CVS Practice Test

In phase 3 of the non-pacemaker action potential, potassium efflux is the primary type of ion movement. During this phase, the voltage-gated potassium channels open, allowing potassium ions to move out of the cell. This movement is crucial because it helps to repolarize the cell membrane after depolarization, which occurs primarily during phase 0 due to sodium influx.

As potassium exits, it counters the positive charge that had entered the cell, leading to a return toward the resting membrane potential. This efflux of potassium ions is essential for restoring the negative charge inside the cell and is key in preparing the membrane for the next action potential.

The other ion movements described do not characterize phase 3. For instance, calcium influx is primarily associated with earlier phases such as phase 2, while sodium influx occurs in phase 0. Chloride influx does not play a significant role during any phase of the non-pacemaker action potential. Therefore, the primary movement of potassium ions out of the cell during phase 3 is critical for the repolarization process and restoring resting potential, making it the correct answer.