# Action potential

A brief electrical signal travelling along a neuron. At rest, the inside is negative (about -70 millivolts). A stimulus opens sodium channels, sodium rushes in and the cell depolarises. Then potassium channels open, potassium leaves and the neuron resets.

Biology (university) · Part 09: Animal physiology · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A line of dominoes falling in order, each tipping the next.

## Example

**Action potential steps**

Rest: about -70 mV.
Na+ in: up to about +40 mV.
K+ out: falls back.
Pump restores ion balance.

## In practice

The signal is all-or-nothing: it either fires at full size or not at all. Stronger stimuli fire more often, not bigger.

## Practice

### Flashcards

- **What causes depolarisation in an action potential?** Sodium ions rushing into the neuron.

### Interview questions

**Describe how a nerve impulse travels along a neuron and across a synapse.**

- Resting potential
- Sodium in, depolarisation
- Potassium out, repolarisation
- Calcium triggers neurotransmitter release
- Receptors on the next cell

## Questions people ask

### Why is the sodium-potassium pump important for nerves?

The pump restores the resting balance of ions after each signal by moving sodium out and potassium in. Without it, ion gradients would gradually run down and the neuron could no longer fire. Each action potential only moves a tiny number of ions, so the pump is needed over the long term.

### What is myelin and why does it speed up signals?

Myelin is a fatty sheath wrapped around many axons. It insulates the axon, so the signal jumps between small gaps in the sheath called nodes of Ranvier. This jumping, called saltatory conduction, is much faster than travelling along bare axons. Multiple sclerosis damages myelin.

## Related concepts

- [Synapse](https://studyvisually.com/learn/science/biology-university/biology-university-part-9/biology-university-part-9-concept-3)

Web page: https://studyvisually.com/learn/science/biology-university/biology-university-part-9/biology-university-part-9-concept-2
Study this card in the app: https://studyvisually.com/app/learn?category=biology-university&card=biology-university-part-9-concept-2