Explain why the voltage -gated Na+ channels can close while the cell is depolarized even though the depolarization was the initial stimulus for the channel opening. Include a discussion on refractory periods and explain why action potentials travel in only one direction.

Question

What will be an ideal response?

Answer 1

The channels have two gates: activation and inactivation gates. (See Fig. 8.10.) At rest, the activation gate is closed
and the inactivation gate is open. Upon depolarization both gates move: the activation gate opens allowing Na+ to
enter the cell and the inactivation gate (with a delay of 0.5 msec) closes stopping the influx of Na+.
At this point during the action potential, the peak has been reached and repolarization occurs due to the K+ ions
leaving the cell. During this time, even if another wave of depolarization occurred, the Na+ channels cannot be
opened because the activation gate is already open and the inactivation gate is closed. This is the absolute
refractory period, when another action potential absolutely cannot occur because the Na+ channels have not reset to
their original positions. The relative refractory period occurs after some of the Na+ channels have reset, but a higher
than normal depolarizing graded potential is necessary to cause another action potential.
Refractory periods also explain why action potentials cannot move backward. (See Fig. 8.15.) The part of the axon
experiencing the action potential has open Na+ channels. An increase in Na+ inside the cell causes depolarization
and perpetuates the action potential toward the axon terminal due to local current flow. The area of the axon
toward the trigger zone, where the action potential (AP) occurred a moment earlier, is in the absolute refractory
period and will not experience another action potential even with a depolarization.