Skeletal muscle cells are formed by hundreds of myoblasts that line up end to end and coalesce into a myotube. each myotube manufactures its own con- tractile elements, myofilaments, which are distinc- tively arranged to form myofibrils, and cytoskeletal components and organelles. skeletal muscle cells:
• May be several centimeters long and 10 to 100 µm in diameter, and
• Are arranged so that they not only are parallel to each other, but also the dark and light bands of adjacent cells are aligned with each other.
the extracellular spaces between neighboring cells are occupied by continuous capillaries.
skeletal muscle strength is a function of the number and diameter of the muscle fibers compos- ing a particular muscle.
• White fibers (e.g., chicken breast) are designed for fast contractility but are easily fatigued.
• Red fibers (e.g., dark meat) contract slower but are not fatigued easily.
• Fibers that are in between red and white are intermediate fibers.
White fibers have a poorer vascular supply, fewer mitochondria, fewer oxidative enzymes, and less of
the oxygen-transporting protein myoglobin than red fibers, but their diameters are larger, and their sarco-
plasmic reticulum is more extensive.
the nerve supply determines whether a muscle fiber is red or white, and switching the fiber of one muscle cell type to that of the other switches the characteristic of the muscle cell to the modality of its new innervation.
the connective tissue elements of skeletal muscle not only harness the contraction-derived energy of the mus cle but also conduct neurovascular ele- ments to each muscle cell and subdi- vide the muscle mass into smaller units, known as fascicles. each fasci- cle, enveloped by its perimysium (see Fig. 8.1), is composed of numerous skeletal muscle fibers, each with its own, slender connective tissue investment—the endomysium, whose reticular fibers interweave with those of adjacent cells. the connec- tive tissue surrounding the entire muscle, the epimy- sium (see Fig. 8.1), is continuous with the tendons and aponeuroses of the whole muscle and is inti- mately related to the reticular fibers of the endomy- sium that interdigitate with the fluted ends of the muscle cell; this relationship is the myotendinous junction.
LIGHT MIcRoScoPy of SKELETAL MuScLE
Along the length of the skeletal muscle fiber, small regenerative cells, known as satellite cells and pos- sessing a single nucleus, are present, sharing the external lamina of the muscle fiber. occasional fibro- blasts are also noted in the endomysium. the cytoplasm of skeletal muscle cells is packed with cylindrical myofibrils.
• Myofibrils are precisely arranged so that their dark and light bands are aligned with those of their neighbors; these bands are aligned along the length of the muscle fiber.
• I bands are transected by a thin Z disk (line).
• Dark bands, A bands, are bisected by a light area, the H band, which is transected by a thin M line.
• the contractile unit of skeletal muscle, the sarcomere, extends from Z disk to Z disk.
• During muscle contraction, the sarcomere shortens; the Z disks are closer to each other, the h band disappears, and the i bands become narrower, but the A band does not change.
KEy WoRDS
• Skeletal muscle
• Myofibrils
• Sarcomere
• Myofilaments
• Muscle contraction
• Neuromuscular junction
• cardiac muscle
• Smooth muscle
-
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Figure 8.1 Diagram of the three types of muscle: skeletal (top), smooth (middle), and cardiac (bottom). (From Gartner LP, Hiatt JL:
Color Textbook of Histology, 3rd ed.
Philadelphia, Saunders, 2007, p 159.) Epimysium
Total muscle
Perimysium
SKELETAL MUSCLE
SMOOTH MUSCLE
CARDIAC MUSCLE
Endomysium
Fascicle Sarcolemma Sarcoplasm
Nucleus
Nucleus in central sarcoplasm
Endomysium
Intercalated disk Endomysium Myofibril Nucleus Sarcoplasm Endomysium
Fiber
cLINIcAL coNSIDERATIoN
temporary myositis is a mild to severe
inflammation of skeletal muscles that results from accidental injury, infection, strenuous exercise, viral infection, or certain prescription drugs.
Symptoms include muscle pain, muscle
weakness, tenderness of the area over the region of the muscle, warmth, and reduced or impaired function. As its name suggests, the condition is not serious; it is temporary, and the problem resolves itself when the offending condition is removed.
Myositis can be a very serious condition that includes numerous inflammatory myopathies—
dermatomyositis, inclusion body myositis, the juvenile form of myositis, and polymyositis. All of these diseases are idiopathic, although they may be autoimmune diseases. The general symptoms for all of these myopathies are painful, weak muscles; general malaise; reduced mobility (especially in climbing stairs and standing up after falling down); and frequently difficulties in
deglutition (dysphagia).
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96 ELEcTRoN MIcRoScoPy of SKELETAL MuScLE the sarcolemma is similar in most respects to other cell membranes except that in skeletal muscle it forms numerous deep, tubular invaginations.
• T tubules (Fig. 8.2) extend into the cytoplasm and interweave, always at the junction of the i and A bands, throughout the interior of the muscle fiber. two t tubules for each sarcomere spread waves of depolarization into the interior of the muscle fiber.
• two terminal cisternae, expanded regions of the sarcoplasmic reticulum that store calcium, flank each t tubule at the i–A junctions (known as a triad) around every myofibril.
• Voltage-gated calcium release channels (ryanodine receptors) of the terminal cisternae are in close association with the voltage-sensitive dihydropyridine-sensitive receptors (DHSR) of the t tubules (this complex is known as junctional feet). As the wave of depolarization is conducted into the interior of the muscle cell, the DhsR causes calcium release channels to open, and calcium leaves the terminal cisternae to enter the sarcoplasm (Fig. 8.3; see Fig. 8.2).
the A and i bands of adjacent myofilaments are closely aligned with each other.
• this relationship is maintained by desmin, which wraps around the Z disks of adjacent myofibrils, fastening them to each other and to Z disks via the assistance of plectin.
• the heat shock protein aB-crystallin protects desmin from stresses placed on it.
• Actin-binding protein dystrophin fixes desmin to the costamere regions of the sarcolemma.
• long, tubular mitochondria occupy spaces among myofilament bundles and the periphery of the sarcoplasm deep to the cell membrane.
the sarcoplasm is rich in myoglobin.
STRucTuRAL oRGANIzATIoN of MyofIbRILS
the dark and light bands seen in light microscopy are due to the presence of parallel, interdigitating:
• Thin myofilaments (1 µm in length, 7 nm in diameter, and composed mainly of actin) and
• Thick myofilaments (1.5 µm long, 15 nm in diameter, and composed principally of myosin II).
thin filaments extend from each side of the Z disk in opposite directions toward the middle of successive sarcomeres. the two Z disks of a single sarcomere have thin filaments pointing toward the center of that sarcomere and pointing toward the center of the sarcomeres to its right and left sides.
if the skeletal muscle cell is not contracted, neither the thin nor the thick filaments extend the entire length of the sarcomere, and the area on either side of a particular Z disk, composed only of thin fila- ments, is the i band of light microscopy.
• An I band is composed of two halves, each belonging to adjacent sarcomeres.
• the area of a particular relaxed sarcomere that is composed of the entire length of the thick filament is the A band. the center of the A band of a relaxed sarcomere is void of thin filaments, and this represents the H band, an area rich in creatine kinase, the enzyme that catalyzes the transfer of high-energy phosphate from creatine phosphate to form adenosine triphosphate (ATP).
• in the center of the h band is the M line, composed mainly of C protein and myomesin, macromolecules that interconnect the thick filaments to each other and assist in maintaining their proper position to permit the interdigitation of the thick filaments with the thin filaments.
When a muscle cell contracts, the thin filaments slide past the thick filaments and drag the Z disks closer to each other, shortening the sarcomere by approximately 0.4 µm. Because a single skeletal mus- cle cell may have 100,000 sarcomeres in sequence, a change in length of 0.4 µm per sarcomere means that the contracted muscle becomes 4 cm shorter. For the thin filaments to be able to interact with the thick filaments as they slide past them, the morphologic arrangements must be very precise.
in mammalian skeletal muscle, each thick fila- ment is surrounded by six thin filaments at 60-degree intervals so that in cross section the thin filaments form a hexagon with a thick filament in the center (Fig. 8.4). Five proteins are responsible for maintain- ing the correct relationships among the sarcomere components:
• two titin molecules, large, elastic proteins extend from each Z disk of the same sarcomere to the M line, ensure that the thick filaments remain in the correct position.
• a-Actinins anchor thin filaments to the Z disk.
• two nebulin molecules extend from the Z disk to the end of each thin filament, ensuring that the thin filaments are in their proper positions, and that they are exactly the correct length.
• the length of the thin filament is also controlled by Cap Z and tropomodulin, molecules that prevent the addition to or deletion of g actin to or from the thin filament. cap Z acts at the barbed plus end (at the Z disk), whereas tropomodulin acts at the pointed minus end of the thin filament (see Fig. 8.4).
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Figure 8.4 A–D, Myofilaments of a sarcomere. (From Gartner LP, Hiatt JL: Color Textbook of Histology, 3rd ed. Philadelphia, Saunders, 2007, p 164.)
Sarcomere Z disk
A
B
C
D
A band H band
M band Nebulin Titin
Tropomodulin Tropomodulin
Tropomyosin Myofilaments
Myosin II molecule Light chain
Heavy meromyosin
S1 S2 Light meromyosin
Actin Troponin Myosin II Figure 8.2 organization of sarcomeres and myofibrils of a skeletal muscle cell. (From Gartner LP, Hiatt JL: Color Textbook of Histology, 3rd ed. Philadelphia, Saunders, 2007, p 161.) Bundle of
muscle fibers
One muscle fiber
Z disk
One myofibril
Sarcomere A band I band
H band
Figure 8.3 organization of triads and sarcomeres of skeletal muscle fibers. (From Gartner LP, Hiatt JL:
Color Textbook of Histology, 3rd ed.
Philadelphia, Saunders, 2007, p 162.)
Nucleus
Z line
A band I bandZ line Transverse
tubule
Terminal cisternae of sarcoplasmic
reticulum Sarcolemma
Myofibril Mitochondrion
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98 Thick filaments
Approximately 300 myosin II molecules, each 2 to 3 nm in diameter and 150 nm long, are present in a thick filament. Myosin ii molecules are composed of:
• two heavy chains
• two pairs of light chains; each pair consists of an essential light chain and a regulatory light chain (Fig. 8.5), and the regulatory light chain can be phosphorylated by myosin light chain kinase (MlcK)
each of the two identical heavy chains resembles a golf club, and the polypeptide chains (handles) of the two form an α-helix as they wrap around each other. each heavy chain can be enzymatically cleaved by trypsin into:
• Rodlike light meromyosin
• heavy meromyosin, two globular heads with a short stalk, composed of two polypeptide chains wrapped around each other; papain cleaves heavy meromyosin into two globular regions (s1) and the short stalk (s2)
each S1 subfragment has three binding sites—
AtP, light-chain myosin, and F actin binding sites.
Myosin molecules are arranged head to tail in a thick filament so that the center of the thick filament is smooth, and the two ends appear barbed because of the projection of the S1 subfragments. Myosin mol- ecules possess two pliant regions—one at the junc- tion of the s1 and s2 moieties, and one at the junction of the heavy and light meromyosins—that allow myosin ii to contact and drag the thin filament toward the center of the sarcomere.
Thin filaments
thin filaments, composed of F actin, tropomyosin, and troponin, have a barbed plus end attached to the Z disk and a pointed minus end capped by tropo- modulin (Fig. 8.6).
• F actin consists of two chains of G actin polymers, which resemble two strands of pearls twisted around each other. the two shallow grooves formed in this fashion are each occupied by 40-nm-long linear tropomyosin molecules arranged head to toe.
• the tropomyosin molecules mask the active site of each g actin molecule so that it is unavailable for contact by the s1 subunit of the myosin ii molecule.
• A tripartite troponin molecule is bound to each tropomyosin. the three components are
troponin C (TnC), which binds free calcium;
troponin T (TnT), which binds the troponin molecule to tropomyosin; and troponin I (TnI), which inhibits the interaction of the s1 subunit with g actin.
• if free calcium ions are available, they bind to tnc causing a conformational change in the troponin molecule that pushes the tropomyosin molecule deeper into the groove of the F actin filament and, by unmasking the active site, allows temporary binding with the s1 subunit.
MuScLE coNTRAcTIoN
Muscle contraction usually occurs after a nervous impulse, and for each individual muscle cell, it follows the all-or-none law, which is that either the cell contracts or it does not. the amount of shorten- ing is a function of the number of sarcomeres in a particular myofibril, and the strength of contraction of an entire muscle depends on the number of muscle cells that are contracting. Myofilaments do not contract; instead, according to the Huxley sliding filament theory, the thin filaments slide past the thick filaments as follows:
• t tubules convey the impulse generated at the myoneural junction to the terminal cisternae.
Voltage-gated calcium release channels of the terminal cisternae open, and ca++ ions, released into the sarcoplasm, bind to tnc, altering its conformation and pushing the tropomyosin deeper into the groove, unmasking the myosin binding site of g actin molecules.
• hydrolysis of AtP on the s1 moiety of myosin ii results in the formation of adenosine diphosphate (ADP) and inorganic phosphate (Pi), both of which remain attached to the s1
moiety. the myosin head swivels, and the entire complex becomes bound to the myosin binding site of g actin (see Fig. 8.6).
• Pi leaves the complex; this not only results in a stronger bond between the myosin and the actin, but also the s1 moiety alters its conformation and releases ADP, and the conformation of the myosin head alters and pulls the thin filament toward the center of the sarcomere. this movement is referred to as the power stroke of muscle contraction.
• the s1 moiety accepts a new AtP, releasing the bond between actin and myosin (see Fig. 8.6).
• For muscle contraction to be complete, the attachment and release cycles must be repeated approximately 200 to 300 times, and each cycle necessitates the hydrolysis of an AtP.
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Figure 8.6 Role of AtP in muscle contraction.
(Modified from Alberts B, Bray D, Lewis J, et al:
Molecular Biology of the Cell. New York, Garland Publishing, 1994.)
Actin
Myosin ATP present on the S1
subfragment is hydrolyzed, and the complex binds to the active site on actin.
Pi is released, resulting in a conformational alteration of the Si subfragment.
A new ATP molecule binds to the S1 subfragment, which causes the release of the bond between actin and myosin.
ADP is also released and the thin filament is dragged toward the center of the sarcomere.
Power Stroke
ATP
ATP
ADP P
ADP
ADP P
Figure 8.5 A–D, thick and thin filaments within a sarcomere. (From Gartner LP, Hiatt JL: Color Textbook of Histology, 3rd ed.
Philadelphia, Saunders, 2007, p 164.) Sarcomere
Z disk
A
B
C
D
A band H band
M band Nebulin Titin
Tropomodulin Tropomodulin
Tropomyosin Myofilaments
Myosin II molecule Light chain
Heavy meromyosin
S1 S2 Light meromyosin
Actin Troponin Myosin II
cLINIcAL coNSIDERATIoN
Mutations in some of the structural proteins that are responsible for the integrity of the myofibrillar organization of skeletal muscle can be devastating.
If the primary structure of the intermediate filament desmin or of the heat shock protein αB-crystallin is altered, the myofibrils cannot be fixed in their proper position in three-dimensional space, and the myofibrils become destroyed under conditions of stressful contractile forces.
rigor mortis is a condition that occurs after death. During muscle contraction in a living individual, ATP on the S1 moiety (myosin head) of myosin II is hydrolyzed into ADP and Pi, but neither ADP nor Pi leaves the myosin head. A change in conformation of myosin II allows the head to contact the myosin binding site of G actin of the thin filament. This contact is followed by the release of Pi and a stronger bond between myosin and actin, and then ADP is released from the
myosin head resulting in the power stroke. New ATP binds to the myosin head releasing the bond between the S1 moiety of myosin II and the G actin of the thin filament. In a dead individual, ATP is not regenerated, and after a while the muscle’s ATP supply becomes exhausted; the sarcoplasmic reticulum can no longer sequester calcium, and muscle contraction continues until ATP is no longer available to detach the S1 moiety of myosin II from the thin filament, and a sustained muscle contraction (i.e., muscle rigidity) ensues. This rigidity is known as rigor mortis. Depending on the ambient temperature, a little while later, lysosomal enzymes escape from the lysosomes and break down the actin and myosin, resolving rigor mortis.
During late spring in temperate zones, rigor mortis begins 3 to 8 hours after death, and the stiffness lasts 16 to 24 hours; by 36 hours after death, the muscles are no longer rigid.
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100 MuScLE RELAxATIoN
the process of muscle contraction requires the pres- ence of free calcium ions in the sarcoplasm. When the neural stimulus ceases, and the t tubules no longer convey the wave of depolarization into the interior of the muscle cell, the voltage-gated calcium release channels of the terminal cisternae close.
• the sarcoplasmic calcium is driven back into the sarcoplasmic reticulum by the action of calcium pumps to be sequestered by calsequestrin.
• Because calcium is no longer abundant, tnc releases its calcium ions and regains its relaxed conformation; the tropomyosin molecule occupies its previous position, hiding the active site of the g actin molecule, and myosin and actin are unable to bind to each other.
INNERvATIoN of SKELETAL MuScLE
skeletal muscle cells receive motor nerve fibers, which induce muscle contraction; sensory nerve fibers, which supply muscle spindles and golgi tendon organs that protect the muscle from injury; and auto- nomic fibers, which control the vascular supply of the muscle. Depending on the degree of fine coordina- tion of a particular muscle, it may have a:
• Rich nerve supply, as in the muscles of the eyes, in which a single motoneuron may control only five muscle cells, or
• Crude nerve supply, as in the muscles of the back, in which a single motoneuron may control several hundred muscle cells.
the motoneuron and all of the muscle cells that it controls are known as a motor unit. All the muscle fibers of a particular motor unit either contract simul- taneously or do not contract at all.
Impulse Transmission at the Neuromuscular Junction
skeletal muscle cells are innervated by the myelin- ated axons of a-motoneurons. these axons use the connective tissue elements of the muscle as they arborize to reach each skeletal muscle cell of their motor unit. As an axon branch reaches its muscle cell, it loses its myelin sheath, but retains its Schwann cell cover, and forms an expanded axon terminal (presynaptic membrane) over the motor end plate (postsynaptic membrane), a modified region of the sarcolemma. the combination of the motor end plate, (primary) synaptic cleft (the space between the presynaptic and postsynaptic membranes), and
axon terminal is known as a neuromuscular junc- tion (Fig. 8.7).
the postsynaptic membrane has numerous folds, and the spaces between these folds are referred to as secondary synaptic clefts (junctional folds). the folds and secondary synaptic clefts are lined by an external lamina. the axon terminal is covered by schwann cells, and it houses mitochondria, sarco- plasmic reticulum, and several hundred thousand syn aptic vesicles that contain the neurotransmitter acetylcholine, proteoglycans, ATP, and various other substances. the presynaptic membrane displays dense bars in the vicinity of which the membrane houses voltage-gated calcium channels. the trans- mission of a stimulus occurs in the following manner:
• A stimulus, traveling along the axon, reaches and depolarizes the presynaptic membrane, causing an opening of the voltage-gated calcium channels and the influx of calcium into the axon terminal.
• With each impulse, approximately 120 synaptic vesicles fuse with the active sites of the presyn- aptic membrane along the dense bars, releasing a quantum of acetylcholine (approximately 20,000 molecules), proteoglycans, and AtP into the primary synaptic cleft (Fig. 8.8).
• Acetylcholine receptors of the postsynaptic (muscle) membrane bind the released acetylcholine, opening ligand-gated sodium channels of the postsynaptic membrane, and the influx of sodium causes depolarization of the sarcolemma and t tubule. the wave of depolarization reaches the terminal cisternae, and calcium is released at the i–A junction to initiate muscle contraction.
• in less than 500 msec, the enzyme
acetylcholinesterase, located in the external lamina of the primary and secondary synaptic clefts, degrades acetylcholine into choline and acetate; the resting membrane potential of the postsynaptic membrane is re-established, preventing a single release of acetylcholine from precipitating multiple contractions.
• the sodium concentration gradient powers a sodium-choline symport to ferry the choline back into the axon terminal where activated acetate, derived from mitochondria, combines with the choline facilitated by the action of the enzyme choline O-acetyltransferase. the acetylcholine is conveyed into synaptic vesicles by a proton gradient powered by antiport carrier proteins.
• the surface area of the presynaptic membrane remains constant because of the membrane- trafficking mechanism.