Muscles are classified as skeletal, cardiac, or smooth. General features are summarized in Table 1-3-1. Skeletal Muscle
Skeletal muscle has three levels of connective tissue: endomysium, perimysium, and epimysium. Skeletal muscle is composed of long cylindrical fibers that have dark (A) bands and light (I) bands. A dark transverse line, the Z line, bisects each I band. Skeletal muscle fibers contain myofibrils, which in turn are composed of sarcomeres.
Sarcomeres have thick and thin filaments. Thick filaments are centrally located in sarcomeres, where they interdigitate with thin filaments. The I band contains thin filaments only, the H band contains thick filaments only, and the A bands contain both thick and thin filaments.
Thin filaments contain three proteins: actin, tropomyosin, and troponin.
Actin forms a double helix, whereas tropomyosin forms an a-helix. Troponin includes three polypeptides: TnT, which binds to tropomyosin; TnC, which binds to calcium ions; and Tnl, which inhibits actin-myosin interaction. Thick filaments are composed of myosin. Myosin has two heavy chains with globular head regions. The heads contain actin-binding sites and have ATPase activity. The transverse tubular system surrounds each myofibril and facilitates excitation-contraction coupling.
Cardiac musde has an arrangement of sarcomeres similar to that in skeletal muscles, but the fibers are coupled through gap junctions.
Smooth muscle is found in the walls of blood vessels and hollow viscera. Gap junctions couple them electrically. Myofilaments of smooth muscles are not arrayed like in skeletal muscles; they are obliquely placed in order to form a latticework. Electrical or chemical signaling via hormones can trigger smooth muscles. Table 1-3-2 summarizes the differences between the three types of muscles.
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