Neurons and the resting potential

Cells (Interaction and interdependence) · Neural signalling · note 1 of 4

Spec C2.2.1, C2.2.2Download all Neural signalling notes (PDF)Download all notes (PDF)6 pages

Neurons and the resting potentialSpec C2.2.1, C2.2.2

In short

Neurons are cells within the nervous system that carry electrical impulses. A cell body with the nucleus has fibres projecting from it: one long axon and many shorter dendrites. The resting potential, about −70 mV, is generated by sodium–potassium pumps using ATP to move Na⁺ out and K⁺ in, establishing and maintaining concentration gradients.

Neurons are cells within the nervous system that carry electrical impulses. The cell body contains the cytoplasm and nucleus. Elongated nerve fibres of varying length project from it.

Axon: a long single fibre that carries impulses away from the cell body. Some are over a metre long.
Dendrites: multiple shorter fibres that receive signals from other neurons.
Electrical impulses are conducted along these fibres.

A motor neuron: impulses travel from the dendrites and cell body along the axon to the axon terminals.

The resting potential

A membrane potential is the voltage across a membrane. A neuron that is not conducting an impulse is polarized: there is a charge difference across its plasma membrane, with the inside negative. This is the resting potential, typically about −70 mV.

Sodium–potassium pumps in the plasma membrane use energy from ATP.
Each cycle pumps three Na⁺ out of the neuron and two K⁺ in: the two ions move in opposite directions.
This establishes and maintains concentration gradients: Na⁺ is more concentrated outside, K⁺ more concentrated inside.

More positive charge is pumped out than in (3 Na⁺ out for 2 K⁺ in).
The membrane is much more permeable to K⁺ than to Na⁺, so K⁺ leaks back out through open channels faster than Na⁺ leaks in.
The cytoplasm contains many negatively charged proteins and other organic ions that cannot leave.
Together these make the inside negative relative to the outside.

Axon membrane at rest: high Na⁺ outside and high K⁺ and negatively charged proteins inside; a sodium–potassium pump uses ATP to move 3 Na⁺ out and 2 K⁺ in; an open K⁺ channel lets K⁺ leak out; the outside is positive, the inside negative, and a voltmeter reads the resting potential of about −70 mV. — The resting potential (about −70 mV): pumps set up the Na⁺ and K⁺ gradients and K⁺ leaks out faster than Na⁺ leaks in.

Written and checked against the IB Biology SL 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.