myofibrils

Myofibrils are long, cylindrical bundles of contractile proteins that run parallel within muscle fibers. They are the primary contractile elements of striated muscle and are organized in series to produce coordinated shortening of the muscle. The repeating contractile units along a myofibril are sarcomeres, defined by Z-discs at each end.

Within each sarcomere, thick filaments made of myosin lie central to the structure, while thin filaments composed of actin surround them. The thin filaments also include regulatory proteins, tropomyosin and the troponin complex, which regulate contraction in response to calcium. Titin spans from the Z-disc to the M-line and provides passive elasticity and helps stabilize sarcomere alignment; nebulin helps organize thin filaments. Desmin and other cytoskeletal proteins link adjacent myofibrils at the Z-discs to maintain cellular integrity, and the M-line anchors the thick filaments.

Contraction occurs via the sliding filament mechanism. Upon stimulation, calcium released from the sarcoplasmic reticulum binds to troponin C, shifting tropomyosin away from myosin-binding sites on actin. Myosin heads attach to actin, hydrolyze ATP, and undergo cross-bridge cycling, pulling thin filaments past thick filaments and shortening the sarcomere. This shortening of many sarcomeres along a myofibril generates the overall force of contraction, transmitted through the muscle cell to connective tissue.

Myofibrils reside in the sarcoplasm of muscle fibers and are associated with transverse tubules and the sarcoplasmic reticulum, which coordinate electrical and chemical signals for contraction. Disorders affecting myofibril structure or function, such as myofibrillar myopathies or nemaline myopathy, reflect disruptions in these contractile proteins.