Question

Nerve fibre gets depolarized when it acquires

• A. +ve charges on inner side
• B. +ve charges on outer side
• C. -ve charges on inner side
• D. $N{a}^{+}$ and $K^{+}$ from outside

Answer 1

Answer: (A)

When a stimulus of adequate strength (threshold stimulus) is applied to a polarized membrane, the permeability of the membrane to $N{a}^{+}$ ions is greatly increased at the point of stimulation. It is due to the fact that the $N{a}^{+}$ channels open and the $K^{+}$ channels remain closed. As a result, sodium channels permit the influx of $N{a}^{+}$ ions by diffusion. Since there are more $N{a}^{+}$ ions entering than leaving. the electrical potential of the membrane changes from -70 mV towards zero. At 0 mV the membrane is said to be depolarized. Throughout depolarization, the $N{a}^{+}$ ions continue to rush inside. Entry of $N{a}^{+}$ leads to depolarization (reversal of polarity) of the nerve membrane, so that the nerve fibre contents become electropositive with respect to the extracellular fluid.