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Patellar reflex. Note that this image includes an interneuron in the pathway of the patellar reflex for purposes of illustration.

The patellar reflex or knee jerk is a monosynaptic reflex. Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon. This stimulates stretch sensory receptors (most importantly, muscle spindles) that trigger an afferent impulse in a sensory nerve fiber of the femoral nerve leading to the lumbar region of the spinal cord. There, the sensory neuron synapses directly with a motor neuron that conducts an efferent impulse to the quadriceps femoris muscle, triggering contraction. This contraction, 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.[How to reference and link to summary or text]

The patellar tendon reflex tests the function of the femoral nerve and spinal cord segements L2-L4.

The patellar reflex is a clinical and classic example of the monosynaptic reflex arc. There is no interneuron. Instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord. (Ganong, 2005)

That was coined by Sir Michael Foster in his textbook of physiology in 1877: "Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk."[1]

See also[]


  • 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.
  • William F. Ganong, MD, Review of Medical Physiology, McGraw Hill, 2005, ISBN 0-8385-8282-6
it:Riflesso patellare
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