sarcomere

A sarcomere is the basic contractile unit of striated muscle, organized as repeating segments along a myofibril. It extends from one Z-disc to the next and shortens when muscle fibers contract, generating force.

Within each sarcomere, thin and thick filaments create distinct bands. Thin filaments are primarily actin, with regulatory proteins tropomyosin and the troponin complex. Thick filaments are primarily myosin, with heads that form cross-bridges during contraction. The A-band contains overlap of thick and thin filaments, while the I-band contains only thin filaments. The center of the sarcomere houses the M-line, and the Z-discs anchor thin filaments.

Two giant proteins contribute to structure and elasticity. Titin spans from the Z-disc to the M-line and provides passive stiffness and alignment. Nebulin acts as a molecular ruler that helps set thin filament length in skeletal muscle. Additional proteins such as myomesin at the M-line help align thick filaments, and desmin links adjacent sarcomeres to the cellular scaffold.

Contraction is driven by the sliding filament mechanism. Calcium released from the sarcoplasmic reticulum binds to troponin C, shifting tropomyosin away from actin’s myosin-binding sites. Myosin heads attach to actin, perform a power stroke as ATP is hydrolyzed, and detach after ATP binds again. This cross-bridge cycling shortens the sarcomere and generates tension.

In cardiac and skeletal muscle, sarcomere length influences force production. Resting lengths that optimize overlap typically lie around 2.0–2.2 micrometers. Mutations or defects in sarcomeric proteins can contribute to muscle diseases and cardiomyopathies.