Explain how the structure of a synapse allows a nerve impulse to pass from one neurone to the next.

WJEC A-Level Biology (Wales) — 3.8 The nervous system · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A nerve impulse reaches the synaptic knob of the presynaptic neuron, causing vesicles containing neurotransmitter to fuse with the presynaptic membrane and release their contents into the synaptic cleft. The neurotransmitter then diffuses across the cleft and binds to receptors on the postsynaptic membrane, generating a new nerve impulse in the postsynaptic neuron.

Examiner tips

Use the sequence of events verbatim from the mark scheme; include ‘synaptic knob’, ‘vesicles’, ‘neurotransmitter’, ‘synaptic cleft’, ‘receptors’, ‘postsynaptic membrane’ and ‘new nerve impulse’.
Show the causal link: arrival of impulse → vesicle fusion → release → diffusion → receptor binding → new impulse.
Use correct terminology and UK spelling (neuron, synapse, neurotransmitter).

Common mistakes

Confusing the presynaptic and postsynaptic neuron; writing that the impulse starts in the postsynaptic neuron.
Omitting the synaptic cleft or the diffusion step; describing only vesicle fusion and receptor binding.
Using vague terms like ‘chemical signal’ instead of ‘neurotransmitter’ or ‘receptor’.

Mark scheme (5 marks)

A nerve impulse arrives at the end of the pre-synaptic neurone (synaptic knob / pre-synaptic terminal)
This causes vesicles containing neurotransmitter to move to and fuse with the pre-synaptic membrane, releasing neurotransmitter into the synaptic cleft
The neurotransmitter diffuses across the synaptic cleft
Neurotransmitter binds to receptors on the post-synaptic membrane
This generates a new nerve impulse in the post-synaptic neurone

Key terms in this question

synapse