Myocardium is the involuntary, striated muscle tissue in the heart between the epicardium and the endocardium. The primary structural proteins of the cardiac muscle are actin and myosin filaments, which interdigitate and slide along each other during contraction in a manner similar to skeletal muscle.

Cardiac Action Potential. At the initiation of an action potential in a cardiomyocyte, the cell membrane rapidly depolarizes as the transmembrane potential rises from -85 mV to +20 mV. The membrane remains depolarized for about 0.2 second, the plateau phase, which is then followed by rapid repolarization. During the plateau phase, the cardiomyocyte cannot be restimulated for about 0.25 to 0.3 second, called the refractory period. This is followed for additional 0.05 second by the relative refractory period, when the myocardium can only be stimulated by a strong excitatory signal.