A person is walking barefoot on grass when they step on a sharp stone. Their foot is pulled away from the stone before they are consciously aware of any pain. Explain how the nervous system brings about this response.

Eduqas A-Level Biology — 3.8 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)

The sharp stone is a stimulus detected by a nociceptor in the skin of the foot.
A sensory neuron carries a nerve impulse up the spinal cord.
At the spinal cord the impulse synapses onto a relay neuron.
The relay neuron sends a nerve impulse down a motor neuron to the leg muscle.
This is a reflex action – the response is automatic and occurs before the brain consciously perceives pain.

Examiner tips

  • Use the term ‘nociceptor’ for the receptor. Show the pathway: sensory → spinal cord → relay → motor → effector. Mention that the brain is not involved in the initial response.
  • common_mistakes
  • :
  • Calling the receptor a general ‘sensory receptor’ instead of a nociceptor. Forgetting to include the relay neuron in the spinal cord. Saying the brain is involved or that the response is conscious.

Mark scheme (5 marks)

  1. The sharp stone acts as a stimulus detected by a receptor (in the skin/foot)
  2. A nerve impulse travels along a sensory neurone to the spinal cord
  3. The impulse passes through a relay neurone (in the spinal cord)
  4. A nerve impulse travels along a motor neurone to the effector (muscle in the leg/foot)
  5. This is a reflex action / the response is automatic and does not involve conscious thought from the brain, which is why the foot is pulled away before the pain is felt

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