Describe the roles of the sensory neurone, relay neurone and motor neurone in carrying a nerve impulse during a reflex action.

Pearson Edexcel International GCSE Biology (4BI1) — 2.15 Coordination and response: humans · Describe · 4 marks · View as Markdown

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

A student accidentally touches a very cold surface and rapidly withdraws their hand before they are consciously aware of the cold sensation.

Model answer (4 marks)

The sensory neurone carries the impulse from the receptor to the spinal cord.
The relay neurone passes the impulse within the spinal cord, linking the sensory and motor neurones.
The motor neurone carries the impulse from the spinal cord to the effector muscle.
The impulse passes across synapses between neurones and the pathway does not go through the conscious brain, allowing a rapid response.

Examiner tips

  • Use the exact terms: sensory, relay, motor neurone. Mention the spinal cord and synapses. Show the pathway does not involve the conscious brain.
  • Keep each point brief – 1 mark each.

Mark scheme (4 marks)

  1. The sensory neurone carries the impulse from the receptor to the spinal cord / relay neurone
  2. The relay neurone passes the impulse within the spinal cord / between the sensory and motor neurones
  3. The motor neurone carries the impulse from the spinal cord / relay neurone to the effector / muscle
  4. The impulse passes across synapses between neurones / the pathway does not pass through the conscious brain (allowing a rapid response)

Key terms in this question

sensory neurone · relay neurone · motor neurone · reflex action

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