Nerve impulses at Higher Level: Shared with SL; at HL the same impulse is explained by voltage-gated channels and the threshold (3.5.8), local currents (3.5.9) and the oscilloscope trace (3.5.10).
Practise this as you read
- Outline why a nerve impulse is electrical.
- Label resting and depolarised regions.
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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.
One spike, step by step
At rest
The inside of the axon is negative (about −70 mV).
Depolarisation
When a strong enough stimulus arrives, the membrane depolarises: sodium ions (Na⁺) rush in and the inside flips to about +40 mV.
Repolarisation
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.
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The big idea: A nerve impulse is an action potential that travels along a nerve fibre. It is electrical because it is a movement of positively charged ions: sodium ions flow in, then potassium ions flow out, region by region along the fibre.
The points to remember
- At rest: outside positive, inside negative.
- At the impulse: Na⁺ flows in, the inside becomes positive (depolarised).
- Just behind: K⁺ flows out, the inside is negative again.
- The impulse moves along the fibre, one way.
- It is electrical because positive ions move.
Remember it as: Sodium in, potassium out, and on to the next bit.
Real example: the squid giant axon. The movements of sodium and potassium ions in a nerve impulse were first measured in the giant axon of the squid, which is up to about 1 mm wide.
Trap: The impulse is not an electric current running down a wire: it is a wave of ion movement across the membrane, repeated along the fibre.
How this is tested: Data questions often use the action-potential trace, and written questions ask 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.
Explain 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.
Exam-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]
Step by step
- 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.
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 works: It names the phase (depolarisation), states the ion and direction (Na⁺ in), and explains the voltage change that follows.
For an '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) |
Another question in the papers' style:
Outline why a nerve impulse is described as electrical.
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