Nerve impulses as action potentials

Cells (Interaction and interdependence) · Neural signalling · note 2 of 7

Spec C2.2.3Download all Neural signalling notes (PDF)Download all notes (PDF)12 pages

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.

Common mistake:

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 HL specification · Updated October 2026

Frequently asked questions

Why is the resting potential negative?

The resting potential is negative because sodium–potassium pumps move three Na⁺ out for every two K⁺ in, the membrane lets K⁺ leak out much faster than Na⁺ leaks in, and negatively charged proteins stay inside the cytoplasm. Together these leave the inside of the neuron about 70 mV more negative than the outside.

How does a nerve impulse cross the synapse?

The impulse depolarizes the presynaptic membrane, so calcium ions enter and cause vesicles to release neurotransmitter by exocytosis. The neurotransmitter, such as acetylcholine, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Positive ions enter and depolarize it; if threshold is reached, a new action potential starts.

Why do myelinated neurons conduct impulses faster?

HL only Myelin insulates the axon, so action potentials can only occur at the nodes of Ranvier, where ion pumps and channels are clustered. Local currents spread under the myelin to the next node, so the impulse jumps from node to node. This saltatory conduction is much faster than depolarizing every part of the membrane in turn.

All 5 questions on Neural signalling