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
Threshold
A stimulus pushes the membrane up to threshold.
Na⁺ channels open
Reaching threshold opens Na⁺ channels.
Depolarisation
Na⁺ rushing in depolarises the membrane.
K⁺ channels open
Then K⁺ channels open.
Repolarisation
K⁺ leaving repolarises the membrane back to rest.
Threshold → Na⁺ in → K⁺ out → 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.
The undershoot: 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.
Below threshold
- A weak stimulus that does not reach threshold triggers nothing
At or above threshold
- Any stimulus at or above threshold triggers a full action potential
- Always the same size (up to about +40 mV)
A stronger stimulus: 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.
Why it travels one way: 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.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
How this is tested: Paper 1B data questions love the action-potential trace, and Paper 2 asks you to explain how a nerve impulse travels.
Reading the 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.
Stimuli of increasing strength: 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.
Paper 2: Explain [up to 7 marks] how a nerve impulse is transmitted along a neuron
Resting potential
Start with the axon at rest.
Threshold
A stimulus reaches threshold.
Depolarisation
Na⁺ in.
Repolarisation
K⁺ out.
Propagation
The next region is depolarised and fires in turn.
Work through every stage in order.
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) |
See how examiners mark answers
Answer exam-style questions with model answers. Learn exactly what earns marks and what doesn't.
How this is tested: Three question types come up: a Paper 2 Outline, a 1-mark Identify, and a Paper 1A data-reasoning item.
What each one needs
- Paper 2, 2-mark Outline: how saltatory conduction transmits a nerve impulse — say the impulse is regenerated only at the nodes of Ranvier and so jumps from node to node
- 1-mark Identify: a myelinated neuron is shown and you say where depolarization occurs — at the nodes (the gaps in the myelin), not under the sheath
- Paper 1A data-reasoning: cross-sections of different axons — say which conducts most slowly (the thin, unmyelinated one), or propose why one fibre is faster (thicker / more myelin)
IB-style question — outline saltatory conduction
Outline how saltatory conduction transmits a nerve impulse along a myelinated axon. [2]
How to score both marks
- Where the impulse forms. The myelin sheath insulates the axon, so the action potential (depolarization) can only be regenerated at the nodes of Ranvier — the gaps in the sheath.
- How it moves. Because of this, the impulse 'jumps' from one node to the next rather than moving continuously, which makes conduction much faster. (Mark 1: depolarization / action potential only at the nodes of Ranvier. Mark 2: impulse jumps node to node — faster than continuous.)
Final answer
The myelin insulates the axon, so the action potential is regenerated only at the nodes of Ranvier; the impulse therefore jumps from node to node, which is faster than continuous conduction.
Why this scores full marks: Both scoring ideas are present: the impulse is only at the nodes of Ranvier, and it jumps between them.
An answer that just says 'it goes fast' without naming the nodes and the jumping would not score the Outline marks.
IB-style question — draw and label a myelinated neuron
Draw a labelled diagram of a myelinated motor neuron and mark where depolarization occurs during an action potential. [3]
How to score all three marks
- Draw the axon and its sheath. Show the long axon carrying the impulse, wrapped in segments of myelin sheath with gaps between them.
- Label the gaps. Label those gaps as the nodes of Ranvier, and label the fatty myelin sheath segments.
- Mark depolarization. Mark that depolarization (the action potential) occurs at the nodes of Ranvier, not under the myelin. (Mark 1: axon with myelin segments and gaps. Mark 2: nodes of Ranvier + myelin sheath labelled. Mark 3: depolarization marked at the nodes.)
Final answer
A myelinated axon with the myelin sheath labelled, the gaps labelled as nodes of Ranvier, and depolarization shown occurring at those nodes.
Depolarization (the action potential) only happens at the nodes of Ranvier — the gaps in the myelin. The impulse 'jumps' from one node to the next, so it travels much faster than along a bare axon.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
| Feature | Myelinated axon | Unmyelinated axon |
|---|---|---|
| Myelin sheath | present (fatty, insulating) | absent |
| Where the impulse forms | only at the nodes of Ranvier (gaps) | continuously, all along the membrane |
| Type of conduction | saltatory ('jumping' node to node) | continuous |
| Speed of impulse | fast | slow |