Explain how a reflex arc allows the body to respond rapidly to a harmful stimulus, naming the structures involved in the pathway.

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 accidentally touches a very hot surface and quickly pulls their hand away before they are consciously aware of the pain. This is an example of a reflex action.

Model answer (4 marks)

A receptor in the skin detects the heat and generates a nerve impulse.
The impulse travels along a sensory neuron to the spinal cord.
In the spinal cord a relay neuron passes the impulse to a motor neuron.
The motor neuron carries the impulse to the muscle, causing the hand to pull away.
The reflex arc bypasses the brain, allowing a rapid response.

Examiner tips

  • Use the word ‘receptor’ and ‘sensory neuron’ first. Show the sequence: receptor → sensory neuron → relay neuron → motor neuron → effector. Mention that the brain is bypassed to explain speed.

Common mistakes

  • Forgetting to name the relay neuron in the spinal cord. Describing the response as ‘pain’ instead of the muscle action. Omitting that the brain is bypassed, missing the speed argument.

Mark scheme (4 marks)

  1. A receptor detects the stimulus (heat/pain) and generates a nerve impulse
  2. The impulse travels along a sensory neurone to the spinal cord / CNS
  3. A relay neurone (in the spinal cord) passes the impulse to a motor neurone
  4. The motor neurone carries the impulse to an effector (muscle) which produces a response (hand pulls away) / the reflex bypasses the brain making it faster

Key terms in this question

reflex arc

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