AEMCA Practice Exam

What characterizes the terminal phase of rapid repolarization in the cardiac action potential?

1. Phase 3

2. Phase 1

3. Phase 4

4. Phase 0

The correct answer is: Phase 3

The terminal phase of rapid repolarization in the cardiac action potential is indeed characterized by Phase 3. During this phase, the cardiac myocytes experience a significant drop in membrane potential, moving back toward the resting state. This is primarily driven by the inactivation of sodium channels and the opening of potassium channels, which allows potassium ions to flow out of the cell. This efflux of potassium contributes to the rapid decrease in the action potential, effectively repolarizing the cell after the depolarization phase. As Phase 3 progresses, the membrane potential continues to rise until it reaches a level close to the resting membrane potential, preparing the cardiac cells for the next cycle of depolarization. Understanding the distinction between phases is crucial; for example, Phase 1 involves initial repolarization, while Phase 4 is the resting phase after repolarization, and Phase 0 represents rapid depolarization due to calcium and sodium influx. Therefore, recognizing the characteristics and ionic movements involved in each phase is essential for comprehending the overall cardiac action potential dynamics.