Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures

Skeletal muscle consists of parallel bundles of muscle fibers, each fiber a single, very large, multinucleated cell, 20 to 100 ^m in diameter, formed from many cells fused together and often spanning the length of the muscle. Each fiber, in turn, contains about 1,000 myofibrils, 2 ^m in diameter, each consisting of a vast number of regularly arrayed thick and thin filaments complexed to other proteins (Fig. 5-31). A system of flat membranous vesicles called the sarcoplasmic reticulum surrounds each myofibril. Examined under the electron microscope, muscle fibers reveal alternating regions of high and low electron density, called the A bands and I bands (Fig. 5-31b, c). The A and I bands arise from the arrangement of thick and thin filaments, which are aligned and partially overlapping. The I band is the region of the bundle that in cross section would contain only thin filaments. The darker A band stretches the length of the thick filament and includes the region where parallel thick and thin filaments overlap. Bisecting the I band is a thin structure called the Z disk, perpendicular to the thin filaments and serving as an anchor to which the thin filaments are attached. The A band too is bisected by a thin line, the M line or M disk, a region of high electron density in the middle of the thick filaments. The entire contractile unit, consisting of bundles of thick filaments interleaved at either end with bundles of thin filaments, is called the sarcomere. The arrangement of interleaved bundles allows the thick and thin filaments to slide past each other (by a mechanism discussed below), causing a progressive shortening of each sarcomere (Fig. 5-32).

The thin actin filaments are attached at one end to the Z disk in a regular pattern. The assembly includes the minor muscle proteins a-actinin, desmin, and vi-mentin. Thin filaments also contain a large protein called nebulin (~7,000 amino acid residues), thought to be structured as an a helix that is long enough to span the length of the filament. The M line similarly organizes the thick filaments. It contains the proteins paramyosin, C-protein, and M-protein. Another class of proteins called titins, the largest single polypeptide chains discovered thus far (the titin of human cardiac muscle has 26,926 amino acid residues), link the thick filaments to the Z disk, providing additional organization to the overall structure. Among their structural functions, the proteins nebulin and titin are believed to act as "molecular

Capillaries Muscle fiber

Sarcoplasmic reticulum

Z disk M line

Capillaries Muscle fiber

Sarcoplasmic reticulum

Z disk M line

Z disk M line Z disk 18

Z disk M line Z disk 18

FIGURE 5-31 Structure of skeletal muscle. (a) Muscle fibers consist of single, elongated, multinucleated cells that arise from the fusion of many precursor cells. Within the fibers are many myofibrils (only six are shown here for simplicity) surrounded by the membranous sarcoplasmic reticulum. The organization of thick and thin filaments in the myofibril gives it a striated appearance. When muscle contracts, the I bands narrow and the Z disks come closer together, as seen in electron micrographs of (b) relaxed and (c) contracted muscle.

Thin Thick filament filament

Thin Thick filament filament

FIGURE 5-32 Muscle contraction. Thick filaments are bipolar structures created by the association of many myosin molecules. (a) Muscle contraction occurs by the sliding of the thick and thin filaments

past each other so that the Z disks in neighboring I bands approach each other. (b) The thick and thin filaments are interleaved such that each thick filament is surrounded by six thin filaments.

rulers," regulating the length of the thin and thick filaments, respectively. Titin extends from the Z disk to the M line, regulating the length of the sarcomere itself and preventing overextension of the muscle. The characteristic sarcomere length varies from one muscle tissue to the next in a vertebrate organism, a finding attributed in large part to the different titin variants in the tissues.

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