myofibrillien

Myofibrillen, known in English as myofibrils, are the basic contractile elements of striated muscle fibers. They are long, rod-like structures that extend along most of the length of a muscle cell and are responsible for the force generated during contraction. In skeletal and cardiac muscle, myofibrils are organized into repeating units called sarcomeres, which account for the characteristic banded (striated) appearance of these tissues.

The sarcomere is the functional unit of a myofibril. It is bounded at each end by a Z-disc and contains interdigitating thin and thick filaments. The thin filaments are primarily actin, while the thick filaments are myosin. The arrangement creates alternating dark (A) bands and light (I) bands, with a central (H) zone within the A band where only myosin is present and a M-line that helps maintain structural alignment. Regulatory proteins, including troponin and tropomyosin on the actin filaments, govern the interaction between actin and myosin in response to calcium ions.

Several proteins stabilize and organize the myofibril. Titin acts as a molecular spring within the sarcomere, helping to restore resting length after contraction. Nebulin helps calibrate thin filament length, while desmin links neighboring myofibrils to form a cohesive cytoskeletal network. Dystrophin connects the cytoskeleton to the extracellular matrix, contributing to membrane stability.

Contraction occurs via the sliding filament mechanism: calcium release enables myosin heads to form cross-bridges with actin, hydrolyze ATP, and pull actin filaments toward the center of each sarcomere, shortening the sarcomere and the muscle fiber as a whole. Myofibrils are essential for muscle strength and endurance and can be affected in various myopathies and dystrophies.