CVS Practice Test 2025 - Free CVS Practice Questions and Study Guide

The pacemaker action potential is characterized by three distinct phases. In pacemaker cells, which are primarily found in the sinoatrial (SA) node of the heart, the action potential is essential for initiating the heart's rhythmic contractions. The three phases of the pacemaker action potential are:

1. **Phase 4 (spontaneous depolarization)**: This phase is unique to pacemaker cells. It represents the gradual depolarization that occurs due to a slow influx of sodium ions and a decrease in potassium conductance. This is what allows these cells to reach the threshold for initiating an action potential without any external stimulus.

2. **Phase 0 (rapid depolarization)**: Once the threshold is reached, there is a rapid influx of calcium ions through voltage-gated calcium channels, leading to a steep rise in the membrane potential, which characterizes this phase.

3. **Phase 3 (repolarization)**: After reaching the peak of depolarization, the cell repolarizes due to the inactivation of calcium channels and an increase in the permeability to potassium ions, allowing potassium to exit the cell. This return to a more negative membrane potential helps reset the cell for the next action potential.

Understanding these phases is