question_answer

Given is the diagrammatic representation of impulse conduction through an axon (at points A and B). View the diagram and arrange the steps  of impulse conduction.

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

II. A stimulus causes disturbance to the membrane at site of A nerve fibre resulting in leakage of \[N{{a}^{+}}\] 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. 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.

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. As a result, a current flows on the inner surface from site A to site B.

The correct option is -

A) \[I\to II\,\,\to IV\,\to \,\,III\]

B)   \[II\,\,\to \,\,I\,\,\to III\to IV\]

C) \[II\,\,\to \,\,I\to IV\,\,\to \,\text{ }III\]

D)   \[I\,\,\to \,\,IV\,\,\to \,\,III\,\,\to \,\,II\]

Correct Answer: C

Solution :

You might be curious to know about the mechanisms of generation of nerve impulse and its conduction along an axon. When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to\[N{{a}_{+}}\]. This leads to a rapid influx of \[N{{a}_{+}}\] followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at the site A is thus reversed and hence depolarised. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as a nerve impulse. At sites 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.