Explain the roles of the sensory neurone, relay neurone, and motor neurone in transmitting a nerve impulse through the central nervous system (CNS) to produce a response.

OCR GCSE Biology A: Gateway Science (J247) — B3.1 Coordination and control — the nervous system · Explain · 4 marks · View as Markdown

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

A student pricks their finger on a drawing pin. The nervous system rapidly coordinates a response without the student having to think about it.

Model answer (4 marks)

1. The sensory neurone carries the impulse from the receptor to the CNS (brain or spinal cord).\n2. The relay neurone connects the sensory neurone to the motor neurone within the CNS.\n3. The motor neurone carries the impulse from the relay neurone to the effector.\n4. The effector (muscle or gland) produces a response, such as the muscle contracting to withdraw the finger.

Examiner tips

  • Use the exact terms: sensory, relay, motor neurone; mention CNS and effector; keep each point short and to the point.
  • Show the flow: receptor → sensory → relay → motor → effector.
  • Use the word ‘carry’ or ‘transmit’ to describe impulse movement.

Common mistakes

  • Confusing the relay neurone with the sensory or motor neurone; not mentioning the CNS; omitting the effector or describing it incorrectly.

Mark scheme (4 marks)

  1. The sensory neurone carries the impulse from the receptor to the CNS (brain or spinal cord).
  2. The relay neurone connects the sensory neurone to the motor neurone within the CNS.
  3. The motor neurone carries the impulse from the relay neurone to the effector.
  4. The effector (muscle or gland) produces a response, such as the muscle contracting to withdraw the finger.

Key terms in this question

sensory neurone · relay neurone · motor neurone · central nervous system (CNS)

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