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Thepatellar reflex, also called theknee reflex orknee-jerk, is astretch reflex which tests the L2, L3, and L4 segments of thespinal cord. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species.[1]
Striking of thepatellar tendon with areflex hammer just below thepatella stretches themuscle spindle in thequadriceps muscle.[2][3] This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of L3 or L4 in the spinal cord, completely independent of higher centres.[4] From there, analpha motor neuron conducts an efferent impulse back to thequadriceps femoris muscle, triggering contraction.[3] This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick.[4] There is a latency of around 18ms between stretch of the patellar tendon and the beginning of contraction of the quadriceps femoris muscle.[5] This is a reflex ofproprioception which helps maintainposture andbalance, allowing to keep one's balance with little effort or conscious thought.
The patellar reflex is a clinical and classic example of the monosynapticreflex arc.[3] There is nointerneuron in the pathway leading to contraction of the quadriceps muscle. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord.[3] However, thereis an inhibitory interneuron used to relax the antagonistic hamstring muscle (reciprocal innervation).
This test of a basic automatic reflex may be influenced by the patient consciously inhibiting or exaggerating the response; the doctor may use theJendrassik maneuver in order to ensure a more valid reflex test.
After the tap of a hammer, the leg is normally extended once and comes to rest. The absence or decrease of this reflex is problematic, and known asWestphal's sign. This reflex may be diminished or absent inlower motor neuron lesions and during sleep. On the other hand, multiple oscillation of the leg (pendular reflex) following the tap may be a sign ofcerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found inupper motor neuron lesions,hyperthyroidism,[6] anxiety or nervousness. The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord.[1]
The patellar reflex is often tested ininfants to test the nervous system.[7]
Wilhelm Heinrich Erb (1840–1921) andKarl Friedrich Otto Westphal (1833–1890) simultaneously reported the patellar tendon or knee reflex in 1875.[8] The termknee-jerk was recorded by SirMichael Foster in hisTextbook ofphysiology in 1877: "Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk."[9]
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The term began to be used figuratively from the early 20th century onwards.O. O. McIntyre, in hisNew York Day-By-Day column inThe Coshocton Tribune, October 1921, wrote: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk."[9]
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