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How do actin and myosin filaments interact during muscle contraction?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Muscle contraction occurs through the interaction of actin and myosin filaments, a process fundamentally powered by ATP.12

In striated muscle, this interaction is regulated by the proteins troponin and tropomyosin. During relaxation, these proteins inhibit the binding of myosin to actin. When a muscle is activated, calcium ions bind to troponin, causing a structural shift in tropomyosin that exposes the binding sites on actin.13

Once exposed, myosin cross-bridges attach to actin. The subsequent breakdown of ATP provides the energy necessary for the cross-bridge cycle, which involves the mechanical sliding of filaments past one another. ATP binding also facilitates the detachment of myosin from actin, allowing the cycle to repeat. In smooth muscle, regulation involves a different mechanism where calcium activates an enzyme that phosphorylates myosin, enabling it to interact with actin. This sliding-filament process serves as the basis for force generation in living muscle.4521