Question

Given is the diagrammatic representation of impulse conduction through an axon (at points A and B). Study the diagram and arrange the events in correct order.

I. The polarity of the membrane at site A is reversed and depolarized.
II. A stimulus causes disturbance in the membrane at site of a nerve fibre resulting in entry of Na+ ions inside the nerve fibre.
III. On the outer surface, current flows from site B to site A to complete the circuit of current flow.
IV. Immediately ahead, the axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface.
The correct option is:
(correct answer + 1, wrong answer - 0.25)

• A. I $\to$ II $\to$ IV $\to$ III
• B. II $\to$ I $\to$ III $\to$ IV
• C. II $\to$ I $\to$ IV $\to$ III
• D. I $\to$ IV $\to$ III $\to$ II

Answer 1

The correct option is C II $\to$ I $\to$ IV $\to$ III

A stimulus causes disturbance to the membrane at site of a nerve fibre resulting in entry of Na+ ions inside the nerve fibre.

The polarity of the membrane at site A is reversed and depolarized, i.e., the outer surface becomes negatively charged and the inner side becomes positively charged generating a nerve impulse.

Immediately ahead, the axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B.

On the outer surface, current flows from site B to site A to complete the circuit of current flow. Hence, the polarity at the site is reversed, and an action potential is generated at site B. The impulse (action potential) generated at site A arrives at site B.

The sequence is repeated along the length of the axon and consequently the impulse is conducted.

II $\to$ I $\to$ IV $\to$ III