Explain what happens in a reflex arc when a person accidentally touches a sharp object with their finger. Your answer should include the roles of receptors, sensory neurones, relay neurones, motor neurones and effectors.

Eduqas A-Level Biology — 4.6 The nervous system · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Receptors in the finger detect the sharp stimulus and generate a nerve impulse.
The impulse travels along a sensory neuron to the spinal cord.
In the spinal cord a relay neuron receives the impulse and immediately passes it to a motor neuron.
The motor neuron carries the impulse back to the effector muscle.
The muscle contracts, withdrawing the finger – the reflex is involuntary and does not involve the brain, making it fast.

Examiner tips

  • Use the exact sequence: receptors → sensory neuron → relay neuron → motor neuron → effector.
  • Mention that the brain is bypassed to justify speed.
  • Use terms ‘spinal cord’ and ‘involuntary’ as they are key points.

Mark scheme (5 marks)

  1. Receptors in the finger detect the stimulus (sharp object / pain)
  2. A nerve 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 (muscle)
  5. The reflex is involuntary / the impulse does not travel to the brain, so the response is faster

Key terms in this question

receptor · sensory neurone · relay neurone · motor neurone · effector · reflex arc

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