What is the absolute refractory period?

• A. The time immediately before an action potential occurs.
• B. The period during which a second action potential can be initiated with a strong stimulus.
• C. The period after an action potential during which no new action potential can be initiated, regardless of stimulus.
• D. The period of hyperpolarization following repolarization.

Answer: (C)

The absolute refractory period is the brief time after an action potential during which a new one cannot be fired, no matter how strong a stimulus. This happens because the voltage-gated sodium channels that opened to depolarize the membrane become inactivated and cannot reopen until they reset to their resting state. Until those channels recover, the membrane can’t reach threshold again, so another action potential cannot occur. Once repolarization proceeds and some channels recover, the neuron enters the relative refractory period, where a stronger stimulus can trigger another AP but requires a larger depolarization due to the membrane still being hyperpolarized and many Na+ channels still recovering.