Resting membrane potential: Voltage-gated Na+ channels are in the resting state and voltage-gated K+ channels are closed.
Stimulus causes depolaritzation to threshold
Absolute refractory period:
a) Voltage-gated Na+ channel activation gates are open
b) Voltatge-gated K+ channels are open; Na+ channels are inactivating
Relative refractory period: voltage-gated K+ channels are still open; Na+ channels are in the resting state.
Resting state:
All voltage-gated Na+ and K+ channels are closed.
Depolarizing phase:
Depolarization to threshold open Na+ channel activation gates. Na+ inflow further depolarizes the membrane, opening more Na+ channel activation gates.
Repolarizing phase:
Na+ channel inactivation gates close and K+ channels open. Outflow of K+ causes repolarization.

Repolarization continues:
K+ outflow restores resting membrane potential. Na+ channel inactivation gates open. Return to resting state when K+ gates close.