# Describe how a reflex arc works when a person accidentally touches a sharp object with their finger. Include the names of the neurones involved and explain why the response does not require a decision from the brain.

> WJEC A-Level Biology (Wales) — 4.6 The nervous system · Describe · 5 marks

## Mark scheme (5 marks)

1. The sharp object acts as a stimulus, which is detected by receptors in the finger.
2. A nerve impulse travels along a sensory neurone to the spinal cord.
3. A relay neurone in the spinal cord passes the impulse to a motor neurone.
4. The impulse travels along the motor neurone to an effector (e.g. a muscle), which produces the response / causes the hand to withdraw.
5. The impulse is processed in the spinal cord rather than the brain, so the response is faster / the brain is not needed for the decision, allowing an almost instantaneous / automatic response to protect the organism.

## 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 "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/describe-how-a-reflex-arc-works-3c09e5ea) · Published by Druglandscape Ltd.