Describe the pathway of a reflex action, from detection of a stimulus to the response of an effector.

AQA GCSE Biology (8461) — 4.5.2 The human nervous system · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Receptors in the skin detect the stimulus and generate a receptor potential.
2. The receptor potential triggers an action potential that travels along a sensory neuron to the spinal cord.
3. In the spinal cord the impulse is transmitted to a relay neuron (interneuron) and then to a motor neuron.
4. The motor neuron carries the impulse to an effector muscle, causing it to contract and produce the reflex response.

Examiner tips

  • Use the word "receptor" and "sensory neuron" for the first step. Mention the relay neuron in the spinal cord. State that the motor neuron carries the impulse to a muscle effector. Keep the answer concise and use the exact terminology.
  • common_mistakes
  • :
  • Confusing the relay neuron with the motor neuron. Omitting the receptor or the sensory neuron. Using vague terms like "nerve" instead of "neuron".

Mark scheme (4 marks)

  1. Receptors detect the stimulus
  2. Electrical impulse travels along a sensory neurone to the CNS (spinal cord)
  3. The impulse passes through a relay neurone in the spinal cord
  4. Impulse travels along a motor neurone to an effector (muscle or gland), which produces the response

Key terms in this question

reflex action

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