Action Potential

The Action Potential

The action potential comprises activation of first a transient Na current and then a slower K current.The Na current depolarizes the membrane from the resting potential close to the reversal potential for K to close the Na reversal potential.

The cell would then hyperpolarize to resting potential following inactivation of the Na current. This process is speeded up by the activation of the voltage activated K current.

We can start by considering the cell's resting condition. The membrane potential is close to the equilibrium potential for K, because K conductances represent the dominant resting selective conductance across the membrane. Let us consider the case where we insert Na selective channels within the membrane. These are voltage gated channels (they open in response to a positive signed change in membrane potential (depolarization). Consequently, at rest they are closed. In this state the membrane potential is unaltered. Opening these channels, see below, will move the membrane potential toward the equilirium potential of Na.

Closing these Na channels once more will allow the membrane potential to return to resting levels once more. However, as Hodgkin and Huxley observed in the 1950s the membrane potential undershoots the resting potential at the end of the action potential. This is because K channels open activated by depolarization but more slowly than the Na channels. So at the end of the action potential there is an after hyperpolarization. This may be for at least two reasons in axons. The capacitance of the axonal membrane would otherwise slow the return of the membrane potential to rest. In addition, the hyperpolarization will reset the H gate component of the Na channel.

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