Sarkomere

A Sarkomere (English: sarcomere) is the fundamental contractile unit of striated muscle, arranged in series within a myofibril and extending from one Z-disc to the next. It repeats along the length of the muscle fiber and is responsible for the characteristic striations observed in skeletal and cardiac muscle.

Structurally, a sarcomere contains alternating bands and zones. Thin filaments composed mainly of actin extend from each Z-disc toward the center, while thick filaments made of myosin reside in the A-band. The I-band contains only thin filaments, and the A-band includes overlapping regions of thick and thin filaments. The center of the sarcomere houses the M-line, which anchors thick filaments, and the H-zone, a region within the A-band with reduced overlap during rest. The Z-discs define the borders of the sarcomere, and the giant elastic protein titin runs from Z-disc to M-line, contributing to elasticity and resting tension. Nebulin helps regulate thin-filament length in skeletal muscle.

Contraction is driven by the sliding filament mechanism. An action potential triggers calcium release from the sarcoplasmic reticulum, Ca2+ binds to the troponin complex on actin, tropomyosin shifts to expose myosin-binding sites, and myosin heads bind, perform power strokes, and hydrolyze ATP to pull actin filaments inward. Relaxation occurs as Ca2+ is pumped back into the SR.

In skeletal and cardiac muscle, sarcomeres determine length-tension relationships; smooth muscle lacks a true sarcomeric organization. Mutations in sarcomeric proteins can cause cardiomyopathies and skeletal myopathies.