During phase 3 of the action potential in contractile myocardial tissue, which primary process occurs?

During phase 3 of the action potential in contractile myocardial tissue, a key event is the closing of sodium (Na+) gates. This phase corresponds with the repolarization of the myocardial cells, which is essential for restoring the resting membrane potential after the cell has depolarized.

Initially, during phase 0 (depolarization), Na+ channels open, leading to an influx of sodium ions and a rapid rise in membrane potential. However, as the action potential progresses into phase 1 and then to phase 2 (the plateau phase), the Na+ channels quickly become inactivated and close. This closure is critical as it helps limit the influx of Na+, allowing the cell to transition towards repolarization.

In phase 3, while several ionic movements are occurring (including the opening of potassium channels to help restore the negative membrane potential), the primary defining characteristic of this phase is indeed the closing of the Na+ channels. This action, alongside the opening of potassium (K+) channels, facilitates the exit of K+ ions from the cell, which is essential for repolarization and the return to the resting membrane potential. Ultimately, this sequence of channel activity ensures that the heart muscle can effectively contract and then prepare for the next electrical