Biology · Neural Control and Coordination · NEET
Yes. NCERT clearly says the electrical potential difference across the membrane at the stimulated site is called the action potential, "which is in fact termed as a nerve impulse." So the two words mean the same event. NEET can ask this directly, so remember: action potential = nerve impulse.
When a stimulus hits a point on the membrane, that point becomes freely permeable to Na+. Sodium ions rush in (influx). Because so many positive Na+ ions enter, the inner side turns positive and the outer side turns negative. This flip of charge is depolarisation.
During the action potential the inside becomes positive and the outside becomes negative. This is the opposite of the resting state. At rest the outer surface is positive and inner surface is negative. So depolarisation reverses the polarity.
At rest the axoplasm has low Na+ and the outside fluid has high Na+, so a concentration gradient exists. The moment the Na+ channels open, Na+ moves down this gradient, from high outside to low inside. No energy is needed for this influx; the gradient does the work.
At site A the polarity is reversed, but the next point (site B) is still resting. A local current flows between A and B. This current makes site B permeable to Na+, so B also depolarises and generates its own action potential. The sequence repeats along the axon, and this is how the impulse is conducted.
The rise in Na+ permeability is extremely short-lived. It is quickly followed by a rise in K+ permeability. K+ diffuses out and restores the resting potential (repolarisation), making the fibre ready for the next stimulus within a fraction of a second.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the reversed electrical charge that appears at a point on the axon membrane when a stimulus makes it permeable to Na+. It is the same as the nerve impulse.
Sodium (Na+). Its rapid influx into the axon causes depolarisation and generates the action potential.
Depolarisation is the reversal of the membrane's resting polarity. The inner side becomes positive and the outer side becomes negative because of Na+ influx.
K+ permeability rises, K+ diffuses out, and the resting potential is restored. This is repolarisation, which prepares the fibre for the next stimulus.
Neural coordination is a high-yield chapter. NEET often tests the direction of Na+ movement, the reversal of polarity, and the fact that action potential equals nerve impulse. One clear diagram usually settles these questions.
