# Explain how the nervous system carries out a reflex arc when a person touches a sharp object. Include the names of the neurones involved and explain why the response occurs so quickly.

> WJEC A-Level Biology (Wales) — 4.4 Application of reproduction and genetics · Explain · 5 marks

> A student accidentally touches the sharp point of a pin. Their hand is pulled away almost instantly, before they are consciously aware of any pain.

## Mark scheme (5 marks)

1. The sharp pin acts as a stimulus which is detected by receptors in the skin
2. An impulse travels along the sensory neurone to the spinal cord
3. The relay neurone passes the impulse to the motor neurone within the spinal cord / CNS
4. The impulse travels along the motor neurone to an effector (muscle), which contracts to pull the hand away
5. The response is fast because the impulse does not travel to the brain / bypasses the brain

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-nervous-system-carries-e790190e) · Published by Druglandscape Ltd.