Figure 103

Organization of a skeletal muscle. A skeletal muscle consists of bundles of muscle fibers called fascicles. In turn, each fascicle consists of a bundle of elongate muscle fibers (cells). The muscle fiber represents a collection of longitudinal units, the myofibrils, which in turn are composed of myofilaments of two types: thick (myosin) filaments and thin (actin) filaments. The myofilaments are organized in a specific manner that imparts a cross-striated appearance to the myofibril and to the fiber. The functional unit of the myofibril is the sarcomere; it extends in both directions from one Z line to the next Z line. The A band marks the extent of the myosin filaments. Actin filaments extend from the Z line into the region of the A band, where they interdigitate with the myosin filaments as shown. The cross sections through different regions of the sarcomere are also shown (from left to right): through thin filaments of the I band; through thick filaments of the H band; through the center of the A band, where adjacent thick filaments are linked to form the M line; and through the A band, where thin and thick filaments overlap. Note that each thick filament is within the center of a hexagonal array of thin filaments.

The functional unit of the myofibril is the sarcomere, the segment of the myofibril between two adjacent Z lines

The sarcomere is the basic contractile unit of striated muscle. It is the portion of a myofibril between two adjacent Z lines. A sarcomere measures 2 to 3 /xm in relaxed mammalian muscle. It may be stretched to more than 4 /xm and, during extreme contraction, may be reduced to as little as 1 /xm (Fig. 10.5). The entire muscle cell exhibits cross-striations because sarcomeres in adjacent myofibrils are in register.

The arrangement of thick and thin filaments gives rise to the density differences that produce the cross-striations of the myofibril

The myosin-containing thick filaments are about 1.5 /xm long and are restricted to the central portion of the sarcomere, i.e., the A band. The thin filaments attach to the Z

line and extend into the A band to the edge of the H band. Portions of two sarcomeres, on either side of a Z line, constitute the I band and contain only thin filaments. In a longitudinal section of a sarcomere, the Z line appears as a zigzag structure, with matrix material, the Z matrix, bisecting the zigzag. The Z line and its matrix material anchor the thin filaments from adjacent sarcomeres to the angles of the zigzag by the actin-binding protein a-actinin. These features are illustrated in Figure 10.3.

F-actin, troponin, and tropomyosin in thin filaments and myosin II in thick filaments are the primary proteins in the contractile apparatus

Thin filaments contain F-actin, tropomyosin, and troponin. Thick filaments contain only myosin II.

G-actin is a small, 42-kDa molecule that polymerizes to form a double-stranded helix, the F-actin filament. These

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