A doctor tests a patient's reflexes by tapping the tendon just below the kneecap. The lower leg kicks forward automatically. Explain how the nervous system brings about this response, naming the components involved.

Eduqas A-Level Biology — 3.8 The nervous system · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A stretch receptor in the quadriceps tendon detects the sudden stretch.
A sensory neuron carries the impulse from the receptor into the spinal cord.
In the spinal cord a relay (interneuron) connects the sensory neuron to a motor neuron.
The motor neuron carries the impulse out of the spinal cord to the quadriceps muscle.
The muscle contracts, producing a kick of the lower leg – a reflex arc that bypasses the brain.

Examiner tips

  • Use the word "receptor" for the stretch detector, "sensory neuron" for the afferent, "relay neuron" for the interneuron, "motor neuron" for the efferent, and "effector" for the muscle. Show the sequence as a one‑way reflex arc. Mention that the brain is not involved in the reflex.
  • common_mistakes
  • :
  • Calling the receptor a muscle instead of a stretch receptor. Forgetting to name the relay neuron. Saying the brain is involved or that the response is voluntary.

Mark scheme (5 marks)

  1. A receptor detects the stimulus (stretch of the tendon/muscle)
  2. A sensory neurone carries the impulse from the receptor to the spinal cord
  3. A relay neurone in the spinal cord connects the sensory neurone to the motor neurone
  4. A motor neurone carries the impulse from the spinal cord to the effector (muscle)
  5. The effector (muscle in the leg) contracts, causing the lower leg to kick forward / this is a reflex arc so it does not require conscious thought from the brain

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