sarcómera

The sarcomere is the basic contractile unit of striated muscle. It is the segment of a myofibril between two adjacent Z-discs and shortens during contraction.

Within the sarcomere, thick filaments made of myosin interdigitate with thin filaments made of actin, along with regulatory proteins. The arrangement creates characteristic bands: the A band corresponds to the length of the thick filament; the I band contains only thin filaments; the Z-discs define sarcomere boundaries; the H zone is the central part where only thick filaments are present; the M line anchors thick filaments. Elastic and structural proteins such as titin span from Z-disc to M-line, contributing to elasticity and alignment, while nebulin helps determine thin-filament length.

Contraction occurs by cross-bridge cycling, in which myosin heads bind actin, perform power strokes, and detach after ATP binding, causing sliding of filaments past one another. This sliding shortens the sarcomere and generates force. Regulation is controlled by calcium ions released from the sarcoplasmic reticulum in response to membrane depolarization; calcium binds to troponin, leading to a shift of tropomyosin and exposure of myosin-binding sites on actin.

Sarcomere length influences force production; resting lengths in skeletal muscle are typically around 2.0–2.2 micrometers, with variation among muscles. Sarcomeres are arranged in series along the myofibril, and proper sarcomeric function is essential for muscle contraction. Mutations in sarcomeric proteins can contribute to cardiomyopathies and congenital myopathies.