CVS Practice Test

In phase 4 of the pacemaker action potential, the notable characteristic is the gradual depolarization that occurs due to the influx of sodium ions. This phase is often referred to as the "resting potential" in pacemaker cells. During this time, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are permeable to sodium, open. The opening of these channels allows sodium ions to enter the cell, leading to a slow depolarization that eventually reaches the threshold required for the next action potential.

While calcium influx plays a significant role during the later phases of the pacemaker potential (specifically phase 0), it is the influx of sodium that primarily drives the depolarization seen in phase 4. Potassium ions also play a crucial role in repolarization during earlier phases but do not contribute to the depolarization in phase 4. Chloride ions typically do not have a significant influence in the pacemaker action potential. Thus, the correct answer highlights the importance of sodium ion influx in facilitating the gradual depolarization that sets the stage for the next heartbeat.