📌Schematic drawings of the structural organization of muscle.
✨ A. A fibrous connective tissue fascia, the "epimysium", surrounds the muscle,which is composed of many bundles, or fascicles. The fascicles are encased in a dense connective tissue sheath, the "perimysium" .
✨ B. The fascicles are composed of muscle fibers, which are long,. cylindrical, multinucleated cells. Between the individual muscle fibers are capillary blood vessels. Each muscle fiber is surrounded by a loose connective tissue called the "endomysium" . Just beneath the endomysium lies the sarcolemma, a thin elastic sheath with infoldings that invaginate the fiber interior. Each muscle fiber is composed of numerous delicate strands, myofibrils, the contractile elements of muscle.
✨ C. Myofibrils consist of smaller filaments,which form a repeating banding pattern along the length of the myofibril. One unit of this serially repeating pattern is called a sarcomere. The "sarcomere" is the functional unit of the contractile system of muscle.
✨D. The banding pattern of the sarcomere is formed by the organization of thick and thin filaments, composed of the proteins myosin and actin, respectively. The actin filaments are attached at one end but are free along their length to interdigitate with the myosin filaments. The thick filaments are arranged in a hexagonal fashion. A cross- section through the area of overlap shows the thick filaments surrounded by six equally spaced thin filaments.
✨ E. The lollipop- shaped molecules of each myosin filament are arranged so that the long tails form a sheaf with the heads, or cross-bridges, projecting from it. The cross-bridges point in one direction along half of the filament and in the other direction along the other half. Only a portion of one-half of a filament is shown here. The cross-bridges are an essential element in the mechanism of muscle contraction, extending outward to interdigitate with receptor sites on the actin filaments. Each actin filament is a double helix, appearing as two strands of beads spiraling around each other. Two additional proteins,tropomyosin and troponin, are associated with the actin helix and play an important role in regulating the interdigitation of the actin and myosin filaments. Tropomyosin is a long polypeptide chain that lies in the grooves between the helices of actin. Troponin is a globular molecule attached at regular intervals to the tropomyosin.
📌Source:
➡️Basic Biomechanics of the Musculoskeletal System
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