Q82 / 200 · MHT CET - Medical 2026 (24 April Shift 2) · Biology

Choose the option that shows the correct sequence of the following steps involved in the generation and conduction of a nerve impulse.
I. The number of cations inside the axon increases.
II. Na+ ions diffuse into the neuron from the ECF.
III. Membrane potential changes from -70 mV to about +30 mV.
IV. Application of stimulus make membrane of nerve fibre more permeable to Na+.

  • A
    II $\rightarrow$ I $\rightarrow$ IV $\rightarrow$ III
  • Correct answer
    B
    IV $\rightarrow$ II $\rightarrow$ I $\rightarrow$ III
  • C
    I $\rightarrow$ II $\rightarrow$ IV $\rightarrow$ III
  • D
    III $\rightarrow$ II $\rightarrow$ I $\rightarrow$ IV

Solution

When a stimulus is applied to a nerve fibre, the resting membrane becomes more permeable to $Na^{+}$ ions.

This increased permeability causes $Na^{+}$ ions to rapidly diffuse into the neuron from the extracellular fluid (ECF).

The influx of $Na^{+}$ ions leads to an increase in the number of cations (positive ions) inside the axon.

As a result of this accumulation of positive charges, the membrane potential changes from the resting value of $-70$ mV to about $+30$ mV, generating an action potential (depolarization).

Thus, the correct sequence of events is IV $\rightarrow$ II $\rightarrow$ I $\rightarrow$ III.

Answer: IV $\rightarrow$ II $\rightarrow$ I $\rightarrow$ III

Hence, the correct answer is option: 'B'.

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