A2 1 nervous transmission — appears in nearly every A2 1 series · 6 marks
Describe how an action potential is generated in an axon membrane.
Marked instantly, no account needed
A2 · A2 1 Physiology, Co-ordination and Control, and Ecosystems
Action potentials: resting potential, depolarisation and the refractory period
The short answer
At rest the axon sits at about −70 mV, maintained by the sodium–potassium pump and low membrane permeability to sodium. A stimulus opens voltage-gated sodium channels; sodium floods in and the membrane depolarises to about +40 mV. Sodium channels close, potassium channels open, and the membrane repolarises and briefly hyperpolarises.
The sequence, in order
- Resting potential (−70 mV): sodium–potassium pump moves 3 Na+ out for every 2 K+ in; the membrane is more permeable to potassium, so the inside is negative.
- Threshold: a stimulus depolarises the membrane to about −55 mV.
- Depolarisation: voltage-gated sodium channels open, Na+ diffuses in down an electrochemical gradient, the inside becomes positive (about +40 mV). This is positive feedback.
- Repolarisation: sodium channels close, voltage-gated potassium channels open, K+ diffuses out and the potential falls.
- Hyperpolarisation: potassium channels are slow to close, so the potential briefly overshoots below −70 mV before the pump restores the resting state.
All-or-nothing and the refractory period
Below threshold, nothing happens; at or above it, a full-size action potential fires. Stimulus strength is therefore coded by frequency of impulses and by how many neurones are recruited, not by the size of each impulse.
During the refractory period the sodium channels cannot reopen. That does three things worth writing down: it makes impulses unidirectional, it separates discrete impulses, and it sets an upper limit on frequency.
What makes conduction faster
- Myelination: saltatory conduction, with depolarisation jumping between nodes of Ranvier.
- Larger axon diameter: less leakage of ions and lower resistance to the local currents.
- Higher temperature: faster ion diffusion, up to the point where proteins denature.
Across the synapse
Depolarisation at the presynaptic membrane opens calcium channels; calcium entry makes vesicles fuse with the membrane and release acetylcholine by exocytosis. The transmitter diffuses across the cleft, binds to receptors on the postsynaptic membrane, opens sodium channels and depolarises it. Acetylcholinesterase then hydrolyses the transmitter so the response stops. Synapses are unidirectional because only the presynaptic neurone has vesicles and only the postsynaptic membrane has receptors.
Phrases that earn the marks
- voltage-gated sodium channels open
- sodium ions diffuse in down an electrochemical gradient
- depolarisation to about +40 mV
- potassium ions diffuse out, causing repolarisation
- refractory period ensures unidirectional impulses / discrete impulses
- sodium–potassium pump restores the resting potential
Where marks get lost
- Writing 'sodium is pumped in' during depolarisation — it diffuses in through channels.
- Giving the stages out of order, or omitting hyperpolarisation.
- Saying a stronger stimulus makes a bigger action potential.
- Describing the synapse without naming calcium ions or acetylcholinesterase.
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These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on action potential a level biology, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 1 nervous transmission — appears in nearly every A2 1 series · 6 marks
Describe how an action potential is generated in an axon membrane.
Command word: Describe — how to answer itMark scheme — 6 creditworthy points
- Resting potential of about −70 mV maintained by the sodium–potassium pump and differential membrane permeability
- A stimulus causes some voltage-gated sodium channels to open and sodium ions diffuse in
- If the threshold (about −55 mV) is reached, more sodium channels open — positive feedback
- Depolarisation to about +40 mV
- Sodium channels close and voltage-gated potassium channels open, so potassium ions diffuse out — repolarisation
- Hyperpolarisation followed by restoration of the resting potential by the sodium–potassium pump; refractory period ensures one-way transmission
Full-mark answer
At rest the membrane is at about −70 mV, maintained by the sodium–potassium pump moving 3 Na⁺ out for every 2 K⁺ in. A stimulus opens some voltage-gated sodium channels and sodium ions diffuse into the axon. If depolarisation reaches the threshold of about −55 mV, many more sodium channels open in a positive feedback cycle and the membrane potential rises to about +40 mV. The sodium channels then close and voltage-gated potassium channels open, so potassium ions diffuse out and the membrane repolarises. Potassium channels close slowly, causing a brief hyperpolarisation, and the sodium–potassium pump restores the resting potential. During the refractory period no new action potential can be generated, so the impulse travels in one direction only.
Examiner insight: Quote the values (−70 mV, −55 mV, +40 mV) — CCEA credits them. The threshold and all-or-nothing idea is a distinct mark from depolarisation itself.
A2 1 explain question — myelination and saltatory conduction · 4 marks
Explain why an action potential travels faster along a myelinated axon than along an unmyelinated axon of the same diameter.
Command word: Explain — how to answer itMark scheme — 4 creditworthy points
- Myelin sheath is an electrical insulator, so ions cannot cross the membrane where it is present
- Depolarisation can only occur at the nodes of Ranvier
- The impulse jumps from node to node — saltatory conduction
- Fewer depolarisations are needed / less of the membrane must be depolarised, so transmission is faster
Full-mark answer
The myelin sheath acts as an electrical insulator, so sodium and potassium ions cannot cross the axon membrane where myelin is present. Depolarisation can therefore occur only at the nodes of Ranvier, and the local currents cause the impulse to jump from one node to the next in saltatory conduction. Because far less of the membrane has to be depolarised, the action potential travels much faster than in an unmyelinated axon of the same diameter.
Examiner insight: 'Saltatory conduction' by itself is one mark. The final marking point — fewer depolarisations therefore faster — must be stated explicitly to gain the fourth mark.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- What is the resting potential of a neurone?
- About −70 mV inside relative to outside, maintained by the sodium–potassium pump and the membrane's greater permeability to potassium ions.
- Why is an action potential described as all-or-nothing?
- Because it only fires once the threshold of about −55 mV is reached, and every action potential in that neurone is the same size regardless of how strong the stimulus was.
- What is the purpose of the refractory period?
- It stops the impulse travelling backwards, keeps impulses separate, and limits the maximum frequency of firing.
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