Practice question · Multiple choice
During the absolute refractory period no stimulus will fire a second action potential, however strong. Why does raising the stimulus strength not help here, when it is exactly what triggers a spike at rest?
Hints
- A closed channel and an inactivated channel both pass no current. Ask what makes them different.
- One of these states can be reopened by depolarising the membrane further. The other cannot.
Show the answer
B. Because the Na+ channels are inactivated, not merely closed.
Why
Na+ channels have two gates, and that is the whole answer: the stimulus acts on the activation gate, which is no longer the obstacle once the inactivation gate has swung shut. Only repolarisation resets it. A single spike barely dents the ion gradients, so the pump has nothing to do with the timing.
Practise Action Potential
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