Which phase of the cardiac action potential is characterized by calcium entry?

In the context of the cardiac action potential, Phase 2 is the phase characterized by significant calcium entry. This particular phase, also known as the plateau phase, sees the influx of calcium ions through L-type calcium channels. The entry of calcium is crucial as it helps to maintain depolarization of the cardiac myocytes and plays a vital role in initiating muscle contraction by triggering the release of additional calcium from the sarcoplasmic reticulum.

In contrast, Phase 1 involves a transient outward potassium current that contributes to the initial repolarization. Phase 3 is primarily characterized by potassium efflux, leading to repolarization of the cell, and Phase 4 represents the resting membrane potential, where the cell is in a polarized state and prepares for the next action potential. Thus, Phase 2 is distinct in that it is during this period that calcium entry predominates, making it critical for cardiac muscle contraction.