thin filaments which overlap in certain regions.

• (c) Thin filaments are actin filaments with one end bound to α-actinin in the Z disc. Thick filaments are bundles of myosin, which span the entire A band and are bound to proteins of the M line and to the Z disc across the I bands by a very large protein called titin, which has springlike domains.

• (d) The molecular organization of the sarcomeres produces staining differences which cause the dark- and light-staining bands seen by light microscopy and TEM. (X28,000)

• (e) With the TEM an oblique section of myofibrils includes both A and I bands and shows hexagonal patterns that indicate the relationships between thin and thick myofilaments and other proteins, as shown in part b of this figure. Thin and thick filaments are arranged so that each myosin bundle contacts six actin filaments. Large mitochondria in cross section and SER cisternae are seen between the myofibrils. (X45,000)

(Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008).

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The A bands contain both the thick filaments and the overlapping portions of thin filaments. Close observation of the A band shows the presence of a lighter zone in its center, the H zone, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments (Figure 10–8c). Bisecting the H zone is the M line (Ger. Mitte, middle; Figure 10–8d), containing a myosin-binding protein myomesin that holds the thick filaments in place, and creatine kinase. This enzyme catalyzes transfer of phosphate groups from phosphocreatine, a storage form of high-energy phosphate groups, to ADP, helping to supply ATP for muscle contraction.

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Despite the many proteins present in sarcomeres, myosin and actin together represent over half of the total protein in striated muscle. The overlapping arrangement of thin and thick filaments within sarcomeres produces in TEM cross sections hexagonal patterns of structures which were important in determining the functions of the filaments and other proteins in the myofibril (Figures 10–8b and 10–8e).

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Sarcoplasmic Reticulum & Transverse Tubule System

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In skeletal muscle fibers the membranous smooth ER, called here sarcoplasmic reticulum, contains pumps and other proteins for Ca²⁺ sequestration and surrounds the myofibrils (Figure 10–9). Calcium release from cisternae of the sarcoplasmic reticulum through voltage-gated Ca²⁺ channels is triggered by membrane depolarization produced by a motor nerve.

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FIGURE 10–9

Organization of a skeletal muscle fiber.

Skeletal muscle fibers are composed mainly of myofibrils. Each myofibril extends the length of the fiber and is surrounded by parts of the sarcoplasmic reticulum. The sarcolemma has deep invaginations called T-tubules, each of which becomes associated with two terminal cisternae of the sarcoplasmic reticulum. A T-tubule and its two associated terminal cisterna comprise a "triad" of small spaces along the surface of the myofibrils. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).

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To trigger Ca²⁺ release from sarcoplasmic reticulum throughout the muscle fiber simultaneously and produce uniform contraction of all myofibrils, the sarcolemma has tubular infoldings called transverse or T-tubules (Figures 10–9 and 10–10). These long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle each myofibril near the aligned A- and I-band boundaries of sarcomeres.

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Adjacent to each T-tubule are expanded terminal cisternae of sarcoplasmic reticulum. In longitudinal TEM sections, this complex of a T-tubule with two terminal cisternae is called a triad (Figures 10–9 and 10–10). The triad complex allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca²⁺ ions into cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres.

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FIGURE 10–10

Transverse tubule system and triads.

Transverse tubules are invaginations of the sarcolemma that penetrate deeply into the muscle fiber around all myofibrils.

• (a) TEM cross section of fish muscle shows portions of two fibers and the endomysium (E) between them. Several transverse or T-tubules (T) are shown, perpendicular to the fiber surface, penetrating between myofibrils (M). (X50,000)

• (b) Higher-magnification TEM of skeletal muscle in longitudinal section shows four membranous triads (Tr) cut transversely near the A-band–I-band junctions. Each triad consists of a central transverse tubule (T) and two adjacent terminal cisterns (TC) extending from the sarcoplasmic reticulum. Centrally located is the Z disc. Besides elements of the triad, sarcoplasm surrounding the myofibril also contains dense glycogen granules (G).

Components of the triad are responsible for the cyclic release of Ca²⁺ from the cisternae and its sequestration again which occurs during muscle contraction and relaxation. The association between SR cisternae and T-tubules is shown diagrammatically in Figure 10–11. (X90,000)

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Mechanism of Contraction

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Figure 10–11 summarizes the key molecular events of muscle contraction. During this process neither the thick nor the thin filaments change their length. Contraction occurs as the overlapping thin and thick filaments of each sarcomere slide past one another.

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FIGURE 10–11

Events of muscle contraction.

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Contraction is induced when an action potential arrives at a synapse, the neuromuscular junction (NMJ), and is transmitted along the T-tubules to terminal cisternae of the sarcoplasmic reticulum to trigger Ca²⁺ release. In a resting muscle, the myosin heads cannot bind actin because the binding sites are blocked by the troponin-tropomyosin complex on the F-actin filaments. Calcium ions released upon neural stimulation bind troponin, changing its shape and moving tropomyosin on the F-actin to expose the myosin-binding active sites and allow crossbridges to form. Binding actin produces a conformational change or pivot in the myosins, which pulls the thin filaments farther into the A band, toward the Z disc (Figure 10–11).

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Energy for the myosin head pivot that pulls actin is provided by hydrolysis of ATP bound to the myosin heads, after which myosin binds another ATP and detaches from actin. In the continued presence of Ca²⁺ and ATP, these attach-pivot-detach events occur in a repeating cycle, each lasting about 50 milliseconds, which rapidly shorten the sarcomere and contract the muscle (Figures 10–11 and 10–12). A single muscle contraction results from hundreds of these cycles.

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FIGURE 10–12

Sliding filaments and sarcomere shortening in contraction.

Diagrams and TEM micrographs show sarcomere shortening during skeletal muscle contraction. (a) In the relaxed state the sarcomere, I band, and H zone are at their expanded length. The springlike action of titin molecules, which span the I band, helps pull thin and thick filaments past one another in relaxed muscle. (b) During muscle contraction, the Z discs at the sarcomere boundaries are drawn closer together as they move toward the ends of thick filaments in the A band. Titin molecules are compressed during contraction. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).

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When the neural impulse stops and levels of free Ca²⁺ ions diminish, tropomyosin again covers the myosin-binding sites on actin and the filaments passively slide back and sarcomeres return to their relaxed length (Figure 10–11). In the absence of ATP, the actin-myosin crossbridges become stable, which accounts for the rigidity of skeletal muscles (rigor mortis) that occurs as mitochondrial activity stops after death.

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Myelinated motor nerves branch out within the perimysium, where each nerve gives rise to several unmyelinated terminal twigs that pass through endomysium and form synapses with individual muscle fibers. Schwann cells enclose the small axon branches and cover their points of contact with the muscle cells (Figure 10–13); the external lamina of the Schwann cell fuses with that of the sarcolemma. Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface, which are part of the synapses termed the neuromuscular junctions, or motor end plates (MEPs) (Figure 10–13). As in all synapses the axon terminal contains mitochondria and numerous synaptic vesicles; here the vesicles contain the neurotransmitter acetylcholine. Between the axon and the muscle is the synaptic cleft. Adjacent to the synaptic cleft, the sarcolemma is thrown into numerous deep junctional folds, which provide for greater postsynaptic surface area and more transmembrane acetylcholine receptors.

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FIGURE 10–13

The neuromuscular junction (NMJ).

Before it terminates in a skeletal muscle, each motor axon bundled in the nerve forms many branches, each of which forms a synapse with a muscle fiber.

• (a) Silver staining can reveal the nerve bundle (NB), the terminal axonal twigs, and the motor end plates (MEPs, also called neuromuscular junctions or NMJ) on striated muscle fibers (S). (X1200)

• (b) An SEM shows the branching ends of a motor axon, each covered by an extension of the last Schwann cell and expanded terminally as an MEP embedded in a groove in the external lamina of the muscle fiber.

• (c) Diagram of enclosed portion of the SEM indicating key features of a typical MEP: synaptic vesicles of acetylcholine (ACh), a synaptic cleft, and a postsynaptic membrane. This membrane, the sarcolemma, is highly folded to increase the number of ACh receptors at the MEP. Receptor binding initiates muscle fiber depolarization, which is carried to the deeper myofibrils by the T-tubules.

(Reproduced, with permission, from Widmaier EP, Raff H, Strang KT. Vander's Human Physiology. 11th ed. New York, NY: McGraw-Hill; 2008).

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When a nerve action potential reaches the MEP, acetylcholine is liberated from the axon terminal, diffuses across the cleft, and binds to its receptors in the folded sarcolemma. The acetylcholine receptor contains a nonselective cation channel that opens upon neurotransmitter binding, allowing influx of cations, depolarizing the sarcolemma, and producing the muscle action potential. Acetylcholine quickly dissociates from its receptors, and free neurotransmitter is removed from the synaptic cleft by the extracellular enzyme acetylcholinesterase, preventing prolonged contact of the transmitter with its receptors.

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As discussed with Figure 10–11, the muscle action potential moves along the sarcolemma and along T-tubules which penetrate deeply into sarcoplasm. At triads the depolarization signal triggers the release of Ca²⁺ from terminal cisterns of the sarcoplasmic reticulum, initiating the contraction cycle.

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An axon from a single motor neuron can form MEPs with one or many muscle fibers. Innervation of single muscle fibers by single motor neurons provides precise control of muscle activity and occurs, for example, in the extraocular muscles for eye movements. Larger muscles with coarser movements have motor axons that typically branch profusely and innervate 100 or more muscle fibers. In this case the single axon and all the muscle fibers in contact with its branches make up a motor unit. Individual striated muscle fibers do not show graded contraction—they contract either all the way or not at all. To vary the force of contraction, the fibers within a muscle fascicle do not all contract at the same time. With large muscles composed of many motor units, the firing of a single motor axon will generate tension proportional to the number of muscle fibers it innervates. Thus, the number of motor units and their variable size control the intensity and precision of a muscle contraction.

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Key features of skeletal muscle cells, connective tissue, contraction, and innervation are summarized in Table 10–1.

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Table Graphic Jump Location

Table 10–1 Important comparisons of the three types of muscle.

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Table 10–1 Important comparisons of the three types of muscle.

	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Fibers	Single multinucleated cells	Aligned cells in branching arrangement	Single small, closely packed fusiform cells
Cell/fiber shape and size	Cylindrical, 10-100 μm diameter, many cm long	Cylindrical, 10-20 μm diameter, 50-100 μm long	Fusiform, diameter 0.2-10 μm, length 50-200 μm
Striations	Present	Present	Absent
Location of nuclei	Peripheral, adjacent to sarcolemma	Central	Central, at widest part of cell
T tubules	Center of triads at A-I junctions	In dyads at Z discs	Absent; caveolae may be functionally similar
Sarcoplasmic reticulum (SR)	Well-developed, with two terminal cisterns per sarcomere in triads with T tubule	Less well-developed, one small terminal cistern per sarcomere in dyad with T tubule	Irregular smooth ER without distinctive organization
Special structural features	Very well-organized sarcomeres, SR, and transverse tubule system	Intercalated discs joining cell, with many adherent and gap junctions	Gap junctions, caveolae, dense bodies
Control of contraction	Troponin C binds Ca2+, moving tropomyosin and exposing actin for myosin binding	Similar to that of skeletal muscle	Actin-myosin binding occurs with myosin phosphorylation by MLCK triggered when calmodulin binds Ca2+
Connective tissue organization	Endomysium, perimysium, and epimysium	Endomysium; subendocardial and subpericardial CT layers	Endomysium and less-organized CT sheaths
Major locations	Skeletal muscles, tongue, diaphragm, eyes, and upper esophagus	Heart	Blood vessels, digestive and respiratory tracts, uterus, bladder, and other organs
Key function	Voluntary movements	Automatic (involuntary) pumping of blood	Involuntary movements
Efferent innervation	Motor	Autonomic	Autonomic
Contractions	All-or-none, triggered at motor end plates	All-or-none, intrinsic (beginning at nodes of conducting fibers)	Partial, slow, often spontaneous, wavelike and rhythmic
Cell response to increased load	Hypertrophy (increase in fiber size)	Hypertrophy	Hypertrophy and hyperplasia (increase in cell/fiber number)
Capacity for regeneration	Limited, involving satellite cells mainly	Very poor	Good, involving mitotic activity of muscle cells

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MEDICAL APPLICATION

Myasthenia gravis is an autoimmune disorder that involves circulating antibodies against proteins of acetylcholine receptors. Antibody binding to the antigenic sites interferes with acetylcholine activation of their receptors, leading to intermittent periods of skeletal muscle weakness. As the body attempts to correct the condition, junctional folds of sarcolemma with affected receptors are internalized, digested by lysosomes, and replaced by newly formed receptors. These receptors, however, are again made unresponsive to acetylcholine by similar antibodies, and the disease follows a progressive course. The extraocular muscles of the eyes are commonly the first affected.

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Muscle Spindles & Tendon Organs

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Striated muscles and myotendinous junctions contain sensory receptors acting as proprioceptors (L. proprius, one's own + capio, to take), providing the central nervous system (CNS) with data from the musculoskeletal system. Among the muscle fascicles are stretch detectors known as muscle spindles, approximately 2-mm long and 0.1-mm wide (Figure 10–14a). A muscle spindle is encapsulated by modified perimysium, with concentric layers of flattened cells, containing interstitial fluid and a few thin muscle fibers filled with nuclei and called intrafusal fibers (Figure 10–14). Several sensory nerve axons penetrate each muscle spindle and wrap around individual intrafusal fibers. Changes in length (distension) of the surrounding (extrafusal) muscle fibers caused by body movements are detected by the muscle spindles and the sensory nerves relay this information to the spinal cord. Different types of sensory and intrafusal fibers mediate reflexes of varying complexity to help maintain posture and to regulate the activity of opposing muscle groups involved in motor activities such as walking.

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FIGURE 10–14

Sensory receptors associated with skeletal muscle.

• (a) The diagram shows both a muscle spindle and a tendon organ. Muscle spindles have afferent sensory and efferent motor nerve fibers associated with the intrafusal fibers, which are modified muscle fibers. The size of the spindle is exaggerated relative to the extrafusal fibers to show better the nuclei packed in the intrafusal fibers. Both types of sensory receptors provide the CNS with information concerning degrees of stretch and tension within the musculoskeletal system. (Reproduced, with permission, from Widmaier EP, Raff H, Strang KT. Vander's Human Physiology. 11th ed. New York, NY: McGraw-Hill; 2008; Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).

• (b) A TEM cross section near the end of a muscle spindle shows the capsule (C), lightly myelinated axons (MA) of a sensory nerve, and the intrafusal muscle fibers (MF). These thin fibers differ from the ordinary skeletal muscle fibers in having very few myofibrils. Their many nuclei can either be closely aligned (nuclear chain fibers) or piled in a central dilation (nuclear bag fibers). Muscle satellite cells (SC) are also present within the external lamina of the intrafusal fibers. (X3600) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).

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A similar role is played by Golgi tendon organs, much smaller encapsulated structures that enclose sensory axons penetrating among the collagen bundles at the myotendinous junction (Figure 10–14a). Tendon organs detect changes in tension within tendons produced by muscle contraction and act to inhibit motor nerve activity if tension becomes excessive. Because both of these proprioceptors detect increases in tension, they help regulate the amount of effort required to perform movements that call for variable amounts of muscular force.

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MEDICAL APPLICATION

Dystrophin is a large actin-binding protein located just inside the sarcolemma of skeletal muscle fibers, which is involved in the functional organization of myofibrils. Research on Duchenne muscular dystrophy revealed that mutations of the dystrophin gene can lead to defective linkages between the cytoskeleton and the extracellular matrix (ECM). Muscle contractions can disrupt these weak linkages, causing the atrophy of muscle fibers typical of this disease.

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Skeletal Muscle Fiber Types

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Skeletal muscles such as those that move the eyes and eyelids need to contract rapidly, while others such as those for bodily posture must maintain tension for longer periods while resisting fatigue. These metabolic differences are possible because of varied expression in muscle fibers of contractile or regulatory protein isoforms and other factors affecting oxygen delivery and use. Different types of fibers can be identified on the basis of (1) their maximal rate of contraction (fast or slow fibers) and (2) their major pathway for ATP synthesis (oxidative phosphorylation or glycolysis). Fast versus slow rates of fiber contraction are due largely to myosin isoforms with different maximal rates of ATP hydrolysis.

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Histochemical staining is used to identify fibers with differing amounts of "fast" and "slow" ATPases (Figure 10–15). Other histological features reflecting metabolic differences among muscle fibers include the density of surrounding capillaries, the number of mitochondria, and levels of glycogen and myoglobin, a globular sarcoplasmic protein similar to hemoglobin which contains iron atoms and allows for O₂ storage.

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FIGURE 10–15

Skeletal muscle fiber types.

Cross section of a skeletal muscle stained histochemically for myosin ATPase at acidic pH, which reveals activity of the "slow" ATPase and shows the distribution of the three main fiber types. Slow oxidative (SO) or type I fibers have high levels of acidic ATPase activity and stain the darkest. Fast glycolytic (FG) or type IIb fibers stain the lightest. Fast oxidative-glycolytic (FOG) or type IIa fibers are intermediate between the other two types (X40). ATPase histochemistry of unfixed, cryostat section, pH 4.2. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008).

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Each of these features exists as a continuum in skeletal muscle fibers, but fiber diversity is divided into three major types:

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• Slow oxidative muscle fibers are adapted for slow contractions over long periods without fatigue, having many mitochondria, many surrounding capillaries, and much myoglobin, all features that make fresh tissue rich in these fibers dark or red in color.

• Fast glycolytic fibers are specialized for rapid, short-term contraction, having few mitochondria or capillaries and depending largely on anaerobic metabolism of glucose derived from stored glycogen, features which make such fibers appear white. Rapid contractions lead to rapid fatigue as lactic acid produced by glycolysis accumulates.

• Fast oxidative-glycolytic fibers have physiological and histological features intermediate between those of the other two types.

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Table 10–2 summarizes these and other characteristics of the three skeletal muscle fiber types. The metabolic type of each fiber is determined by the rate of impulse conduction along its motor nerve supply, so that all fibers of a motor unit are similar. Most skeletal muscles receive motor input from multiple nerves and contain a mixture of fiber types (Figure 10–15). Determining the fiber types in needle biopsies of skeletal muscle helps in the diagnosis of specific myopathies (myo + Gr. pathos, suffering), motor neuron diseases, and other causes of muscle atrophy. Different fiber types also exist in cardiac muscle at various locations within the heart and in smooth muscle of different organs.

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Table Graphic Jump Location

Table 10–2 Major characteristics of skeletal muscle fiber types.

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Table 10–2 Major characteristics of skeletal muscle fiber types.

	Slow, Oxidative Fibers (Type I)	Fast, Oxidative-Glycolytic Fibers (Type IIa)	Fast, Glycolytic Fibers (Type IIb)
Mitochondria	Numerous	Numerous	Sparse
Capillaries	Numerous	Numerous	Sparse
Fiber diameter	Small	Intermediate	Large
Size of motor unit	Small	Intermediate	Large
Myoglobin content	High (red fibers)	High (red fibers)	Low (white fibers)
Glycogen content	Low	Intermediate	High
Major source of ATP	Oxidative phosphorylation	Oxidative phosphorylation	Anaerobic glycolysis
Glycolytic enzyme activity	Low	Intermediate	High
Rate of fatigue	Slow	Intermediate	Fast
Myosin-ATPase activity	Low	High	High
Speed of contraction	Slow	Fast	Fast
Typical major locations	Postural muscles of back	Major muscles of legs	Extraocular muscles

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Source: https://accessmedicine.mhmedical.com/content.aspx?sectionid=190280039

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