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

When a resting axonal membrane is stimulated, voltage-gated Na⁺ channels open. The influx of Na⁺ causes depolarisation (inner side becomes positively charged). It is quickly followed by a rise in permeability to K⁺ due to the opening of voltage-gated K⁺ channels. Efflux of K⁺ causes repolarisation of the axonal membrane (inner side again becomes negatively charged).