What is the first phase of the myocardial action potential?

The first phase of the myocardial action potential is characterized by rapid depolarization. This phase occurs due to the influx of sodium ions (Na+) through voltage-gated sodium channels in the myocardial cell membrane. As the membrane potential becomes less negative and crosses a threshold, these channels open rapidly, allowing Na+ to enter the cell. This swift movement of positively charged sodium ions depolarizes the cell membrane, leading to a sharp increase in membrane potential. This initial phase is essential as it sets the stage for subsequent phases of the action potential, which include a plateau phase and repolarization, ultimately allowing the heart muscles to contract effectively.

In contrast, early repolarization, final repolarization, and restoration of resting potential all occur after the rapid depolarization phase and involve different ionic movements and channel activities that contribute to returning the cell to its resting state.