sarkomeres

Sarkomeres are the functional units of striated muscle tissue, responsible for muscle contraction. They are composed of thin filaments of actin and thick filaments of myosin, arranged in a repeating pattern along the muscle fiber. The term "sarkomere" comes from the Greek words "sarx" meaning flesh and "meros" meaning part.

Each sarkomere is bounded by a Z disc on either side and contains an A band in the center, which is further divided into an H zone and an M line. The H zone contains the thick filaments of myosin, while the M line marks the midpoint of the A band and is composed of the protein titin. The I band lies between the A band and the Z discs and contains the thin filaments of actin.

The sliding filament theory explains muscle contraction. During contraction, the cross-bridges between actin and myosin filaments shorten, pulling the Z discs closer together and reducing the length of the sarcomere. This movement is powered by the hydrolysis of adenosine triphosphate (ATP) molecules. Relaxation occurs when ATP is replenished and the cross-bridges detach, allowing the sarcomeres to return to their original length.

Sarkomeres play a crucial role in muscle function, enabling the generation of force and movement. They are also involved in various physiological processes, such as blood pressure regulation and posture maintenance. Disorders affecting sarkomeres can lead to muscle weakness, rigidity, or other movement disorders.