What marks the end of the refractory period in cardiac action potential physiology?

The completion of repolarization marks the end of the refractory period in cardiac action potential physiology. During the cardiac cycle, the action potential has distinct phases, and the refractory period is critical for ensuring that the heart muscle has time to recover and prevents the occurrence of spontaneous and potentially dangerous heart rhythms.

At the peak of the action potential, the cardiac muscle is in a depolarized state due to the influx of sodium and calcium ions. As the cardiac action potential progresses, repolarization occurs, restoring the resting membrane potential and allowing the cells to become excitable again. The end of the refractory period coincides with the completion of this repolarization process. Once repolarization is finished, the cardiac muscle can respond to a new stimulus, which is essential for maintaining a proper heart rhythm.

While the other options mention important phases of the cardiac action potential, they do not specifically identify the precise point at which the refractory period concludes. For instance, achieving the resting membrane potential does occur after repolarization but is not the defining moment of the refractory period's conclusion. Therefore, recognizing the significance of repolarization's completion provides a clearer understanding of the cardiac cycle's electrochemical dynamics.