The resting potential is the voltage difference across an axon membrane when it is not transmitting a nerve impulse. The inside of the axon is more negatively charged than the outside.

It is typically around -70 mV in human neurones.

1. Active transport of three sodium ions (Na⁺) out of the axon and two potassium ions (K⁺) into the axon by the sodium-potassium pump
2. K⁺ channels are open, so K⁺ diffuses out of the axon along an electrochemical gradient
3. Na⁺ channels are closed, so Na⁺ cannot diffuse into the axon along an electrochemical gradient

An action potential is the temporary reversal of the resting potential that occurs when a nerve impulse is transmitted. The inside of the axon becomes positively charged.

1. Stimulus - voltage-gated Na⁺ channels open, so more Na⁺ flows into the axon making the inside less negative
2. Depolarisation - if the threshold potential is reached, more Na⁺ channels open causing an influx of Na⁺
3. Repolarisation - Na⁺ channels close and K⁺ channels open, so K⁺ flows out of the axon
4. Hyperpolarisation - an excess of K⁺ leaves the axon, dropping the potential below the resting level
5. Refractory period - ion pumps and channels work together to restore the membrane back to the resting potential

Depolarisation is when the inside of the axon becomes positively charged relative to the outside due to Na⁺ influx.

Repolarisation occurs when K⁺ diffuses out of the axon. This makes the inside of the axon more negatively charged than the outside.

Hyperpolarisation is a temporary overshoot of repolarisation where the inside of the axon becomes more negative than it is at the resting potential.

The threshold potential is the level of stimulus required to trigger an action potential.

The all-or-nothing principle states that if the level of the stimulus is below the threshold potential, no action potential is generated.

Above the threshold potential, an action potential is always generated.

If a stimulus is strong enough to be above the threshold potential, an action potential of the same size is always generated.

Although the strength of the stimulus does not affect the size of the action potential, a stronger stimulus does increase the frequency of action potentials.