Membrane 0 Potential

Pulse 1

Pulse 2

1 msec

FIGURE 19 Recovery from inactivation. The second of two depolarizing pulses activates less Na+ conductance as the interval between the pulses decreases. Once the Na+ channels become inactivated by the first pulse, several milliseconds are required before they recover completely.

one action potential, if one attempts to initiate a second action potential soon after the first, the Na+ permeability will not have recovered from inactivation, making a second depolarization ineffective in initiating a voltage-dependent change in Na+ permeability. If there is no voltage-dependent change in Na+ permeability, no action potential will be produced. Thus, the absolute refractory period is understood most simply in terms of this process of recovery from Na+ inactivation. The recovery from Na+ inactivation may contribute to the relative refractory period as well. During this recovery time, the threshold for an action potential will be higher because greater depolarization will be required to activate sufficient Na+ influx to exceed the K+ efflux.

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