Patellar reflex
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2008-06-22T12:32:32Z
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[[Image:Patellar-knee-reflex.png|right|thumb|300px|Patellar reflex.]]
The '''patellar reflex''' or '''knee-jerk''' is a [[stretch reflex]] and is a [[myotatic reflex]].
==Mechanism==
Striking the [[patella]]r [[tendon]] with a [[tendon hammer]] just below the patella stretches the [[quadriceps]] [[tendon]]. This stimulates stretch [[sensory receptors]] (most importantly, [[muscle spindle]]s) that triggers 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. This reflex helps maintain [[posture]] and [http://ofinterest.net/patellar/ balance], allowing one to walk without consciously thinking about each step.
The patellar reflex is a clinical and classic example of the [[reflex arc|monosynaptic reflex arc]]. There is no [[interneuron]] in the pathway leading to flexion of the quadriceps muscle. Instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord.<ref>William F. Ganong, MD, Review of Medical Physiology, McGraw Hill, 2005, ISBN 0-8385-8282-6</ref> However, there ''is'' an inhibitory interneuron used to relax the antagonistic hamstring muscle.
==Purpose of Testing==
Testing the [[patellar tendon]] reflex tests the function of the [[femoral nerve]] and [[spinal cord]] segments L2-L4.
The absence or decrease of this reflex is known as [[Westphal's sign]].
==History==
That term ''knee-jerk'' was coined by Sir [[Michael Foster (physiologist)|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."<ref name="phrases">[http://www.phrases.org.uk/meanings/220600.html Knee-jerk reaction<!-- Bot generated title -->]</ref>
== Popular culture ==
The term began to be used figuratively from the early 20th century onwards. [[O. O. McIntyre]], in his New York Day-By-Day column in [[The Coshocton Tribune]], October 1921, wrote: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk." <ref name="phrases"> </ref>
== Notes ==
{{reflist}}
== References ==
*{{cite journal |author=Gurfinkel' VS, Lipshits MI, Popov KE |title=Is the stretch reflex a basic mechanism in the system of regulation of human vertical posture? |language=Russian |journal=Biofizika |volume=19 |issue=4 |pages=744–8 |year=1974 |pmid=4425696}}
== See also ==
{{wiktionary|knee-jerk}}
*[[Tonic vibration reflex]]
*[[Motor control]]
*[[Jendrassik maneuver]]
{{Reflex}}
[[Category:Reflexes]]
[[de:Patellarsehnenreflex]]
[[it:Riflesso patellare]]
[[nl:Kniepeesreflex]]
[[pl:Odruch rzepkowy]]
[[sv:Patellarreflex]]