During which period after an action potential is the neuron unable to fire another action potential at normal threshold?

During which period after an action potential is the neuron unable to fire another action potential at normal threshold?

• A. Relative refractory period
• B. Hyperpolarization
• C. Absolute refractory period
• D. Depolarization

Answer: (C)

After an action potential, the neuron cannot fire another one during the absolute refractory period because the voltage-gated sodium channels become inactivated right after the peak. With these channels inactivated, no amount of stimulus can generate the depolarization needed to reach threshold, so another spike cannot occur regardless of how strong the input is. Once the channels recover from inactivation and the membrane repolarizes, a stronger-than-normal stimulus can trigger another action potential, but only during the relative refractory period when some Na+ channels are back to a state where they can open again and the membrane is briefly hyperpolarized. Depolarization is the rising phase of the action potential, not a refractory period, and hyperpolarization describes the temporary undershoot of membrane potential, not the period when firing at normal threshold is impossible.

After an action potential, the neuron cannot fire another one during the absolute refractory period because the voltage-gated sodium channels become inactivated right after the peak. With these channels inactivated, no amount of stimulus can generate the depolarization needed to reach threshold, so another spike cannot occur regardless of how strong the input is. Once the channels recover from inactivation and the membrane repolarizes, a stronger-than-normal stimulus can trigger another action potential, but only during the relative refractory period when some Na+ channels are back to a state where they can open again and the membrane is briefly hyperpolarized. Depolarization is the rising phase of the action potential, not a refractory period, and hyperpolarization describes the temporary undershoot of membrane potential, not the period when firing at normal threshold is impossible.