Question
NEET 2023: Which of the following is correctly matched? (a) Resting potential — membrane more permeable to Na⁺ (b) Depolarisation — voltage-gated K⁺ channels open (c) Repolarisation — K⁺ efflux (d) Hyperpolarisation — Na⁺ influx continues.
Solution — Step by Step
At rest, the membrane is more permeable to K⁺, not Na⁺. That’s why resting potential sits close to K⁺ equilibrium. Option (a) is wrong.
Depolarisation happens because voltage-gated Na⁺ channels open, not K⁺. Option (b) is wrong.
During repolarisation, Na⁺ channels close and K⁺ channels open — K⁺ flows OUT down its gradient, restoring negative interior. Option (c) is correct.
Hyperpolarisation is the brief overshoot below mV because K⁺ channels stay open a bit too long. Na⁺ is NOT still flowing in. Option (d) is wrong.
Final answer: Correct answer: (c) Repolarisation — K⁺ efflux.
Why This Works
The action potential is really just three gates opening in sequence: Na⁺ in (up), Na⁺ close, K⁺ out (down). If you remember this sequence, any NEET MCQ on membrane potential becomes a direct-match exercise.
Alternative Method
Sketch a graph: -axis time, -axis mV. Mark the rising phase (Na⁺ in), peak, falling phase (K⁺ out), undershoot (K⁺ still out). Then map each option to a phase.
Most neural-system numericals come down to unit hygiene and remembering what each gate does at each phase. Draw the action potential graph before reading the question — it saves time.
Common Mistake
Students mix up (b) and (c) because both involve K⁺. Fix: remember that depolarisation = going UP = Na⁺, repolarisation = going DOWN = K⁺.
Do not confuse passive channels (follow the gradient) with active pumps (fight the gradient). This single distinction clears half of all neural-system doubts.