Explain how a reflex arc allows the body to respond rapidly to a painful stimulus, such as touching a hot surface.

Pearson Edexcel International GCSE Biology (4BI1) — 2.15 Coordination and response: humans · Explain · 4 marks · View as Markdown

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

When a person accidentally touches a hot surface, the hand is withdrawn very quickly — often before the person is consciously aware of the pain.

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 the impulse is relayed by a relay neuron to a motor neuron.
The motor neuron carries the impulse to the muscle, which contracts and withdraws the hand – the response does not require conscious thought, making it fast.

Examiner tips

Use the exact sequence: receptor → sensory neuron → spinal cord → relay neuron → motor neuron → effector.
Mention that the brain is not involved, which explains the speed.
Show the pathway in a clear, linear order.

Common mistakes

Mixing up the order of neurons (e.g., putting motor before sensory).
Forgetting to mention the relay neuron in the spinal cord.
Including unnecessary detail about the brain or higher centres.

Mark scheme (4 marks)

A receptor detects the stimulus (heat / pain) and generates a nerve impulse
The nerve impulse travels along a sensory neurone to the spinal cord
The impulse is relayed via a relay neurone (in the spinal cord) to a motor neurone
The motor neurone carries the impulse to the effector (muscle), which contracts to withdraw the hand — the response does not require conscious thought from the brain, making it faster

Key terms in this question

reflex arc

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