Which channels open first during the action potential phase 0?

During action potential phase 0, the first channels that open are the sodium channels. This phase is characterized by a rapid depolarization of the cardiac myocyte membrane potential. When the membrane potential reaches a certain threshold, voltage-gated sodium channels open, allowing an influx of sodium ions (Na+) into the cell.

This influx causes a dramatic change in the membrane potential, leading it to become more positive and initiating the action potential. The opening of sodium channels is a crucial step in the conduction of electrical impulses in the heart, as it sets off the chain of events that eventually leads to cardiac muscle contraction.

The other channels, such as potassium channels, typically open later during the repolarization phase, while calcium channels contribute to the plateau phase of the action potential but are not the first to open at phase 0. Chloride channels play a minimal role in the action potential process in cardiac cells. Therefore, recognizing that sodium channels are responsible for the initial depolarization helps clarify their critical role in the initiation of the cardiac action potential.