FIGURE 11.7 • Electron micrograph of skeletal muscle and corresponding molecular structure of a sarcomere. a. This high-magnification electron micrograph shows a longitudinal section of the myofibrils. The I band, which is bisected by the Z line, is composed of barely visible, thin (actin) filaments. They are attached to the Z line and extend across the I band into the A band. The thick filaments, composed of myosin, account for the full width of the A band. Note that in the A band there are additional bands and lines. One of these, the M line, is seen at the middle of the A band; another, the less electron-dense H band, consists only of thick filaments. The lateral parts of the A band are more electron dense and represent areas where the thin filaments interdigitate with the thick filaments. 35,000. b. Diagram illustrating the distribution of myofilaments and accessory proteins within a sarcomere. The accessory proteins are titin, a large elastic molecule that anchors the thick (myosin) filaments to the Z line; -actinin, which bundles thin (actin) filaments into parallel arrays and anchors them at the Z line; nebulin, an elongated inelastic protein attached to the Z lines that wraps around the thin filaments and assists -actinin in anchoring the thin filament to Z lines; tropomodulin, an actin-capping protein that maintains and regulates the length of the thin filaments; tropomyosin, which stabilizes thin filaments and, in association with troponin, regulates binding of calcium ions; and myomesin and C proteins, myosin-binding proteins that hold thick filaments in register at the M line. The interactions of these various proteins maintain the precise alignment of the thin and thick filaments in the sarcomere.
 FOLDER 11.2 Clinical Correlation: Muscular Dystrophies— Dystrophin and Dystrophin-Associated Proteins
Dystrophin is a rod-shaped cytoskeletal protein with a short head and a long tail that is located just beneath the skeletal muscle cell membrane. F-actin is bound at the end portion of the tail. Two groups of transmembrane proteins— and -dystroglycans and -, -, -, and -sarcoglycans— participate in a dystrophin–glycoprotein complex that links dystrophin to the extracellular matrix proteins laminin and agrin. Dystroglycans form the actual link between dystrophin and laminin; sarcoglycans are merely associated with the dystroglycans in the membrane. Distribution of dystrophin in healthy individuals is visualized using immunostaining methods (Fig F11.2.1).