Biology · Neural Control and Coordination · NEET
Depolarisation is the event; the action potential is the electrical potential difference it creates. NCERT says when a stimulus is applied, the membrane becomes freely permeable to Na+, Na+ rushes in, and the polarity reverses — this reversal is depolarisation, and the potential difference across the membrane at that moment is called the action potential (the nerve impulse). So depolarisation is the cause and the action potential is the measured result of it.
Sodium (Na+) causes depolarisation. When a stimulus hits the membrane, it becomes freely permeable to Na+, and rapid influx (inflow) of Na+ reverses the polarity. Potassium (K+) is for the next step, repolarisation, where K+ diffuses out. A common mistake is swapping the two ions — remember Na+ IN for depolar, K+ OUT for repolar.
The high permeability to Na+ is extremely short-lived. It is quickly followed by a rise in permeability to K+. Within a fraction of a second K+ diffuses outside the membrane, which restores the resting potential at that site. This restoring step is repolarisation, and it makes the fibre responsive to further stimulation again.
During depolarisation the inside became positive because Na+ came in. In repolarisation, K+ leaves the cell, so positive charge moves out again and the outer surface goes back to positive, inner surface back to negative — the same polarised (resting) state. That is why NCERT says K+ diffusion outside 'restores the resting potential of the membrane at the site of excitation.'
Depolarisation comes first (Na+ influx, polarity reversed), then repolarisation follows (K+ outflux, resting potential restored). The order is fixed: stimulus → depolarisation → action potential → repolarisation → resting potential again. NEET often frames this as a sequence-ordering question.
No. The sodium-potassium pump (3 Na+ out, 2 K+ in) maintains the resting ionic gradients before any impulse — it sets up the polarised resting state. Depolarisation is caused by fast Na+ influx through the membrane becoming permeable to Na+, not by the pump. Do not confuse the slow pump (rest) with the fast channels (impulse).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Depolarisation = Na+ influx reverses membrane polarity (makes the action potential); repolarisation = K+ outflux restores the resting potential.
Sodium (Na+) rapidly moves into the axon during depolarisation.
Potassium (K+) moves out of the axon during repolarisation.
Yes. The action potential is the full sequence: depolarisation (rising Na+ permeability) followed by repolarisation (rising K+ permeability) that returns the membrane to resting potential.
NEET frequently tests the exact ion for each phase and the correct order (depolarisation before repolarisation), so mixing up Na+ and K+ or the sequence directly loses marks.
