Explain how a nerve impulse travels from a sensory neurone to a motor neurone across a synapse, and describe one structural adaptation of a neurone that allows nerve impulses to travel quickly.
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 Ca²⁺ channels open and Ca²⁺ enters the cell. This triggers synaptic vesicles to fuse with the membrane and release the neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft.
It binds to receptors on the postsynaptic membrane, opening ion channels and depolarising the postsynaptic neuron.
The depolarisation generates a new action potential that travels down the motor neuron.
One structural adaptation that speeds up nerve impulses is the myelin sheath around the axon, which provides electrical insulation and allows saltatory conduction between nodes of Ranvier.
The neurotransmitter diffuses across the synaptic cleft.
It binds to receptors on the postsynaptic membrane, opening ion channels and depolarising the postsynaptic neuron.
The depolarisation generates a new action potential that travels down the motor neuron.
One structural adaptation that speeds up nerve impulses is the myelin sheath around the axon, which provides electrical insulation and allows saltatory conduction between nodes of Ranvier.
Examiner tips
- Use the sequence of events in the correct order; include Ca²⁺ influx and vesicle fusion. Mention the specific receptors (e.g., ligand‑gated ion channels). State the adaptation and explain how it increases conduction velocity. Use UK spelling and correct terminology (e.g., ‘synaptic cleft’, ‘postsynaptic’).
Common mistakes
- Confusing the direction of impulse (saying it travels from motor to sensory neuron). Omitting the role of Ca²⁺ or vesicle fusion. Describing the myelin sheath as simply ‘long’ instead of insulating and enabling saltatory conduction.
Mark scheme (5 marks)
- When an impulse arrives at the presynaptic neurone, a neurotransmitter is released into the synaptic cleft
- The neurotransmitter diffuses across the synaptic cleft
- The neurotransmitter binds to receptors on the postsynaptic neurone
- This triggers a new nerve impulse in the postsynaptic neurone
- One structural adaptation named and correctly linked to speed — e.g. the axon has a myelin sheath which provides electrical insulation and speeds up the impulse; OR the axon is long so the impulse travels over large distances without crossing a synapse (which would slow it down); OR dendrites pick up impulses from other neurones (allowing rapid reception)
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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