View source for Biology:Patellar reflex

{{Short description|Monosynaptic reflex}} [[File:Patellar-knee-reflex.png|thumb|Schematic representation of patellar tendon reflex (knee jerk) pathway]]
The '''patellar reflex''', also called the '''knee reflex''' or '''knee-jerk''', is a [[Biology:Stretch reflex|stretch reflex]] which tests the L2, L3, and L4 segments of the [[Biology:Spinal cord|spinal cord]]. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species. <ref name="Weiner2010">{{cite book|author=William J. Weiner|title=Neurology for the Non-Neurologist|url=https://books.google.com/books?id=eQa9c3-roD8C&pg=PA499|access-date=4 July 2011|date=30 July 2010|publisher=Lippincott Williams & Wilkins|isbn=978-1-60547-239-3|pages=499–}}</ref>

==Mechanism==
[[File:Patellar tendon reflex arc.png|thumb|upright=1.6|Representation of the patellar reflex pathway.]]
Striking of the patellar tendon with a [[Biology:Reflex hammer|reflex hammer]] just below the [[Biology:Patella|patella]] stretches the [[Biology:Muscle spindle|muscle spindle]] in the quadriceps muscle.<ref>{{Citation|last=Pedroso|first=F. S.|title=Reflexes|date=2008-01-01|url=http://www.sciencedirect.com/science/article/pii/B978012370877900133X|encyclopedia=Encyclopedia of Infant and Early Childhood Development|pages=11–23|editor-last=Haith|editor-first=Marshall M.|place=San Diego|publisher=Academic Press|language=en|doi=10.1016/b978-012370877-9.00133-x|isbn=978-0-12-370877-9|access-date=2020-12-23|editor2-last=Benson|editor2-first=Janette B.}}</ref><ref name=":0">{{Citation|last1=Ramahi|first1=A. A.|title=Lower Motor Neuron Lesions|date=2014-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780123851574011593|encyclopedia=Encyclopedia of the Neurological Sciences (Second Edition)|pages=918–922|editor-last=Aminoff|editor-first=Michael J.|place=Oxford|publisher=Academic Press|language=en|doi=10.1016/b978-0-12-385157-4.01159-3|isbn=978-0-12-385158-1|access-date=2020-12-23|last2=Katirji|first2=B.|last3=Devereaux|first3=M.|editor2-last=Daroff|editor2-first=Robert B.}}</ref> 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.<ref name=":1">{{Citation|last=Johns|first=Paul|title=Chapter 4 - Sensory and motor pathways|date=2014-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780443103216000047|work=Clinical Neuroscience|pages=49–59|editor-last=Johns|editor-first=Paul|publisher=Churchill Livingstone|language=en|doi=10.1016/b978-0-443-10321-6.00004-7|isbn=978-0-443-10321-6|access-date=2020-12-23}}</ref> From there, an alpha motor neuron conducts an efferent impulse back to the quadriceps femoris muscle, triggering contraction.<ref name=":0" /> This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick.<ref name=":1" /> There is a latency of around 18 [[Millisecond|ms]] between stretch of the patellar tendon and the beginning of contraction of the quadriceps femoris muscle.<ref>{{Citation|last=Brown|first=William F.|title=Electromyography and Disorders of the Central Nervous System|date=1984-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780409950427500177|work=The Physiological and Technical Basis of Electromyography|pages=459–489|editor-last=Brown|editor-first=William F.|publisher=Butterworth-Heinemann|language=en|doi=10.1016/b978-0-409-95042-7.50017-7|isbn=978-0-409-95042-7|access-date=2020-12-23}}</ref> This is a reflex of [[Biology:Proprioception|proprioception]] which helps maintain [[Medicine:Neutral spine|posture]] and [[Physics:Balance (ability)|balance]], allowing to keep one's balance with little effort or conscious thought.

The patellar reflex is a clinical and classic example of the monosynaptic [[Biology:Reflex arc|reflex arc]].<ref name=":0" /> There is no [[Biology:Interneuron|interneuron]] in the pathway leading to contraction of the quadriceps muscle. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord.<ref name=":0" /> However, there ''is'' an inhibitory interneuron used to relax the antagonistic hamstring muscle ([[Biology:Reciprocal innervation|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 the [[Biology:Jendrassik maneuver|Jendrassik maneuver]] in order to ensure a more valid reflex test.

==Clinical significance==
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 as Westphal's sign. This reflex may be diminished or absent  in [[Medicine:Lower motor neuron lesion|lower motor neuron lesion]]s and during sleep. On the other hand, multiple oscillation of the leg (pendular reflex) following the tap may be a sign of cerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found in [[Medicine:Upper motor neuron lesion|upper motor neuron lesion]]s, [[Medicine:Hyperthyroidism|hyperthyroidism]],<ref>{{cite book|archive-url=https://web.archive.org/web/20160622083355/http://www.utmb.edu/pedi_ed/CORE/Endocrine/page_32.htm|archive-date=22 June 2016|chapter-url=http://www.utmb.edu/pedi_ed/CORE/Endocrine/page_32.htm|chapter=Endocrinology - Hyperthyroidism|title=Core Concepts of Pediatrics
|website=www.utmb.edu}}</ref> anxiety or nervousness. The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord.<ref name="Weiner2010" />

The patellar reflex is often tested in infants to test the nervous system.<ref>{{Citation|last=Volpe|first=Joseph J.|title=Chapter 9 - Neurological Examination: Normal and Abnormal Features|date=2018-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780323428767000090|work=Volpe's Neurology of the Newborn (Sixth Edition)|pages=191–221.e8|editor-last=Volpe|editor-first=Joseph J.|publisher=Elsevier|language=en|doi=10.1016/b978-0-323-42876-7.00009-0|isbn=978-0-323-42876-7|access-date=2020-12-23|editor2-last=Inder|editor2-first=Terrie E.|editor3-last=Darras|editor3-first=Basil T.|editor4-last=de Vries|editor4-first=Linda S.}}</ref>

==History==
Wilhelm Heinrich Erb (1840–1921) and Carl Friedrich Westphal (1833–1890) simultaneously reported the patellar tendon or knee reflex in 1875.<ref>{{cite book|last1=Fine|first1=Edward J.|chapter=Chapter 16 History of the development of the neurological examination|date=2009-01-01|series=Handbook of Clinical Neurology|volume=95|pages=213–233|editor-last=Aminoff|editor-first=Michael J.|publisher=Elsevier|doi=10.1016/s0072-9752(08)02116-7|pmid=19892119|last2=Ziad Darkhabani|first2=M.|editor2-last=Boller|editor2-first=François|editor3-last=Swaab|editor3-first=Dick F.|title=History of Neurology|isbn=9780444520098}}</ref> The term ''knee-jerk'' was recorded by Sir Michael Foster in his Textbook of [[Biology:Physiology|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">{{cite web|url=http://www.phrases.org.uk/meanings/220600.html|title='Knee-jerk reaction' - the meaning and origin of this phrase|first=Gary|last=Martin|website=Phrasefinder}}</ref>

== In 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" />

==See also==
* {{Annotated link |Tonic vibration reflex}}
* {{Annotated link |Motor control}}
* {{Annotated link |Jendrassik maneuver}}

== References ==
{{reflist}}

==Sources==
* {{cite journal |vauthors=Gurfinkel' VS, Lipshits MI, Popov KE |title=Is the stretch reflex a basic mechanism in the system of regulation of human vertical posture? |language=ru |journal=Biofizika |volume=19 |issue=4 |pages=744–8 |year=1974 |pmid=4425696}}
* {{cite book|vauthors=Pinnock CA, Lin ES, Smith T |title=Fundamentals of Anaesthesia, 2nd Edition |publisher=Greenwich Medical Media Ltd. |chapter=Physiology of the Nervous System |year=2003 |edition=2nd}}

{{Reflex}}

{{DEFAULTSORT:Patellar Reflex}}
[[Category:Reflexes]]

{{Sourceattribution|Patellar reflex}}