A person is walking barefoot on a beach and steps onto a sharp shell. Nerve impulses travel rapidly along a pathway, and the person's foot is pulled away before they are consciously aware of the pain. Explain how this response is coordinated, referring to the structures involved in the reflex arc.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Reflex actions allow the body to respond rapidly to harmful stimuli without conscious thought. The pathway involved passes through the spinal cord rather than the brain.
Model answer (5 marks)
The sharp shell is the stimulus, detected by a nociceptor in the skin of the foot.
1. The receptor generates a sensory impulse.
2. The impulse travels along a sensory neuron to the spinal cord.
3. In the spinal cord the impulse synapses onto a relay (interneuron) which acts as the coordinator.
4. The relay neuron sends an impulse along a motor neuron to the muscle in the leg/foot.
5. The muscle contracts, pulling the foot away. The response occurs before conscious awareness because the impulse does not need to reach the brain first.
1. The receptor generates a sensory impulse.
2. The impulse travels along a sensory neuron to the spinal cord.
3. In the spinal cord the impulse synapses onto a relay (interneuron) which acts as the coordinator.
4. The relay neuron sends an impulse along a motor neuron to the muscle in the leg/foot.
5. The muscle contracts, pulling the foot away. The response occurs before conscious awareness because the impulse does not need to reach the brain first.
Examiner tips
- Use the exact terms – receptor, sensory neuron, relay/interneuron, motor neuron, effector.
- Show the sequence clearly – 1‑5 – and note that the brain is bypassed.
- Mention that the response is rapid because it is a spinal reflex.
Mark scheme (5 marks)
- The sharp shell acts as a stimulus, which is detected by a receptor (in the skin/foot).
- A nerve impulse travels along a sensory neurone towards the spinal cord (coordinator/relay neurone).
- The impulse is passed to a relay neurone (within the spinal cord), which acts as the coordinator.
- The impulse then travels along a motor neurone to the effector (muscle in the leg/foot).
- The effector (muscle) contracts, producing the response of pulling the foot away / the response occurs before conscious awareness because the impulse does not need to travel to the brain first.
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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