During which phase does early, rapid depolarization occur?

The phase during which early, rapid depolarization occurs is known as Phase 0. This phase is characterized by a swift increase in the membrane potential due to the rapid influx of sodium ions (Na+) into the cardiac myocytes, primarily through voltage-gated sodium channels that open in response to a stimulus. This rapid depolarization is critical for initiating an action potential in cardiac cells and leads to the subsequent phases of the cardiac action potential.

Phase 1 follows Phase 0 and is marked by a brief repolarization due to the closing of sodium channels and the opening of potassium channels, but it is not the phase where early, rapid depolarization occurs. In Phase 2, known as the plateau phase, calcium channels open, allowing calcium ions to enter the cell, leading to sustained depolarization. Phase 3 is characterized by repolarization as potassium ions exit the cell, returning the membrane potential to its resting state.

Understanding these phases is essential for comprehending how the cardiac action potential propagates and how it influences heart contractions, which is fundamental in cardiac rhythm management.