Answer: Option D
:
D
When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na⁺.This leads to a rapid influx of Na+ 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. This change in membrane potential spreads to the nearby sites by changing the membrane permeability to Na⁺ in these regions, causing the membrane depolarisation, which is then propagated further down an axon and is in fact termed as a nerve impulse.