Final answer:
The inactivation of Na+ channels occurs when the inactivation gate closes after depolarization. This prevents more sodium from entering the cell.
Step-by-step explanation:
The activation and inactivation of Na+ channels play a crucial role in the depolarization and repolarization of an action potential. The inactivation of Na+ channels is caused by the closing of the inactivation gate after a specific period of time. This closure occurs during the peak of depolarization, preventing more sodium from entering the cell. After the membrane potential passes a certain threshold, the inactivation gate re-opens, allowing the channel to start the process again.