Ok, so a nerve impulse is fired if it opens its gates to allow the Na+ in and the K+ out, which move the way they do because they move down their concentration gradient. That makes sense to me.
What I don’t understand is, once they are there, and the impulse has been fired, how do they revert? How is it down their concentration gradient to go the opposite direction?
Through the action of the sodium pump, which entails active transport through the cell membrane.
Yeah, there are several active transport ion channels on the cell membrane. The Na/K ATPase is probably the biggest. It uses ATP (adenosine triphosphate, a source of fuel for several cellular reactions) to move the Na back out and K back in. By moving both at the same time, the charges balance out, so it only works against a concentration gradient, not an electro-chemical one.
It’s also worth noting that calcium also plays a large roll in triggering action potentials in excitable cells. Different cells rely more on Na, some more on Ca.