Describe how a reflex arc works to produce a rapid response when a person touches a very hot object, naming each component involved in the pathway.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.3 Nervous system and reflex action · Describe · 5 marks · View as Markdown

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

Reflex actions are automatic, rapid responses to stimuli that help protect the body from harm. They do not involve conscious thought.

Model answer (5 marks)

A receptor in the skin detects the heat and sends a signal via a sensory neuron to the spinal cord. In the spinal cord a relay neuron connects the sensory neuron to a motor neuron. The motor neuron carries the impulse back to the muscle, which contracts and withdraws the hand from the hot object.

Examiner tips

Use the exact terms: receptor, sensory neuron, relay neuron, motor neuron, effector (muscle).
Show the pathway in order: receptor → sensory neuron → relay neuron → motor neuron → effector.
Mention the rapid withdrawal of the hand as the final action.
Keep the answer concise – 5 marks allow a short paragraph or numbered points.

Common mistakes

Mixing up sensory and motor neuron roles.
Omitting the relay neuron in the spinal cord.
Using vague terms like ‘nerve’ instead of specific neuron types.

Mark scheme (5 marks)

A receptor in the skin detects the stimulus (heat/pain)
A sensory neurone carries the impulse from the receptor to the spinal cord / central nervous system
A relay neurone in the spinal cord connects the sensory neurone to the motor neurone
A motor neurone carries the impulse from the spinal cord to the effector (muscle)
The effector (muscle) responds by contracting so the hand is withdrawn / moved away from the heat

Key terms in this question

reflex arc