Muscle Contraction - Cross Bridge Cycle, Animation.

Muscle contraction is fundamental to all skeletal movements. Skeletal muscles are comprised of muscle fibers, which contain repetitive functional units called sarcomeres. Sarcomeres contain overlapping thin (actin) and thick (myosin) filaments.

Muscle contraction occurs when actin and myosin filaments slide past each other, shortening the sarcomere and the muscle. This is known as the sliding filament theory, with cross-bridge cycling forming its molecular basis.

Muscle contraction begins when nerve impulses stimulate muscle fibers, leading to the release of calcium ions. These calcium ions bind to troponin units on the actin myofilaments, displacing tropomyosin and exposing myosin binding sites.

Initially, myosin heads are bound to ADP and phosphate. Upon exposure of binding sites, myosin heads release phosphates and bind to actin. The myofilaments then glide past each other as myosin units move in a head-first motion, releasing ADP and utilizing stored chemical energy.

The gliding motion stops when ATP molecules bind to myosin heads, detaching them from actin. ATP is then decomposed into ADP and phosphate, storing energy in the myosin heads. This allows myosin heads to return to their starting positions and initiate a new cycle if calcium ions are present.