What happens when after an action potential depolarizes the plasma membrane of the synaptic ...

during depolarization, a stimulus opens some sodium channels Na+ inflow through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. there are three more steps that happen after depolarization. the first is the "rising" phase of the action potential. Depolarization opens most sodium channels, while the potassium channels remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. Then comes the "falling" phase. most sodium channels become inactivated, blocking Na+ inflow. Most potassium channels open, permitting K+ outflow which makes the inside of the cell negative again. the last step before it returns to the resting state is the "undershoot" phase. the sodium channels close, but some potassium channels are still open. As these potassium channels close and the sodium channels become unblocked. the membrane then returns to its resting state.