Final answer:
Na+ ions enter a neuron through the voltage-gated Na+ channels when an action potential is initiated, involving the sequential opening and closing of activation and inactivation gates in these channels.
Step-by-step explanation:
Na+ ions enter a neuron when an action potential is initiated through the opening of voltage-gated Na+ channels. This happens because these channels have two gates—an activation gate that opens when the membrane potential reaches approximately -55 mV, allowing Na+ to rush into the cell, and an inactivation gate that soon afterwards closes to stop the influx of Na+. When the cell repolarizes and the membrane potential returns to near resting levels, the activation gate closes and the inactivation gate reopens, resetting the channel for the next potential action.