In the resting state, the axonal membrane is nearly impermeable to sodium ions (Na⁺). The fluid outside the axon contains a high concentration of Na⁺ while the axoplasm inside has a low concentration, thus forming a concentration gradient across the membrane.

When a stimulus is applied at a site on the polarised membrane, the membrane at that site becomes freely permeable to Na⁺. This leads to a rapid influx of Na⁺ into the axon. As a result, the polarity of the membrane is reversed — the outer surface becomes negatively charged while the inner side becomes positively charged. This reversal of polarity is called depolarisation.

The electrical potential difference across the plasma membrane at this site is called the action potential, which is in fact termed as a nerve impulse. The rise in stimulus-induced permeability to Na⁺ is extremely short-lived.