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

Eduqas A-Level Biology — 4.2 Inheritance · 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 synaptic cleft.
It binds to specific receptors on the postsynaptic membrane.
Binding opens ion channels, causing a postsynaptic potential that, if it reaches threshold, generates a new action potential.
Thus the impulse is propagated along the nervous pathway.

Examiner tips

Use the sequence: arrival → release → diffusion → binding → postsynaptic potential → new impulse.
Mention Ca²⁺ influx and vesicle fusion to show understanding of release mechanism.
Show that the postsynaptic potential can be excitatory or inhibitory but must reach threshold to propagate.

Common mistakes

Omitting the role of Ca²⁺ and vesicle fusion in neurotransmitter release.
Failing to state that the postsynaptic potential must reach threshold to generate a new impulse.
Using vague terms like "signal" instead of "neurotransmitter" or "action potential."

Mark scheme (5 marks)

When a nerve impulse arrives at the presynaptic neurone, a neurotransmitter is released
The neurotransmitter diffuses across the synaptic cleft
The neurotransmitter binds to receptors on the postsynaptic neurone
This triggers / generates a new nerve impulse in the postsynaptic neurone
This process means the impulse is propagated / continues along the nervous pathway

Key terms in this question

synapse · nerve impulse