CVS Practice Test 2026 - Free CVS Practice Questions and Study Guide

The depolarization phase of the pacemaker action potential is primarily due to the influx of calcium ions. In pacemaker cells of the heart, such as those found in the sinoatrial (SA) node, action potentials begin with a gradual depolarization known as the diastolic depolarization phase.

When the membrane potential reaches a certain threshold, voltage-gated calcium channels open, allowing calcium ions to enter the cells. This influx of positive calcium ions is what drives the membrane potential up, causing depolarization. The rapid entry of calcium creates a significant positive change in the membrane potential, leading to the firing of the action potential.

This process is crucial for the heart's automaticity, enabling pacemaker cells to generate rhythmic contractions. Understanding this ion movement is essential for comprehending how the heart's electrical conduction system operates, ensuring the coordinated contraction of the heart muscle.