Nerve impulses as action potentialsSpec C2.2.3
In short
A nerve impulse is an action potential that is propagated along a nerve fibre. An action potential is a rapid reversal of the membrane potential, from negative inside to positive inside, followed by a return to the resting potential. Nerve impulses are electrical because they involve movement of positively charged ions, Na⁺ and K⁺, across the membrane.
A nerve impulse is an action potential that travels along a nerve fibre. At any one point on the fibre the action potential is a brief, rapid change in membrane potential.
- Depolarization: Na⁺ diffuses into the fibre, so the inside becomes positive.
- Repolarization: K⁺ diffuses out, so the inside becomes negative again.
- Each part of the fibre that depolarizes causes the next part to depolarize, so the action potential is propagated along the fibre.
A nerve impulse is electrical because it involves the movement of positively charged ions across the plasma membrane. Electrons do not flow along the axon as they do in a wire.
Do not write that an impulse is 'electricity flowing down the nerve'. Write that an action potential involves movement of Na⁺ and K⁺ ions across the membrane.
Written and checked against the IB Biology SL specification · Updated October 2026