Figure 18-11 Phases of cellular action potentials and major associated currents in ventricular myocytes. The initial phase zero spike and overshoot (1) are caused by a rapid inward Na current, the plateau phase (2) by a slow Ca current through L-type Ca channels, and repolarization (phase 3) by outward K currents. Phase 4, the resting potential (Na efflux, K influx), is maintained by Na-K-ATPase. The Na-Ca exchanger is mainly responsible for Ca extrusion. In specialized conduction system tissue, spontaneous depolarization takes place during phase 4 until the voltage resulting in opening of the Na channel is reached. (From LeWinter MM, Osol G: Normal physiology of the cardiovascular system. In Fuster V [ed]: Hurst's The Heart, 10th ed. New York, McGraw-Hill, 2001, pp 63–94.)