Depolarisation of neuron is due to

When a stimulus is applied at a site on the polarised membrane, the membrane at the site of stimulus becomes freely permeable to Na⁺. Due to rapid influx of Na⁺ ions, reversal of the polarity occurs on the axonal membrane, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. As the polarity is reversed, it is said to be depolarised.