Question

The relative refractory period coincides with
A. Depolarizing phase
B. Repolarizing phase
C. Hyperpolarizing phase
D. All the three phases

Answer 1

A refractory period is a time during which an organ or cell is incapable of repeating a particular action or produce a second action potential. The amount of time an excitable membrane takes to be ready for a second stimulus once it returns to its resting state is called the relative refractory period. It most commonly refers to electrically excitable muscle cells or neurons.

Complete answer:
Depolarizing phase: Incorrect
During depolarization, the negative internal charge of the cell temporarily shifts to being positive or less negative. This shift occurs during several processes, including an action potential. This phase is triggered when the channels open to let the positive ions flow into the cell.
Repolarizing phase: Incorrect
In the repolarization stage of an action potential, the cell experiences a decrease of voltage due to the efflux of potassium ions along its electrochemical gradient. At the peak of depolarization, the sodium channels close and potassium channels open. Thus, it occurs after the cell reaches its highest voltage from depolarization.
Hyperpolarizing phase: Correct
In the Hyperpolarizing phase, the membrane potential becomes negative at a particular spot on the neuron's membrane, while in the depolarization phase the membrane potential becomes less negative or more positive.
All three phases: Incorrect
The absolute refractory period corresponds to depolarization and repolarization, whereas the relative refractory period corresponds to hyperpolarization. Thus this statement is incorrect.

Hence, the correct answer is option (C).

Note: The absolute refractory period is when a new action potential cannot be generated no matter how large the stimulus is. At this point, the sodium channels are inactivated. However, in the relative refractory period, a new action potential can be generated under the correct circumstances.