Final answer:
Na⁺ ions enter a neuron through voltage-gated Na⁺ channels when an action potential is initiated, initiating the depolarization process of the neuron; this influx of Na⁺ ions is critical for the propagation of the action potential. Option number 3 is correct.
Step-by-step explanation:
Na⁺ ions enter a neuron during the initiation of an action potential primarily through voltage-gated Na⁺ channels. These channels have two gates—an activation gate and an inactivation gate. The activation gate opens when the membrane potential reaches approximately -55 mV, allowing Na⁺ to rush into the neuron, causing depolarization. Following a brief period, the inactivation gate closes to stop the influx of Na⁺, aiding in repolarization as K⁺ channels open and potassium ions exit the cell.
After the action potential has passed, the Na⁺/K⁺-ATPase helps to restore the resting membrane potential by pumping Na⁺ out of the cell and K⁺ back in, although this pump is not directly involved in the action potential initiation.