Why don't sodium Voltage Gated Channels open during Repolarization?
Voltage-gated sodium channels have three basic states that represent different conformations of the protein: closed, open, and inactivated.
When a voltage-gated sodium channel is open at depolarized voltage, there is space for part of the channel to shift into a position that blocks the pore. When this happens, it gets stuck blocking the flow of sodium, and it requires a negative membrane potential to remove the block. We call this the &quot;inactivated&quot; state, and this model is called the ball and chain model of sodium channel inactivation.
The inactivation gating process is also referred to as the &quot;h-gate&quot;, because h is the variable representing the gating in the classic Hodgkin-Huxley model.
Wikipedia currently has a useful table on the sodium channel page showing the different states that a sodium channel goes through during different phases of an action potential.