There are three gates that open an close to generate the action potential. First, the sodium gate (red top) opens, allowing current to enter the membrane to initiate a depolarization. Later, the sodium inactivation gate closes (red, bottom), truncating the inward flow of sodium At the same time the potassium gate opens (green, top) allowing current to flow out of the membrane, causng a hyperpolarization. The action potential undershoots, because the potasium gate is a bit sluggish and responds to changes in voltage with a delay (Why?) The oscilloscope in the background plots the membrane potential as a function of time as the different gates open and close.

The signal that activates (opens) these channels is the membrane voltage itself, the opening of the channels generates an inward current that affects the membrane voltage. So this is a positive feedback loop that reinforces itself. The events that are locked together in this loop are membrane voltage ---- channel opening----current flow----membrane voltage....and so on.

Inactivation occurs when a globular portion of the protein swings up and occludes the pore

This is a more realistic view of the conformational changes in the channels and the inactivation gate that swings up to occlude the channel