CBSE Class 10 Answered

The normal membrane potential inside the axon of nerve cells is –70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called the action potential:

An action potential has 2 main phases called depolarisation and repolarisation. (At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron.)

Depolarisation: A stimulus can cause the membrane potential to change a little. The voltage-gated ion channels can detect this change, and when the potential reaches –30mV the sodium channels open for 0.5ms. The causes sodium ions to rush in, making the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside).

Repolarisation: At a certain point, the depolarisation of the membrane causes the sodium channels to close.  As a result the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation.  As the polarity becomes restored, there is a slight ‘overshoot’ in the movement of potassium ions (called hyperpolarisation).  The resting membrane potential is restored by the Na+K+ATPase pump.