A student accidentally touches a very hot pan. Before the student is consciously aware of the pain, their hand has already been pulled away. Explain how the nervous system brings about this rapid, automatic response.

Pearson Edexcel A-Level Biology A (9BI0) — 4.3 Nervous system and reflex action · Explain · 5 marks · View as Markdown

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

When a person touches something painfully hot, a protective response occurs almost instantly — faster than conscious thought.

Model answer (5 marks)

The heat is a stimulus detected by a thermoreceptor in the skin. A sensory neuron carries a nerve impulse to the spinal cord, where it synapses with a relay neuron. The relay neuron connects the sensory neuron to a motor neuron, which carries the impulse to the effector muscle. The muscle contracts, pulling the hand away. This is a reflex action – it is rapid and automatic because it bypasses the brain and does not require conscious thought.

Examiner tips

  • Use the exact terms: receptor, sensory neuron, relay neuron, motor neuron, effector, reflex action
  • Show the pathway in order: stimulus → receptor → sensory neuron → spinal cord → relay neuron → motor neuron → effector
  • Mention that the brain is bypassed for speed

Common mistakes

  • Confusing the relay neuron with the sensory neuron
  • Omitting the effector muscle from the pathway
  • Failing to state that the brain is bypassed

Mark scheme (5 marks)

  1. The heat acts as a stimulus, detected by a receptor in the skin
  2. A nerve impulse is sent along a sensory neurone to the spinal cord (relay neurone)
  3. The relay neurone in the spinal cord connects the sensory neurone to the motor neurone
  4. The impulse travels along a motor neurone to the effector (muscle)
  5. The response is a reflex action — it is rapid and automatic because it bypasses the brain / does not require conscious thought

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