Sarcomere

A sarcomere is the repeating contractile unit of striated muscle and the basic structural unit of a myofibril. Sarcomeres occur in skeletal and cardiac muscle and are arranged end to end along the length of each myofibril. A single sarcomere extends from one Z disc to the next, and the highly ordered arrangement of its proteins produces the characteristic striped appearance of striated muscle .^1,2

The main contractile structures within a sarcomere are thin filaments, composed primarily of actin, and thick filaments, composed primarily of myosin. Thin filaments extend inward from the Z discs, while thick filaments occupy the central region and are organized around the M line. Their arrangement creates recognizable regions called the A band, I band, and H zone. Regulatory proteins including troponin and tropomyosin are associated with the thin filaments .^1,2

During contraction, calcium binding to troponin helps permit interactions between myosin and actin. Myosin heads then undergo ATP-dependent cross-bridge cycling, generating force. When mechanical conditions allow the sarcomere to shorten, the thin filaments slide toward the center of the sarcomere. The filaments themselves do not substantially shorten; instead, their overlap increases and the Z discs move closer together .^1,2

As a sarcomere shortens, the I band and H zone become narrower, while the A band remains essentially constant in length because the thick filaments retain their length. Coordinated shortening of many sarcomeres can contribute to shortening of the muscle fiber, while cross-bridge cycling can generate force whether or not substantial shortening occurs .^1,2