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

The correct answer is that the non-pacemaker action potential consists of five distinct phases. This type of action potential is typically observed in cardiac myocytes and has a more complex series of events compared to pacemaker action potentials.

The five phases are as follows:

1. **Phase 0 (Depolarization)** - In this phase, there is a rapid influx of sodium ions (Na+) due to the opening of voltage-gated sodium channels, leading to a sharp increase in membrane potential.

2. **Phase 1 (Initial Repolarization)** - This phase features a brief repolarization, where sodium channels begin to inactivate and potassium channels open, allowing potassium ions (K+) to exit the cell.

3. **Phase 2 (Plateau Phase)** - A unique aspect of cardiac action potentials, this phase is characterized by a balance between inward calcium ion (Ca2+) flow through calcium channels and outward potassium ion flow. It is crucial for maintaining contraction in cardiac muscle cells.

4. **Phase 3 (Repolarization)** - This phase involves the closure of calcium channels and increased permeability to potassium, allowing for a rapid repolarization back toward the resting membrane potential.

5. **Phase 4 (Resting Mem