Myofibrilleihin

Myofibrilleihin is Finnish for the illative plural form of myofibrille (myofibril). In English-language texts, the topic corresponds to myofibrils. This article describes the structure, organization and function of myofibrils in striated muscle.

Myofibrils are long, thread-like organelles that run parallel to the axis of a muscle fiber. They are composed of a series of repeating units called sarcomeres, which extend from one Z-disc to the next. Each sarcomere contains thick filaments made of myosin and thin filaments made of actin, arranged in a precise lattice. The thick filaments are anchored at the M-line, while thin filaments are attached to Z-discs. Within the sarcomere, the A-band marks the length of the thick filaments and overlapping regions with thin filaments, the I-band contains only thin filaments, and the H-zone is the central region of the A-band containing only myosin during rest.

A network of proteins supports this arrangement. Titin extends from the Z-disc to the M-line, contributing elasticity and alignment; nebulin helps regulate thin filament length; desmin and other cytoskeletal components connect adjacent myofibrils to maintain cellular integrity.

Contraction occurs via the sliding filament mechanism. Calcium ions released from the sarcoplasmic reticulum bind to troponin C on actin, moving tropomyosin away from myosin-binding sites. Myosin heads hydrolyze ATP and form cross-bridges with actin, pulling filaments toward the sarcomere center and shortening the myofibril. Relaxation follows a decrease in intracellular calcium and detachment of cross-bridges.

Myofibrils underlie the contractile strength and speed of skeletal and cardiac muscles and are central to studies in physiology and muscle pathology, including myofibrillar myopathies caused by mutations in sarcomeric and cytoskeletal proteins.