Final answer:
The rising phase of the action potential is terminated by the closure of the Na+ inactivation gate, which occurs after the depolarization peak, preventing further entry of Na+ ions and starting the process of repolarization.
Step-by-step explanation:
The rising phase of the action potential is stopped by the closure of the Na+ inactivation gate. During the depolarization phase of an action potential, the Na+ activation gate opens when the membrane potential crosses -55 mV, allowing Na+ ions to enter the cell. However, shortly after depolarization peaks, the Na+ inactivation gate closes, preventing further influx of Na+ and thus contributing to the peak of the action potential. Following this, K+ channels open, allowing K+ ions to leave the cell, facilitating repolarization.
The inactivation of the Na+ channel is a crucial step in ensuring the transient nature of the action potential and setting the stage for the refractory period. During the refractory period, the neuron cannot fire another action potential until the Na+ inactivation gate is reset to its initial state, allowing the cycle to potentially begin again if the neuron is sufficiently stimulated.