Biology · Neural Control and Coordination · NEET
| State of membrane | Polarised (charged, at rest) | Depolarised (polarity reversed) |
| Charge inside neuron | Negative | Positive |
| Charge outside neuron | Positive | Negative |
| Ion movement | Na+ kept mostly outside; membrane not permeable to Na+ | Membrane becomes freely permeable to Na+; rapid Na+ influx |
| When it occurs | When no impulse is passing (neuron at rest) | When a stimulus is applied at a site on the axon |
| Also called | Resting potential | Action potential / nerve impulse |
| Recovery | Maintained by ion balance and Na+/K+ pump | K+ moves out to restore resting potential |
At resting potential the OUTER surface of the membrane is positive and the INNER surface is negative. So the inside of the neuron is negatively charged compared to the outside. This charge difference across the resting membrane is called the resting potential. NCERT calls this state polarised.
No. The neuron is very much alive and ready. Resting potential just means no impulse is passing at that moment. The sodium-potassium pump and the ions keep the membrane polarised so the neuron is set to fire the instant a stimulus comes. Think of it as a charged battery waiting, not a dead one.
Sodium (Na+). When a stimulus hits point A on the membrane, that site suddenly becomes freely permeable to Na+. A rapid influx (rush inward) of Na+ flips the charge, making the inside positive and the outside negative. This reversal is the action potential. Potassium (K+) then flows out afterwards to bring the membrane back to resting potential.
Yes. NCERT states clearly that the action potential is in fact termed as a nerve impulse. When the polarity at a site reverses due to Na+ influx, that electrical change is the action potential, and it travels along the axon as the nerve impulse.
Polarised means the two sides of the membrane carry opposite charges in the resting way: outside positive, inside negative (resting potential). Depolarised means that difference is reversed at the stimulated site: outside becomes negative and inside becomes positive because Na+ rushed in. Resting potential = polarised; action potential = depolarised (reversed polarity).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Resting potential is the polarised state of a neuron at rest (inside negative), while action potential is the reversed, depolarised state during a nerve impulse (inside positive due to Na+ influx).
During resting potential the inside is negative and outside positive. During action potential the inside becomes positive and outside negative at the stimulated site because Na+ rushes in.
NEET repeatedly tests the polarised (resting) versus depolarised (action) states and which ion moves in each. Knowing that Na+ influx creates the action potential and K+ efflux restores resting potential lets you answer statement-based and match-the-column questions correctly.
No. Once a segment has fired it enters a refractory phase, so the action potential moves only forward, never backward. This one-way movement is a common NEET point.
After depolarisation, K+ rapidly diffuses out of the neuron. This efflux of K+ restores the resting potential of the membrane at that site, making the fibre ready to fire again.
