A student touches a very hot surface and quickly pulls their hand away before they are consciously aware of the pain. Describe the pathway of nerve impulses that produces this rapid response, naming the neurones involved in order.

AQA A-Level Biology (7402) — 3.6.1 Stimuli and Responses · Describe · 5 marks · View as Markdown

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

Reflex actions allow the body to respond to harmful stimuli without waiting for the brain to process the information consciously.

Model answer (5 marks)

A receptor in the skin detects the heat and sends a signal via a sensory neurone to the spinal cord.
A relay neurone in the spinal cord connects the sensory neurone to a motor neurone.
The motor neurone carries the impulse from the spinal cord to the muscle.
The muscle contracts, pulling the hand away before the brain becomes consciously aware of the pain.

Examiner tips

  • Use the exact sequence: receptor → sensory neurone → relay neurone → motor neurone → effector.
  • Mention that the brain is not involved in the initial reflex pathway.
  • Show the pathway in order and include the effector action.

Common mistakes

  • Omitting the relay neurone in the spinal cord.
  • Confusing the sensory and motor neurones.
  • Failing to state that the brain is only aware after the response.

Mark scheme (5 marks)

  1. A receptor in the skin detects the stimulus (heat / pain)
  2. A sensory neurone carries the impulse to the spinal cord
  3. A relay neurone in the spinal cord connects the sensory neurone to the motor neurone
  4. A motor neurone carries the impulse from the spinal cord to the effector (muscle)
  5. The effector (muscle) contracts, causing the hand to be pulled away / the response occurs without conscious thought / the brain is only aware of pain after the response

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