Figure 1.

Schematic diagram showing the important phases of the ICC pacemaker cycle as described by the Sanders' Hypothesis. (Phase 1) Discharge of pacemaker units operating at the dominant pacing frequency. (Phase 2) Transient unitary potential depolarizations from the dominant pacemaker unit discharge activates the V_m-dependent Ca²⁺ currents; specifically the dihydropyridine-resistant T-Type Ca²⁺ current (I_Ca(T)). (Phase 3) Ca²⁺ diffuses throughout the cell into pacemaker units that are not discharged during the upstroke phase, resulting in advancement of their oscillatory cycle. The staggered firing of these pacemaker units forms the slow wave's plateau phase. (Phase 4) Cellular repolarization, which has been suggested to occur via activation of the K⁺ currents, from the plasma membrane and intracellular Ca²⁺ handling mechanisms, such as the Ca²⁺ extrusion pumps (I_(Ca(Ext)), restores the cell to its original resting state allowing for repetition of the pacemaker cycle.