Explain how a nerve impulse is transmitted from one neurone to the next across a synapse.

WJEC A-Level Biology (Wales) — 4.3 Variation and evolution · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

When a nerve impulse reaches the presynaptic terminal, voltage‑gated Ca²⁺ channels open, Ca²⁺ enters and triggers vesicle fusion, releasing neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane.
Binding opens ion channels, causing a change in membrane potential that, if it reaches threshold, generates a new action potential.
Thus the impulse is propagated to the next neuron and continues along the pathway.

Examiner tips

  • Use the sequence: arrival → Ca²⁺ influx → vesicle fusion → release → diffusion → receptor binding → postsynaptic potential → new impulse.
  • Mention voltage‑gated Ca²⁺ channels and ion channel opening to show understanding of the mechanism.

Common mistakes

  • Omitting the role of Ca²⁺ or ion channels; describing only neurotransmitter release without the trigger for the postsynaptic impulse.
  • Using vague terms like ‘signal’ instead of ‘action potential’ or ‘nerve impulse’.

Mark scheme (5 marks)

  1. When a nerve impulse arrives at the end of the presynaptic neurone, a neurotransmitter is released
  2. The neurotransmitter diffuses across the synaptic cleft
  3. The neurotransmitter binds to receptors on the postsynaptic neurone
  4. This triggers a new nerve impulse in the postsynaptic neurone
  5. This means the impulse is propagated / continues along the nervous pathway

Key terms in this question

synapse

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