Muscle contraction and relaxation theory

Contraction and relaxation of striated muscle appears to be related to the interaction between actin, myosin and ATP. The theory that explains how the actin and myosin work together to cause a forceful shortening of muscle cells is called the sliding-filament theory.

The energy released by the calcium-activated enzymic dephosphorylation of adenosine triphosphate (ATP) brings about muscle contraction through the sliding action of the actin filaments over the myosin filaments, forming contractile actomyosin. On relaxation, the reverse process occurs.

The sliding filament theory suggests the myosin/actin bond moves toward the center of the sarcomere, releasing energy, creating tension, and pulling muscle fibers together, thus creating a muscular contraction.

This cycle of myosin ATP splitting, myosin/actin attaching and pulling continues until the stimulus is no longer active or substrates of energy are depleted.

In this theory, the lengths of the actin an myosin filaments do not change. Rather, myosin cross bridges, or the myosin heads, extend out from the helical part of the molecule, detach, rotate and attach again to the active site on the actin filament, which results in actin being pulled across the myosin filament.
Muscle contraction and relaxation theory