The ability of animals to move is due to the presence of specific cells that have become highly differentiated, so that they function almost exclusively in con-

traction. The contractile process has been harnessed by the organism to permit various modes of movement and other activities for its survival. Some of these activities depend on

•quick contractions of short duration;

•long-lasting contractions without the necessity for rapid actions,

•powerful, rhythmic contractions that must be repeated in rapid sequences.

These varied needs are accommodated by three types of muscle, namely, skeletal, smooth, and cardiac. There are basic similarities among the three muscle types (see Table 6-1).

•They are all mesodermally derived and are elongated parallel to their axis of contraction;

•they possess numerous mitochondria to accommodate their high energy requirements, and

•all contain contractile elements known as myofilaments, in the form of actin and myosin, as well as additional contractile-associated proteins.

Myofilaments of skeletal and cardiac muscles are arranged in a specific ordered array that gives rise to a repeated sequence of uniform banding along their length—hence, their collective name, striated muscle.

Since muscle cells are much longer than they are wide, they are commonly referred to as muscle fibers. However, it must be appreciated that these fibers are living entities, unlike the nonliving fibers of connective tissue. Neither are they analogous to nerve fibers, which are living extensions of nerve cells.

•Often, certain unique terms are used to describe muscle cells; thus, the muscle cell membrane is sarcolemma (although earlier use of this term included the attendant basal lamina and reticular fibers), cytoplasm is sarcoplasm, mitochondria are sarcosomes, and endoplasmic reticulum is sarcoplasmic reticulum (SR).

SKELETAL MUSCLE

Skeletal muscle (see Graphics 6-1 and 6-2) is invested by dense collagenous connective tissue known as the

•epimysium, which penetrates the substance of the gross muscle, separating it into fascicles.

•Each fascicle is surrounded by perimysium, a looser connective tissue.

•Finally, each individual muscle fiber within a fascicle is enveloped by fine reticular fibers, the endomysium.

The vascular and nerve supplies of the muscle travel in these interrelated connective tissue compartments.

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There are three types of skeletal muscle fibers: red, white, and intermediate depending on their contraction velocities, mitochondrial content, and types of enzymes the cell contains (see Table 6-2).

Each gross muscle, for example, biceps, usually possesses all three types of muscle cells.The innervation of a particular muscle cell determines whether it is red, white, or intermediate. Each skeletal muscle fiber is roughly cylindrical in shape, possessing numerous elongated nuclei located at the periphery of the cell, just deep to the sarcolemma.

Longitudinally sectioned muscle fibers display intracellular contractile elements, which are the parallel arrays of longitudinally disposed myofibrils.

•This arrangement of myofibrils produces an overall effect of cross-banding of alternating light and dark bands traversing each skeletal muscle cell. The dark bands are A bands, and the light bands are I bands.

•Each I band is bisected by a thin dark Z disc, and the region of the myofibril extending from Z disc to Z disc, the sarcomere, is the contractile unit of skeletal muscle cell.

•The A band is bisected by a paler H zone, the center of which is marked by the dark M disc.

During muscle contraction, the various transverse bands behave characteristically, in that the width of the A band remains constant, the two Z discs move closer to each other approaching the A band, and the I band and H zone become extinguished. Each Z disc is surrounded by intermediate filaments, known as desmin. The desmin filaments are bound to each other and to the Z discs by plectin filaments.

•Desmin filaments insert into the costameres which are regions of the sarcolemma that are dedicated for the attachment of these intermediate filaments.

•The heat shock protein, aB-crystallin, protects the desmin intermediate filaments by binding to them at their contact with the Z disc.

•The desmin-plectin-aB-crystallin complex, along with the costameres, ensures that the myofibrils of a muscle cell are aligned in the appropriate fashion so that the contraction of all of the myofibrils of each muscle cell occurs in a synchronized fashion.

Myofilaments

Electron microscopy has revealed that banding is the result of interdigitation of thick and thin myofilaments. The I band consists solely of thin filaments, whereas the A band, with the exception of its H and M components, consists of both thick and thin filaments. During contraction, the thick and thin filaments slide past each other (see below), and the Z discs are brought near the ends of the thick filaments.

•Thin filaments (7 nm in diameter and 1 mm in length) are composed of F actin, double-helical polymers of G actin molecules, resembling a pearl necklace twisted upon itself. Each groove of the helix houses linear

tropomyosin molecules positioned end to end.

Associated with each tropomyosin molecule is a troponin molecule composed of three

polypeptides—troponin T (TnT), troponin I (TnI), and troponin C (TnC).

TnI binds to actin, masking its active site (where it is able to interact with myosin);

TnT binds to tropomyosin; and

TnC (a molecule similar to calmodulin) has a high affinity for calcium ions.