cardiac action potential, the transmembrane potential in the heart, consisting of five phases: 0, the upstroke or rapid depolarization, which initiates the heartbeat in response to an influx of Na+; 1, early rapid repolarization; 2, plateau in response to an influx of Ca2+; 3, final rapid repolarization in response to an influx of K+; and 4, resting membrane potential and diastolic depolarization. Abnormalities of the heart or its conduction system that alter the cardiac action potential lead to the development of cardiac arrhythmias.
