The patellar reflex or knee jerk is a reflex employing only three neurons. Striking the patellar ligament just below the patella stretches the quadriceps tendon. This stimulates sensory receptors (most importantly, muscle spindles) that trigger an impulse in a sensory neuron that leads to the lumbar region of the spinal cord. There, the sensory neuron synapses directly with an interneuron, then to a motor neuron that conducts the impulse to the quadriceps, triggering contraction. This coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick. It has been thought that this type of reflex helps maintain the upright posture. However, Russian scientists Gurfinkel, Lipshits and Popov showed that different control mechanisms govern human vertical posture.

The spinal level of the reflex is L3-4.

See also

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• Tonic vibration reflex
• Motor control

The Patellar reflex is a clinical and classic example of the Monosynaptic reflex arc. There is no interneuron. Instead the bipolar senosry neuron syapses directly on a motor neuron in the spinal cord. (see William F. Ganong MD Review of Medical Physiology, McGraw Hill ISBN 0-8385-8282-6)

References

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• Gurfinkel' VS, Lipshits MI, Popov KE (1974) Is the stretch reflex a basic mechanism in the system of regulation of human vertical posture? Biofizika. 1974 Jul-Aug;19(4):744-748.

Reflexes

Patellarsehnenreflex