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Figurę 7-9 Voltage-gated Na* channels have two gates that respond in opposite ways to depolarization. In the resting State the activa-tion gate is closed and the inactivation gate is open (1). A depolarizing stimulus results in opening of the activation gate. allowing Na* to flow through the channel. followed by closing of the inactivation gate (2). Once the inactiva-tion gate has closed. the channel enters the inactivated State (3). On repolarization the inactivation gate opens and the activation gate closes as the channel returns to the resting State (1).The channel is only open during the brief period when both the activation and inac-tivation gates are open (2).

Extracellular

side

+ + +

Cytoplasmic

side

Depolaruration

Activation

gate

lnactivaiion

gate

Repolanzation

3 lnactivated (channel closed)

Maintained

depolarization