THE MUSCLE TISSUE



THE MUSCLE TISSUE

Ahmad Aulia Jusuf, MD, PhD

Depart of Histology

Faculty of Medicine University of Indonesia

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INTRODUCTION

Muscle (Fig-1) is one of the four basic tissue characterized by its specific properties, the ability to convert chemical energy into mechanical work and contractility that permit the locomotion, constriction, pumping and other propulsive movement of the muscle to be occured.

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Fig-1 The muscle

There are two major type of muscle according to the present of repeating dark and light cross-bands or striation (Fig-2); the

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Fig-2 The striated and smooth muscles

striated muscle and and smooth muscle. The striated muscle cells display characteristic alternation of light and dark cross-bands.

There are two types of striated muscle; skeletal muscle accounting for most voluntary muscle mass of the body and involuntary cardiac muscle limited almost exclusively to the heart. The skeletal muscle (Fig-3) is associated with the bony skeleton and consists of cylindrical fibers that are multinucleated.

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Fig-3 Skeletal Muscle

The cardiac muscle (Fig-4) consists of separate cellular units and is uninucleate. Furthermore cardiac muscle is characterized by rhythmic, involuntary contractions controlled by autonomic innervation.

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Fig-4 Cardiac Muscle

The smooth muscle (Fig-5) consists of spindle-shape, fusiform, uninucleate cell that do not exhibit striations. Smooth muscle is

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Fig-5 Smooth muscle

involuntary and is innervated by the autonomic nervous system. Smooth muscle is widely distributed throughout the digestive tube, in the tubular portions of many organs and in the walls of many blood vessels.

Unique terms are often used in describing the component of muscle cells (Fig-6). Muscle membran is referred to as sarcolema, the cytoplasm as sarcoplasm, the smooth endoplasmic reticulum as sarcoplasmic reticulum and occasionally the mitochondria as sarcosomes. The muscle cells frequently

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Fig-6 The Organels of muscle fiber

are called as muscle fiber because they are much longer than they are wide. Unlike the

collagen fibers however they are living entities.

All three muscle types are derived from mesoderm. Cardiac muscle originates in splanchnopleuric mesoderm most smooth muscle is derivated from splanchnic and somatic mesoderm, and most skeletal muscles originate from somatic mesoderm.

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Fig-7 Skeletal, smooth and cardiac muscle

SKLETAL MUSCLE

An anatomically named muscle such as deltoid muscle consists of many muscle bundle or fascicles (Fig-8) which is surrounded by the connective tissue called as epimysium. Each muscle bundle consists

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Fig-8 Organization of skeletal muscle

of a variable number of muscle fibers surrounded or delineated by the connective tissue, the part of epimysium that extended inward, surrounding the muscle bundles or fascicles called as perimysium. Muscle fiber is the basic structural unit of skeletal muscle composed by a long, cylindrical and multinucleate structure. The muscle fiber is surrounded by the extension of connective tissue, the perimysium inward called as endomysium.

All of the connective tissue conducts the blood vessels, lymphatic vessels, and nerve into the interior of the muscle, bringing them close to the individual muscle fibers.

Muscle fiber (Fig-6 and 9) is a long, multinucleated and cylindrical structure with 1-40 mm in long and 10-100 mikrometer in wide. Numerous nuclei, space along the

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Fig-9 The muscle fiber

length of the fiber are displaced to the periphery by the column of myofibril that occupies the bulk of the sarcoplasm. They are flattened against the sarcolemma. The cytoplasmic surface of the sarcolemma in skeletal muscle is coated with the 400 kD protein dystrophin which appears to provide mechanical reinforcement to the membrane, thereby protecting it against stresses developed during muscular contraction.

All of the common cell organelles (Fig-6 and 9) are represented in the sarcoplasm, such as Golgi complex, mitochondria ect. The sarcoplasm also contains the myoglobin, an oxygen binding protein which is largely responsible for the slightly brown color of muscle. Myoglobin is present in low concentration and it possibly of little functional significance in the relatively pale muscles of humans. As required, oxygen dissociates from myoglobin and becomes available for oxidations.

The interior of muscle fiber (Fig-9) contains a variable number of longitudinally oriented structural units called as myofibril with usually range from 1 to 2 mikrometer in diameter. Myofibril consists of many myofilament (more than 100) which are oriented longitudinally within the myofibril. There are two types of myofilament; the thick and thin.

THE LIGHT MICROSCOPY OF SKELETAL MUSCLE FIBER

In the light microscopy, the hemato xyllin-eosin stained striated muscle illustrates alternate light and dark transverse binding along the fibers. The hematoxyllin stainned dark bands (Fig-9and 10) are known as A bands (anisotropic or biferingent/double refractile with polarized

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Fig-10 The dark (A) and light (I) bands

light) with approximately 1.5 um in length, while the alternate bands that do not stainned with hematoxyllin-eosin are the I bands (isotropic or singly refractile with polarized light) with approximately 1 um in length. The center of each A band is occupied by a pale area, the H band, which is bisected by a thin M line. The I bands is bisected by a thin dark line, the Z disk (Z line) . The region of the myofibril between two successive Z disks, known as a sarcomere, is 2.5 um in length and is considered to be the contratile unit of skeletal muscle fibers (Fig-10).

The term sarcomer refers to the unit of distance between adjacent Z lines and is the fundamental unit of contraction.

In a relaxed skeletal muscle fiber (Fig. 9-10), the thick filaments do not extend the entire length of the sarcomere, whereas the

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thin filaments projecting from the two Z disks of the sarcomere meet in the midline. Therefore, there are regions of each sarcomere, on either side of each Z disk, where only thin filaments are present, known as I band which can be seen by the light microscopy. The region of each sarcomere that encompasses the entire length of the thick filaments is the A band. The zone in the middle of the A band, which is devoid of thin filament, is the H band. As noted earlier, the H band is bisected by the M line, which consists of myomesin, C protein, and other proteins that interconnect thick filaments to maintain their specific lattice arrangement.

During muscle contraction (Fig-9,10) the various transverse bands behave characteristically. During contraction individual thick and thin filaments do not shorten, instead, the two Z disk are brought closer together as the thin filaments slide past the thick filaments (sliding filaments theory). Thus when contraction occurs, the motion of the thin filaments toward the center of the sarcomere creates a greater overlap between the two groups of filaments, effectively reducing the width of the I and H bands without influencing the width of the A band.

The arrangement of the thick and thin filaments bears a specific and constant relationship. In mammalian skeletal muscle each thick filament is surrounded equidistantly by six thin filaments (Fig. 9-10). Cross-sections through the region of over-lapping thin and thick filaments display a hexagonal pattern, with a thick filament is surrounded by six thin filaments.

ULTRASTRUCTURE OF STRIATED MUSCLE FIBER

The fine structure of the sarcolemma is similar to that of other cell membranes. However the distinguishing feature of this membrane is that it is continued within the skeletal muscle fiber as numerous T tubules (transverse tubules). T-tubules (Fig.8 and 11) is a long, tubule extending inward from the sarcolemma that penetrate deep into the interior of the muscle fiber crossing many myofibrils.

T tubules pass transversely across the fiber and lie specifically in the plane of the junction of the A and I bands in mammalian skeletal muscle. One sarcomere has two sets of T tubules; one at each interface of the A and I bands. T-tubules extend deep into the interior of the fiber and facilitate the conduction of waves of depolarization along the sarcolemma.

Sarcoplasmic reticulum (Fig. 8 and 11) is a membrane-bounded tubules that form a continuous network occupying the narrow spaces between the myofibrils throughout the muscle fiber. Although it corresponds to the endoplasmic reticulum of other cells, it is largely devoid of associated ribosome and is specialized for a different function. The sarcoplasmic reticulum forms a meshwork around each myofibril and displays dilated terminal cisternae at each A-I junction. Thus two of these cisternae are always in close apposition to a T-tubule forming a Triad in which a T tubule is flanked by two cisternae.

This arrangement permits a wave of depolarization to spread, almost instantaneously, from the surface of the sarcolemma throughout the cell, reaching the terminal cisternae, which have voltage-gated Ca2+ release channel.

The sarcoplasmic reticulum regulates muscle contraction by controlled sequestering (leading to relaxation) and release (leading to contraction) of Ca2+ ions within the sarcopalsm. The wave of depolarization transmitted by T tubules triggers the opening of the calcium release channels of the terminal custernae, resulting in release of calcium into the cytosol in the vicinity of the myofibrils.

Myofibrils are held in register with each other by the intermediate filament desmin and vimentin (Fig.12) which secure the periphery of the Z disks of neighboring myofibrils to each other. These bundles of myofibrils are attached to the cytoplasmic aspect of the sarcolemma by various proteins, including dystrophin, a protein that binds to actin.

Deep to the sarcolemma, and inter-spersed between and among myofibrils are numerous elongated mitochondria with many highly interdigitating cristae. Moreover, numerous mitochondria are located just deep to the sarcoplasm.

STRUCTURAL ORGANIZATION OF MYOFIBRILS

Electron microscopy demonstrates the presence of parallel, interdigitating thick and thin rod-like myofilaments.

THICK FILAMENT

The thick filaments (15 nm in diameter and 1.5 um long) are composed of myosin. These filaments form parallel arrays interdigitating with the thin filaments in a specific fashion. The myosin thick filament is slightly wider in the middle than at either end.

Every thick filament consists of 200 to 300 myosin molecules. Each myosin molecule (150 nm long; 2 to 3 nm in diameter) is composed of two identical heavy chains and two pairs of light chains.

The heavy chains resemble two golf clubs, whose rod-like polypeptide chains are wrapped around each other in an alpa-helix. The heavy chains can be cleaved by trypsin into a rod-like tail, light meromyosin, and a globular head, heavy meromyosin. Heavy meromyosin is cleaved by papain into two globular (S1) moieties and a short, helical, rod-like segment (S2) (Fig-13). The S1 subfragment binds adenosine triphosphate (ATP) and functions in the formation of cross-bridges between thick and thin myofilaments. The heavy chains has two hinges at two different regions: one is at the junction of the LMM and HMM, and the other is at the neck region near the two globular heads (Fig-14).

Light chains are of two types,and one of each is associated with each S1 subfragment of the myosin molecule. Each heavy chain has two light chains, and a myosin molecule is composed of two heavy chains and four light chains.

The 200-300 myosin molecule in a thick filament are bundle together such that one half of the molecules have their heads pointing toward the opposite end (Fig-14). This arrangement result in a bare zone in the center of the A band where there are no myosin heads. This molecule organization explains in part why two sets of thin filaments in a sarcomere are pulled together toward each other that is toward the center of the A band .

THE THIN FILAMENT

The thin filaments (7nm in diameter and 1.0um long) are composed primarily of actin (Fig-13).

Thin filaments originate at the Z disk and project toward the center of the two adjacent sarcomeres, thus pointing in opposite directions. A single sarcomere will have two groups of parallel arrays of thin filaments, each attached to one Z disk. All of these filaments point toward to the middle of the sarcomere (Fig-13).

The major component of each thin filament is F-actin, a polymer of globular G-actin unit. Although G-actin molecules are globular, they all polymerize in the same spatial orientation, imparting to the filament a distinct polarity. The plus end of each filament is bound to the Z disk by α-actinin, the minus end extends toward the center of the sarcomere. Each G-actin molecule also contains an active site where the head region (S1 subfragment) of myosin binds. Two chains of F-actin are wound around each other in a tight helix (36-nm periodicity) like two strands of pearls (Fig-13).

There are shallow grooves along the length of the F-actin double-stranded helix. Pencil-shaped like tropomyosin molecules about 40 nm long, polymerize to form head-to-tail filaments that occupy the shallow grooves in the actin filaments. Bound tropomyosin masks the active sites on the actin molecules by partially overlapping them (Fig-13).

Approximately 25 to 30 nm from the beginning of each tropomyosin molecule is a single troponin molecule (Fig.13-15), composed of three globular poly peptides, TnT, TnC, and TnI. The TnT subunit binds the entire troponin molecule to tropomyosin; TnC has a great affinity for calcium; and TnI binds to actin, preventing the interaction between actin and myosin (Fig-16)

Binding of calcium by TnC induces a conformational shift in tropomyosin, exposing the previously blocked active sites on the actin filament, so that myosin heads can bind.

The structural organization of myofibrils is maintained largely by three proteins, titin, α−actinin, and nebulin. Thick filaments are positioned precisely within the sarcomere with the assistance of titin, a large, linear, elastic protein (Fig-13). A titin molecule extends from each half of a thick filament to the adjacent Z disk, thus anchoring the filaments between the two Z disks of each sarcomere. Thin filaments are held in register by the rod-shaped protein α−actinin, a component of the Z disk that can bind thin filaments in parallel arrays (Fig-13). In addition, two molecules of nebulin-a long, nonelastic protein-are wrapped around the entire length of each thin filament, further anchoring it in the Z disk and ensuring the maintenance of the specific array (Fig-13).

MUSCLE CONTRACTION AND RELAXATION

Contraction effectively reduces the resting length of the muscle fiber by an amount that is equal to the sum of all shortenings that occur in all sarcomeres of that particular muscle cell. The process of contraction, usually triggered by neural impulses, obeys the “all-or-none law”in that a single muscle fiber will either contract or not contract as a result of stimulation. The strength of contraction of a gross anatomical muscle, such as the biceps, is a function of the number of muscle fibers that undergo contraction. The stimulus is transferred at the neuromuscular junction. During muscle contraction the thin filaments slide past the thick filaments, as proposed by Huxleys sliding filament theory.

The following sequence of the events leads to contraction in skeletal muscle:

1. Impulse, generated along the sarcolemma, is transmitted into the interior of the fiber via the tubules, where it is conveyed to the terminal cisternae of the sarcoplasmic reticulum(see Fig-8,11).

2. Calcium ions leave the terminal cisternae through voltage-gated calcium-release channels, enter the cytosol, and bind to the TnC subunit of troponin, altering its conformation (Fig-16)

3. Conformational change in troponin shifts the position of tropomyosin deeper into the groove, unmasking the active site(myosin-binding site) on the actin molecule (Fig-16)

4. ATP present on the S1 fragment of myosin is hydrolyzed, but both adenosine diphosphate (ADP) and inorganic phosphate (P1) remain attached to the S1 fragment, and the complex binds to the active site on actin(Fig-16).

5. P1 is released, resulting not only in an increased bond strength between the actin and myosin but also in a conformational alteration of the S1 fragment.

6. ADP is also released, and the thin filament is dragged toward the center of the sarcomere(“power stroke”).

7. A new ATP molecule binds to the S1 fragment, which causes the release of the bond between actin and myosin.

The attachment and release cycles must be repeated numerous times for contraction to be completed. Each attachment and release requires ATP for the conversion of chemical energy into motion.

As long as cytosolic calcium concentration remains high enough, actin filament will remain in the active stateand contraction cycles will continue. However once the stimulation impulses cease, muscle relaxation occurs involving a reverssal of the step that led to contraction. Fisrt calcium pump in the membrane of the sarcoplasmic reticulum actively drive Ca2+ back into the terminal cisternae, where the calcium ions are bound by the protein calsequestrin. The reduced level of Ca2+ in the cytosol cause TnC to loose its bound Ca2+, tropomyosin then reverts to the position in which it masks the active site of actin, preventing the interaction of actin and myosin.

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