What characterizes the repolarization phase of an action potential?

The repolarization phase of an action potential is primarily characterized by the reestablishment of the polarity of the resting membrane potential. During this phase, after depolarization has occurred (where the inside of the cell became more positive due to sodium ion influx), potassium channels open, allowing potassium ions to flow out of the cell. This efflux of positively charged potassium ions causes the internal charge of the cell to become more negative again, returning toward the resting potential.

This return to resting potential is essential for the neuron to be ready for the next action potential. By restoring the resting membrane potential, the cellular environment becomes ready for the standard ionic gradients that are necessary for normal cellular excitability, enabling the neuron to respond to subsequent stimuli.

In contrast, during the repolarization phase, the sodium concentration inside the cell decreases as sodium channels close, and thus, the option indicating a high sodium concentration remaining inside the cell does not accurately describe this phase. Similarly, while the end of the absolute refractory period does occur as the membrane potential returns to resting levels, it is the restoration of the resting potential itself that best characterizes the repolarization phase. Therefore, reestablishing polarity is a direct and defining characteristic of what happens during repolarization