During an action potential, depolarisation of the axonal membrane occurs due to the

When a stimulus is applied at a site on the polarised axonal membrane, the membrane at that site becomes freely permeable to Na⁺ due to the opening of many voltage-gated Na⁺ channels. 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 (depolarisation).