# Describe the pathway of a reflex arc from the moment a stimulus is detected to the moment a response occurs. Include the names of the neurones involved and describe how the nerve impulse crosses the gap between neurones.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.3 Nervous system and reflex action · Describe · 5 marks

> A student accidentally touches a very hot surface and quickly withdraws their hand before they are consciously aware of any pain.

## Mark scheme (5 marks)

1. A receptor detects the stimulus (heat/pain) and generates a nerve impulse
2. The nerve impulse travels along a sensory neurone to the spinal cord
3. The impulse passes through a relay neurone (in the spinal cord)
4. The nerve impulse crosses the synapse by means of chemical neurotransmitters diffusing across the gap
5. The impulse travels along a motor neurone to an effector (muscle), which brings about the response (withdrawing the hand)

## Key terms

- [reflex arc](https://www.gradenine.co.uk/glossary/reflex-arc)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-pathway-of-a-reflex-5a0da601) · Published by Druglandscape Ltd.