The visible bands in skeletal and cardiac muscles are an important clue to the mechanism that produces muscle contractions. Whole muscles are made up of parallel bundles of individual muscle fibers (Fig. 37.3). Recall that a muscle fiber is a muscle cell. Each muscle fiber in turn contains hundreds of long, rodlike structures called myofibrils. Myofibrils contain parallel arrays of actin and myosin filaments that cause a muscle to contract.

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Let’s consider the structural organization of myosin and actin filaments in skeletal muscles in more detail (Fig. 37.4). Each myosin molecule consists of two long polypeptide chains coiled together, each ending with a globular head. Consequently, the myosin molecule resembles a double-headed golf club. The myosin molecules are arranged in parallel to form a thick filament, with the numerous myosin heads extending out from their flexible necks along the myosin filament. Actin subunits are arranged as a double helix to form a thin filament. The protein tropomyosin runs in the grooves formed by the actin helices.

The thin filaments are attached to protein backbones called Z discs that are regularly spaced along the length of the myofibril (Fig. 37.5a). The region from one Z disc to the next is called the sarcomere. Sarcomeres are the functional units of muscles. In other words, the shortening (contraction) of a muscle is ultimately the result of the shortening of thousands of sarcomeres along a myofibril.

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Sarcomeres are arranged in series along the length of the myofibril. Sets of actin thin filaments extend from both sides of the Z discs toward the midline of the sarcomere (Fig. 37.5a). In the middle of the sarcomere, not directly contacting the Z discs, are myosin thick filaments. The thin filaments overlap with the myosin thick filaments, forming two regions of overlap within a sarcomere. Toward either end of the sarcomere, as well as in the middle, are regions where the actin and myosin filaments do not overlap. The regions that overlap appear dark under an electron microscope, while the regions of non-overlap appear lighter in color. A third large protein, called titin, links the myosin filaments to the Z discs at the ends of the sarcomere. Titin is believed to help with assembly and protect the sarcomeres from being overstretched, thus contributing to muscle elasticity.

As we have seen, the regular pattern of actin and myosin filaments within sarcomeres along the length of the fiber gives skeletal muscles their striated appearance (Fig. 37.5a). In cross section, thick and thin filaments are arranged in a hexagonal lattice. This arrangement allows each thick filament to interact with six adjacent thin filaments (Fig. 37.5b). The precise geometry of thin and thick filaments is critical to their interaction, as we discuss next.