Figure 5. Schematic depiction of normal cardiac action potential, as described in text.

The fast movement of positive Na+ ions into the myocyte depolarizes the resting membrane potential and initiates an action potential. This is followed by outwardly directed potassium currents: Ito, the initial outward current and Ik, the delayed rectifier current which move positive K+ ions out of the cells repolarizing the myocytes back to their resting membrane potential. An inward calcium current, ICa,L temporarily delays the repolarization of the membrane potential by bringing positive Ca2+ into the myocytes producing the plateau of the action potential; this is called the L-type Ca2+ current. The orderly occurrence of these currents creates the action potentials, which couple the electrical and mechanical functions of the heart resulting in its rhythmic contractions. Fatal arrhythmias occur when the electrical signals become chaotic and the heart can no longer function as a pump.