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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student accidentally touches a very hot surface and quickly withdraws their hand before they are consciously aware of any pain.
Model answer (5 marks)
A receptor in the skin detects the heat and generates a nerve impulse.
The impulse travels along a sensory neuron to the dorsal horn of the spinal cord.
In the spinal cord the impulse is transmitted to a relay (interneuron).
The impulse crosses the synapse by chemical neurotransmitters diffusing across the synaptic cleft.
The impulse then travels along a motor neuron to the muscle, causing it to contract and withdraw the hand.
The impulse travels along a sensory neuron to the dorsal horn of the spinal cord.
In the spinal cord the impulse is transmitted to a relay (interneuron).
The impulse crosses the synapse by chemical neurotransmitters diffusing across the synaptic cleft.
The impulse then travels along a motor neuron to the muscle, causing it to contract and withdraw the hand.
Examiner tips
- Use the term "receptor" and "sensory neuron" first. Mention the dorsal horn as the site of the relay neuron. State "chemical neurotransmitters" for synaptic transmission. Finish with "motor neuron to effector muscle" to show the response.
Common mistakes
- Confusing sensory and motor neurons. Forgetting to mention the synaptic cleft. Using vague terms like "nerve signal" instead of "nerve impulse".
Mark scheme (5 marks)
- A receptor detects the stimulus (heat/pain) and generates a nerve impulse
- The nerve impulse travels along a sensory neurone to the spinal cord
- The impulse passes through a relay neurone (in the spinal cord)
- The nerve impulse crosses the synapse by means of chemical neurotransmitters diffusing across the gap
- The impulse travels along a motor neurone to an effector (muscle), which brings about the response (withdrawing the hand)
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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