Skeletal Muscle; Structure, Function, and Repair

Abstract

Skeletal muscle has a specialized structure due to its distinctive function. Skeletal muscle fibers (muscle cells) have cross-band appearance because of the intracellular contractile proteins called the myofilaments. Bundles of thick and thin myofilaments form myofibrils; bundles of myofibrils form muscle fibers; and finally, bundles of muscle fibers constitute skeletal muscle tissue. Thick myofilaments contain myosin protein and thin myofilaments contain mainly actin protein. Sarcoplasmic reticulum stores Ca2+ ions, which are essential for muscle contraction, and T-tubules provide the transmission of muscle innervation. Release of acetylcholine from the axon ending into the synaptic cleft initiates the depolarization of sarcolemma (muscle fiber membrane), and this leads to release of Ca2+ ions from the terminal sacs of sarcoplasmic reticulum into the sarcoplasm (cytoplasm). Ca2+ binds the troponin proteins found on the thin myofilaments. This initiates the attachment of actin to the binding site of myosin and thin myofilaments slide over thick myofilaments and the muscle contraction process occurs. Skeletal muscle injuries are due to direct (sharp, blunt, and penetrating) or indirect (rapid eccentric contraction) trauma. The repair process of skeletal muscle includes necrosis/degeneration, inflammation, proliferation and differentiation of certain cells, formation of scar tissue, and remodeling phases.