A doctor tests a patient's knee-jerk reflex by tapping the tendon just below the kneecap. The lower leg kicks forward rapidly without the patient deciding to move it. Explain how the nervous system produces this rapid, automatic response, naming the components of the reflex arc involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The knee-jerk reflex is a spinal reflex. When the tendon is tapped, the leg muscles are briefly stretched, triggering a response that does not require the brain to initiate the movement.
Model answer (5 marks)
A receptor in the muscle spindle detects the stretch of the quadriceps tendon.
A sensory neurone carries the action potential to the spinal cord.
The spinal cord acts as the coordinator, relaying the impulse via a interneuron to a motor neurone.
The motor neurone carries the impulse from the spinal cord to the quadriceps muscle.
The muscle contracts, producing the rapid, automatic knee‑jerk response without conscious control.
A sensory neurone carries the action potential to the spinal cord.
The spinal cord acts as the coordinator, relaying the impulse via a interneuron to a motor neurone.
The motor neurone carries the impulse from the spinal cord to the quadriceps muscle.
The muscle contracts, producing the rapid, automatic knee‑jerk response without conscious control.
Examiner tips
- Use the exact terms: receptor, sensory neurone, spinal cord, interneuron, motor neurone, effector.
- Show the sequence clearly – list each component in order.
- Mention that the brain is not involved in this reflex.
- Use the word ‘stretch’ to describe the stimulus.
Mark scheme (5 marks)
- A receptor detects the stimulus (stretch of the muscle/tendon)
- A sensory neurone carries the electrical impulse to the spinal cord (coordinator)
- The spinal cord acts as the coordinator and passes the impulse to a motor neurone (via a relay/connector neurone)
- A motor neurone carries the impulse from the spinal cord to the effector (muscle)
- The effector (muscle) contracts, producing the rapid, automatic response without conscious control
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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