Nerve impulse

When a stimulus hits a site on the polarised membrane (site A), the membrane there becomes freely permeable to Na⁺.

Na⁺ rushes in, and the polarity at that site reverses: the outer surface turns negative and the inner surface positive. The site is depolarised.

The potential difference across the membrane at site A is the action potential; this is the nerve impulse.

Just ahead, at site B, the membrane is still positive outside and negative inside. So current flows along the inner surface from A to B, and along the outer surface from B to A, completing the circuit.

This reverses the polarity at B and an action potential forms there: the impulse has moved from A to B. Repeated along the axon, the impulse is conducted.

The rise in Na⁺ permeability lasts only a very short time. It is quickly followed by a rise in K⁺ permeability; within a fraction of a second K⁺ diffuses out, restoring the resting potential at the site, which can then respond to a new stimulus.

The impulse is thus a wave of depolarisation followed by repolarisation travelling along the axon membrane.