What is the difference between absolute and relative refractory periods?

• A. Absolute: hyperpolarization only; Relative: depolarization only
• B. Absolute: no AP; Relative: AP possible with stronger stimulus during hyperpolarization.
• C. Absolute: only occurs in myelinated fibers; Relative: only in unmyelinated.
• D. Absolute: rapid repolarization; Relative: plateau phase

Answer: (B)

The fundamental idea is how excitability changes after an action potential. In the absolute refractory period, the neuron cannot fire another spike no matter how strong a stimulus is, because the voltage-gated sodium channels are inactivated and cannot reopen until the membrane resets. This covers the entire depolarization and most of the repolarization phase, so the cell is completely unresponsive to new input.

Once the membrane has repolarized and then briefly hyperpolarizes, some of the sodium channels have recovered but the membrane is still more negative than resting. In this relative refractory period, a stronger-than-usual stimulus can trigger another action potential because enough channels are back in a state where they can open, but it requires extra push against the hyperpolarized membrane.

That’s why the statement describing absolute as no AP and relative as AP possible with a stronger stimulus during hyperpolarization is the best fit. The other options mix up the timing or mechanisms (for example, refractory periods aren’t defined by depolarization vs hyperpolarization alone, nor do they depend on whether fibers are myelinated in that simple way, and the plateau phase isn’t the defining feature of these periods).