Mon Prof Augmenté
Knee Jerk Stretch Reflex
Age 18 · Biology
A tap on the tendon below the kneecap stretches the quadriceps and its muscle spindles. The spindle sends a signal along one sensory neuron to the spinal cord, where a motor neuron carries it straight back to make the quadriceps contract and the lower leg kick forward.
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In the picture
- Muscle spindle in the quadriceps: the stretch receptor that detects the tendon tap.
- Sensory neuron fibre: carries the stretch signal from the spindle to the spinal cord.
- Spinal cord junction: the synapse where the sensory fibre meets the motor neuron.
- Motor neuron fibre: carries the signal from the spinal cord back to the quadriceps.
- Quadriceps muscle: contracts and pulls the lower leg forward.
Examples
- A doctor tapping your knee with a rubber reflex hammer.
- Your lower leg kicking forward automatically when the tendon is tapped.
Key takeaways
- The knee-jerk reflex uses a sensory neuron and a motor neuron with one synapse in the spinal cord.
- An automatic contraction of the quadriceps makes the lower leg kick forward.
Quiz: Knee Jerk Stretch Reflex (Age 18)
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Which pathway describes the knee-jerk stretch reflex?
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What directly makes the lower leg kick forward during the knee-jerk reflex?
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What does the sensory neuron fibre do in the knee-jerk reflex?
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