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The graph in the question stem illustrates an action potential typical of cardiac pacemaker cells, such as those found in the sinoatrial node.  These action potentials are different from non-pacemaker cardiac action potentials in that phases 1 and 2 are absent and phase 4 represents a slow depolarization phase.

Pacemaker cells are unique in that they exhibit automaticity (an inherent ability to depolarize without any external influences).  Automaticity is made possible by a inward, mixed sodium-potassium current (the funny current) that occurs during phase 4 of the action potential (Choice E).  This current gradually brings the membrane potential closer to threshold, at which point phase 0 depolarization occurs due to a large increase in calcium influx (arrow).  Pacemaker cell depolarization is then followed by phase 3 (repolarization), which is caused by an outward potassium current (Choice B).

In cardiomyocytes and Purkinje cells (non-pacemaker cells), phase 4 corresponds to the resting membrane potential and is typically stable at approximately -90 mV (near the equilibrium potential of potassium).  Phase 0 of the non-pacemaker action potential is mediated by rapid sodium influx (Choice A), in contrast to the slower calcium-mediated depolarization of pacemaker cells.  Phase 1 is an early short repolarization occurring immediately after depolarization that is caused by increased outflow of potassium and decreased sodium conductance.  Phase 2 is the plateau phase of the action potential, during which an inward calcium current counteracts the outward movement of potassium ions that started in phase 1 (Choice F).  Phase 3 (repolarization) occurs as the inward calcium current ceases while potassium continues to rush out of the cell.

Educational objective:
In cardiac pacemaker cells, phase 0 depolarization is mediated by an inward flux of calcium.  This differs from phase 0 of cardiomyocytes and Purkinje cells, which results from an inward sodium current.