Biology · Neural Control and Coordination · NEET
At rest the membrane is almost impermeable to Na+, so Na+ is kept mostly outside and its concentration is high outside, low inside. When a stimulus is applied at a point, the membrane at that point becomes freely permeable to Na+. Because Na+ is more concentrated outside AND the inside is negative, both the chemical gradient and the electrical pull push Na+ inward — this rapid inflow is called Na+ influx.
Na+ carries positive charge. As positive Na+ pours in, the inside of the membrane (which was negative at rest) becomes positive and the outside becomes negative. This flip is called reversal of polarity or depolarisation. The membrane potential at that spot is now the action potential.
Sodium ENTERS (moves in). This is the key point students mix up. Na+ influx = depolarisation (rising phase). Later, potassium (K+) moves OUT to restore the resting state (repolarisation). So Na+ in, then K+ out.
They are two different things. During the action potential Na+ rushes in PASSIVELY through open channels (no ATP). The sodium-potassium pump works separately and continuously, using ATP to push 3 Na+ out and 2 K+ in, keeping the resting concentration gradient ready. The pump SETS UP the gradient; the influx USES the gradient.
When one point depolarises (inside becomes positive), it makes the very next point on the axon permeable to Na+. So Na+ influx happens at the next point, then the next — the action potential travels along the axon as a wave of depolarisation.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Passive. During the action potential Na+ flows in through channels down its concentration and electrical gradient, without using ATP. Only the sodium-potassium pump (a separate process) uses ATP.
NCERT describes the resting membrane as polarised, with the outside positive and the inside negative. When Na+ rushes in, this polarity reverses at that site — that reversed state is the action potential.
Right after depolarisation, the membrane becomes more permeable to K+ and less to Na+. K+ diffuses out, so the outside becomes positive again and the resting potential is restored. This is repolarisation.
Neural Control and Coordination is a scoring chapter. NEET repeatedly asks which ion moves in which direction and which phase (depolarisation vs repolarisation) it belongs to. Knowing Na+ IN = depolarisation prevents easy direction-swap traps.
