Which phase indicates the end of depolarization?

The end of depolarization in cardiac action potential is indicated by Phase 2. This phase corresponds to the plateau of the action potential, where there is a balance between calcium influx and potassium efflux. During this time, the cell remains depolarized due to the sustained opening of calcium channels that allows calcium ions to enter the cell, while potassium channels start to open, but their effect is not yet dominant. This prolonged depolarization is essential for ensuring adequate contraction of cardiac muscle and allowing sufficient time for the heart to pump blood effectively.

In contrast, Phase 0 represents the rapid depolarization, primarily due to sodium influx. Phase 1 is associated with an initial repolarization phase, where there is a slight return toward a more negative membrane potential as potassium channels open. Phase 3 is primarily where repolarization occurs, with a significant outflow of potassium ions, returning the membrane potential to its resting state. Thus, while the subsequent phases are important for the overall cardiac action potential, Phase 2 is specifically critical for indicating the end of depolarization.