The absolute refractory period of a neuron corresponds to which phase of the action potential?

• A Repolarisation phase (phase 3) when K+ channels are open
• B Hyperpolarisation (after-potential) phase
• C Threshold potential just before the action potential fires
• D Depolarisation and peak of the action potential when voltage-gated Na+ channels are inactivated (h-gate closed) ✓

Explanation

Voltage-gated Na+ channels transition through three states: closed (resting, activatable), open (during depolarisation), and inactivated (h-gate closed, unable to open). The absolute refractory period occurs while channels are in the inactivated state — during the peak and early repolarisation of the action potential. No stimulus, however strong, can generate another action potential during this period. The relative refractory period corresponds to the later repolarisation/hyperpolarisation phase when some channels have recovered but membrane potential is still negative.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.