The big idea: A neuron sends a message as an action potential — a brief, fast change in the membrane potential that sweeps along the axon.
At rest the inside of the axon is negative (about −70 mV). When a strong enough stimulus arrives, the membrane depolarises: sodium ions (Na⁺) rush in and the inside flips to about +40 mV.
Straight away the membrane repolarises: potassium ions (K⁺) move out and the inside returns to about −70 mV. That whole up-and-down spike is one action potential — the nerve impulse.
An action potential: the membrane rests at about −70 mV, depolarises to about +40 mV as Na⁺ ions rush in, then repolarises back down as K⁺ ions move out.
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- Membrane potential
- The voltage difference across the axon membrane (between the inside and the outside). It is measured in millivolts (mV).
- Resting potential
- The membrane potential of a neuron that is not firing — about −70 mV, with the inside negative compared with the outside.
- Action potential
- A rapid, temporary reversal of the membrane potential (from about −70 mV to +40 mV and back) that travels along the axon as a nerve impulse.
- Depolarisation
- The phase in which Na⁺ ions enter the axon, making the inside less negative and then positive — the membrane potential rises.
- Repolarisation
- The phase in which K⁺ ions leave the axon, returning the membrane potential back to the resting value.
- Threshold
- The membrane potential that a stimulus must reach to trigger an action potential. Below it, nothing fires; at or above it, a full action potential fires.
Why the names make sense: Depolarise = removing the polarity (the −70 mV difference) as the inside turns positive.
Repolarise = restoring the polarity, bringing the inside back to negative.
The ion that drives each phase is the giveaway: Na⁺ IN to go up, K⁺ OUT to come back down.
An action potential is not one event but a quick sequence of phases, each driven by ion channels opening and closing in turn.
Follow the cause and effect: a stimulus pushes the membrane up to threshold, that opens Na⁺ channels, Na⁺ rushing in depolarises the membrane, then K⁺ channels open and K⁺ leaving repolarises it back to rest.
| Phase | What the ion gates do | Effect on membrane potential |
|---|---|---|
| Resting | Na⁺ and K⁺ gates mostly closed; the membrane is held at about −70 mV | Stays at the resting potential (inside negative) |
| Depolarisation | Voltage-gated Na⁺ channels open; Na⁺ ions rush IN | Rises sharply from −70 mV up to about +40 mV (inside becomes positive) |
| Repolarisation | Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ ions move OUT | Falls back down towards −70 mV (inside negative again) |
| Back to resting | The Na⁺/K⁺ pump restores the original ion balance | Returns to the resting potential, ready for the next impulse |
Depolarisation — Na⁺ rushes IN: Once the stimulus reaches threshold, voltage-gated sodium channels open and Na⁺ ions flood into the axon.
Sodium is positive, so the inside of the axon quickly becomes less negative and then positive — the membrane potential rises from −70 mV to about +40 mV.
This is the steep upstroke you see on the trace.
Repolarisation — K⁺ moves OUT: At the peak the Na⁺ channels close and voltage-gated potassium channels open, so K⁺ ions move out of the axon.
Losing positive ions makes the inside negative again, so the membrane potential falls back towards −70 mV — the downstroke on the trace.
There is often a brief undershoot below −70 mV before the Na⁺/K⁺ pump restores the resting state, ready for the next impulse.
Reading the trace: the rising part is depolarisation (Na⁺ in), the peak is +40 mV, and the falling part is repolarisation (K⁺ out) returning the membrane to its resting potential.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Depolarisation
- Na⁺ ions rush IN
- Voltage-gated Na⁺ channels open
- Inside becomes positive (up to about +40 mV)
- The rising part of the trace
Repolarisation
- K⁺ ions move OUT
- Voltage-gated K⁺ channels open
- Inside becomes negative again (back to −70 mV)
- The falling part of the trace
All-or-none — every spike is the same size: An action potential either fires fully or not at all — this is the all-or-none principle.
A weak stimulus that does not reach threshold triggers nothing. Any stimulus at or above threshold triggers a full action potential, always the same size (up to about +40 mV).
So a stronger stimulus does not make a bigger spike — it makes the neuron fire action potentials more frequently instead. On a graph of responses to increasing stimulus strength, the height of each spike stays constant.
Travelling along the axon (propagation): An action potential in one region depolarises the next region of membrane, which then fires its own action potential, and so on — the impulse is regenerated all the way along the axon without getting smaller.
It travels one way only, because the region just behind is briefly recovering (refractory) and cannot fire again straight away. This keeps the nerve impulse moving away from the cell body towards the axon terminals.
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How this is tested: Paper 1B data questions love the action-potential trace. You might be given an oscilloscope trace and asked to explain the membrane potential at a labelled point (is it resting, depolarising or repolarising?), or to annotate the part of the trace where depolarisation occurs.
A graph of responses to stimuli of increasing strength tests the all-or-none principle — the conclusion is that the size of the action potential stays the same.
On Paper 2, an Explain [up to 7 marks] question asks how a nerve impulse is transmitted along a neuron — work through resting potential, threshold, depolarisation (Na⁺ in), repolarisation (K⁺ out), and propagation to the next region.
IB-style question — explain a point on the trace
An oscilloscope trace of an action potential is recorded. At point Y the membrane potential is rising steeply from −70 mV towards +40 mV. Explain what is happening to the axon membrane at point Y. [3]
How to score all three marks
- Name the phase. Point Y is on the rising part of the trace, so the membrane is undergoing depolarisation.
- Say which channels open and which ion moves. Voltage-gated sodium channels open, so Na⁺ ions move INTO the axon.
- Link the ions to the voltage. Because Na⁺ is positive, the inside of the axon becomes less negative and then positive, so the membrane potential rises from −70 mV towards +40 mV. (Mark 1: depolarisation. Mark 2: Na⁺ channels open / Na⁺ enters. Mark 3: inside becomes positive, so potential rises.)
Final answer
Point Y is depolarisation: voltage-gated Na⁺ channels open and Na⁺ ions move into the axon, making the inside positive, so the membrane potential rises from −70 mV towards +40 mV.
✓ Why this scores full marks: It names the phase (depolarisation), states the ion and direction (Na⁺ in), and explains the voltage change that follows.
For a 3-mark 'explain' on a trace, link the ion movement to the change in membrane potential — do not just say 'the line goes up'.
| Feature | Depolarisation | Repolarisation |
|---|---|---|
| Which ion moves | Sodium (Na⁺) | Potassium (K⁺) |
| Direction of movement | INTO the axon | OUT of the axon |
| Channels involved | Voltage-gated Na⁺ channels open | Voltage-gated K⁺ channels open |
| Membrane potential | Rises (−70 mV → about +40 mV) | Falls (about +40 mV → −70 mV) |
| Inside of axon becomes | Less negative, then positive | Negative again (back to resting) |