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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The pin is a stimulus detected by nociceptors in the skin.
1. A sensory (afferent) neuron carries the impulse to the spinal cord.
2. In the spinal cord a relay (interneuron) transmits the signal to a motor (efferent) neuron.
3. The motor neuron sends the impulse to the hand muscle, causing it to contract and pull the hand away.
4. The response is rapid because the impulse never reaches the brain; it is processed entirely within the spinal cord.
1. A sensory (afferent) neuron carries the impulse to the spinal cord.
2. In the spinal cord a relay (interneuron) transmits the signal to a motor (efferent) neuron.
3. The motor neuron sends the impulse to the hand muscle, causing it to contract and pull the hand away.
4. The response is rapid because the impulse never reaches the brain; it is processed entirely within the spinal cord.
Examiner tips
- Use the word "sensory" for the afferent neuron and "motor" for the efferent neuron. Mention the relay neuron in the spinal cord. Explain that the brain is bypassed, giving a fast response.
Common mistakes
- Confusing sensory with motor neuron. Forgetting to mention the relay neuron. Saying the impulse goes to the brain instead of the spinal cord.
Mark scheme (5 marks)
- The sharp pin acts as a stimulus which is detected by receptors in the skin
- An impulse travels along the sensory neurone to the spinal cord
- The relay neurone passes the impulse to the motor neurone within the spinal cord / CNS
- The impulse travels along the motor neurone to an effector (muscle), which contracts to pull the hand away
- The response is fast because the impulse does not travel to the brain / bypasses the brain
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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