Actin-myosin interaction in myocardial contractility

A model of the sliding filament theory of muscular contraction. Panel A shows myosin in the relaxed state having just been bound by ATP. Panel B shows the first step by which myosin is energized via hydrolysis of an ATP high energy phosphate bond, resulting in the formation of ADP and inorganic phosphate (Pi). The myosin head then binds actin at the myosin binding site, forming the actin-myosin crossbridge as shown in Panel C. Panel D shows release of ADP and inorganic phosphate, plus flexion of the myosin molecule at its hinge point, causing the filaments to slide past each other. The cross-bridge remains until myosin is again bound by ATP, which allows dissociation of the myosin and actin and relaxation of the myosin molecule. The actin-myosin complex returns to the state in Panel A except that the filaments have now moved relative to each other.