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

The action potential is "created" when the positively-charged $S 4$ unit of the voltage-gated $N {a}^{+}$ channel moves outward as a result of the $- 40 m V$ membrane potential. This rapidly depolarizes the membrane, where ${\lim}_{{V}_{m} \to {E}_{N a}} {V}_{m}$.
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 ${\lim}_{{V}_{m} \to {E}_{K}} {V}_{m}$.