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  4. Nerve fibre gets depolarized when it acquires

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Q. Nerve fibre gets depolarized when it acquires

UP CPMTUP CPMT 2008

Solution:

When a stimulus of adequate strength (threshold stimulus) is applied to a polarized membrane, the permeability of the membrane to $Na^+$ ions is greatly increased at the point of stimulation. It is due to the fact that the $Na^+$ channels open and the $K^+$ channels remain closed. As a result, sodium channels permit the influx of $Na^+$ ions by diffusion. Since there are more $Na^+$ 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 $Na^+$ ions continue to rush inside. Entry of $Na^+$ leads to depolarization (reversal of polarity) of the nerve membrane, so that the nerve fibre contents become electropositive with respect to the extracellular fluid.