sarcomeres

Sarcomeres are the repeating structural and functional units that make up the myofibrils of striated muscle fibers and are responsible for muscle contraction. Each sarcomere extends from one Z-disc to the next, with the M-line at its center. The I-band lies between the A-bands of adjacent sarcomeres and contains only thin filaments, while the A-band contains the entire length of thick filaments. The central region of the A-band houses the H-zone, where only thick filaments are present. Thick filaments are primarily myosin, and thin filaments are primarily actin. Regulatory proteins, tropomyosin and troponin, lie along the actin filament and control access to myosin-binding sites in response to calcium.

During contraction, sarcomeres shorten as thick and thin filaments slide past one another in a process known as the sliding filament mechanism. Cross-bridges form between myosin heads and actin, driven by ATP. The cycle includes attachment, the power stroke, detachment with ATP binding, and re-energizing of the myosin head by ATP hydrolysis. Calcium ions released from the sarcoplasmic reticulum bind to troponin C, causing tropomyosin to move and expose actin's myosin-binding sites, thereby enabling cross-bridge formation.

Neuromuscular coupling initiates contraction: an action potential from a motor neuron triggers acetylcholine release at the neuromuscular junction, depolarizing the muscle fiber membrane and propagating into T-tubules to trigger calcium release. The A-band length remains relatively constant, while the I-band shortens and the H-zone may disappear during maximal contraction. The sarcomere length in relaxed skeletal muscle is typically about 2.2–2.5 micrometers, varying with muscle type.