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

WJEC A-Level Biology (Wales) — 4.2 Inheritance · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

When an impulse reaches the presynaptic terminal, voltage‑gated calcium channels open, allowing Ca²⁺ to enter and trigger vesicle fusion, releasing neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft.
It binds to specific receptors on the postsynaptic membrane.
Binding opens ion channels, causing a change in membrane potential.
This depolarisation initiates a new nerve impulse in the postsynaptic neuron, propagating the signal.

Examiner tips

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

Common mistakes

  • Omitting the calcium‑mediated vesicle fusion step.
  • Confusing the synaptic cleft with the cell membrane.
  • Failing to state that the postsynaptic impulse is a depolarising event rather than just a generic signal.

Mark scheme (5 marks)

  1. When an impulse arrives at the presynaptic neurone, a neurotransmitter is released
  2. The neurotransmitter is released into the synaptic cleft
  3. The neurotransmitter diffuses across the synapse
  4. The neurotransmitter binds to receptors on the postsynaptic neurone
  5. This triggers a nerve impulse in the postsynaptic neurone, propagating the impulse

Key terms in this question

synapse

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