The Physiologic Basis of the Cardiac Action Potential

(Adapted from Klabunde RE, ed. Cardiovascular Physiology Concepts. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011.)

Phase 0 represents rapid depolarization as a result of opening of Na+ channels and closing of K+ channels. Phase 1 is the period of initial repolarization that results from closure of Na+ and opening of K+ channels. Phase 2 is the plateau phase that results from the sustained Ca++ current that began with the initial depolarization. Phase 3 is repolarization due to opening of K+ and closure of Ca++ channels. Phase 4 is the resting potential during which time K+ channels are open and Na+ and Ca++ channels are closed. The effective refractory period (ERP) is the time during which the cell cannot be depolarized again.