The nerve impulse

A nerve impulse is a small electric current, a change in voltage, moving along the axon. Neurons can carry it because their membranes are polarised: the inside and outside carry different charges. The membrane has ion channels that let certain ions through and not others.

At rest:

• The membrane lets K⁺ through fairly easily, hardly lets Na⁺ through, and does not let the large negatively charged proteins inside the cell out at all.
• So inside the axon there is a lot of K⁺ and negative protein and little Na⁺; outside there is a lot of Na⁺ and little K⁺.
• A sodium-potassium pump keeps it that way, pushing 3 Na⁺ out for every 2 K⁺ in.
• The result: the outside is positive and the inside negative. This charge difference is the resting potential.

When a stimulus arrives at a point on the membrane (call it A), the membrane there suddenly lets Na⁺ through freely. Na⁺ rushes in, and the charges flip: now the inside is positive and the outside negative. This is depolarisation, and the voltage at that point is the action potential: the nerve impulse.

Current then flows from A to the next point, B, along the inside (and from B to A on the outside), so B flips too, then the next point, and the impulse travels along the axon.

The flood of Na⁺ lasts only a moment. Soon the membrane lets K⁺ flow out, which brings back the resting potential. The fibre is then ready to carry the next impulse.