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 m in relaxed mammalian muscle. It may be stretched to more than 4 m and, during extreme contraction, may be reduced to as little as 1 m (Fig. 11.6). 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 m 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 -actinin. These features are illustrated in Figures 11.4 and 11.6.
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-kilodalton molecule that polymerizes to form a double-stranded helix, the F-actin filament. These actin filaments are polar; all G-actin molecules are oriented in the same direction. The plus end of each filament is bound to the Z line by -actinin; the minus end extends toward the M line and is protected by an actin-capping protein. Each G-actin molecule of the thin filament has a binding site for myosin.
Tropomyosin is a 64-kilodalton protein that also consists of a double helix of two polypeptides. It forms filaments that run in the groove between the F-actin molecules in the thin filament. In resting muscle, tropomyosin and its regulatory protein, the troponin complex, mask the myosin-binding site on the actin molecule.
Troponin consists of a complex of three globular subunits. Each tropomyosin molecule contains one troponin complex. Troponin-C (TnC) is the smallest subunit of the troponin complex (18 kilodaltons). It binds Ca2, the essential step in the initiation of contraction (see illustration that follows). Troponin-T (TnT), a 30-kilodalton subunit, binds to tropomyosin, anchoring the troponin complex.
FIGURE 11.6 • Sarcomeres in different functional stages. In the resting state (middle), interdigitation of thin (actin) and thick (myosin) filaments is not complete; the H and I bands are relatively wide. In the contracted state (bottom), the interdigitation of the thin and thick filaments is increased according to the degree of contraction. In the stretched state (top), the thin and thick filaments do not interact; the H and I bands are very wide. The length of the A band always remains the same and corresponds to the length of the thick filaments; the lengths of the H and I bands change, again in proportion to the degree of sarcomere relaxation or contraction.
Troponin-I (TnI), also a 30-kilodalton subunit, binds to actin, thus inhibiting actin–myosin interaction.
 FOLDER 11.1 Functional Considerations: Muscle Metabolism and Ischemia