Explain how the nervous system carries out a reflex arc when a person touches a sharp object. Include the names of the neurones involved and the role of the effector in your answer.

WJEC A-Level Biology (Wales) — 4.3 Variation and evolution · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The sharp object is the stimulus, detected by receptors in the skin. The sensory neurone carries the impulse to the spinal cord. A relay neurone in the spinal cord passes the impulse to a motor neurone. The motor neurone carries the impulse to the effector – a muscle – which contracts to withdraw the hand. The reflex does not involve the brain, so the response is rapid.

Examiner tips

  • Use the exact terms: receptor, sensory neurone, relay neurone, motor neurone, effector, muscle.
  • Show the sequence clearly – stimulus → receptor → sensory neurone → spinal cord → relay neurone → motor neurone → effector.
  • Mention that the brain is not involved to justify the speed of the response.
  • Use UK spelling (neuron, reflex).

Common mistakes

  • Mixing up sensory and motor neurone names; writing ‘motor receptor’ instead of ‘sensory receptor’.
  • Omitting the relay neurone or the fact that the brain is not involved.

Mark scheme (5 marks)

  1. The sharp object acts as a stimulus and is detected by receptors (in the skin)
  2. An impulse travels along the sensory neurone to the spinal cord
  3. The relay neurone in the spinal cord passes the impulse to the motor neurone
  4. The impulse travels along the motor neurone to the effector (a muscle)
  5. The effector (muscle) contracts/responds to remove the hand from the sharp object / the reflex does not involve the brain so the response is faster

Key terms in this question

effector

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