During which phase of an action potential does depolarization occur?

Depolarization during an action potential is primarily characterized by the rapid influx of sodium ions (Na+) into the neuron. This process occurs when voltage-gated sodium channels open in response to a change in the membrane potential, allowing Na+ to flow down its electrochemical gradient into the cell. As Na+ enters, the inside of the neuron becomes more positively charged compared to the outside, leading to depolarization, which is essential for the propagation of the action potential along the axon.

When considering the context of an action potential, it’s important to note that depolarization is a key phase, occurring after the membrane potential has reached a certain threshold. At this point, the influx of Na+ quickly shifts the membrane potential from a negative value toward a more positive value, thereby signaling the start of the action potential.

The other phases mentioned, such as after K+ exits the cell or once the membrane is hyperpolarized, refer to different aspects of the action potential or the subsequent repolarization phase, which is not related to depolarization itself. Thus, the focus on Na+ entering the cell clearly identifies it as the driving force behind the depolarization phase of the action potential.