Question

If depolarizing membrane potentials open voltage-gated sodium channels, what closes them?

Answer 1

Actually, EPSP's push the membrane potential to threshold, which initiates an action potential.

The action potential is "created" when the positively-charged `S4` unit of the voltage-gated `Na^+` channel moves outward as a result of the `-40mV` membrane potential. This rapidly depolarizes the membrane, where `lim_(V_m to E_(Na))V_m`.

Then, they nearly instantaneously inactivate via an inactivation gate closing the channel intracellularly.

Contemporaneously, voltage-gated potassium channels (mainly the delayed rectifiers) open as a result of `V_m > 0`. These facilitate a large efflux of `K^+`, where `lim_(V_m to E_K)V_m`.

I like cellular-molecular neuroscience. :)