Patellar reflex

Patellar reflex

The patellar reflex or knee-jerk is a stretch reflex and is a myotatic reflex.

Mechanism

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 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 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. [William F. Ganong, MD, Review of Medical Physiology, McGraw Hill, 2005, ISBN 0-8385-8282-6] 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

The term "knee-jerk" 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." [http://www.phrases.org.uk/meanings/220600.html Knee-jerk reaction ] ]

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."

Notes

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

*Tonic vibration reflex
*Motor control
*Jendrassik maneuver


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Look at other dictionaries:

  • patellar reflex — n KNEE JERK * * * the knee jerk, in which stretching the muscle at the front of the thigh by tapping its tendon below the knee cap causes a reflex contraction of the muscle, so that the leg kicks. This is a test of the connection between the… …   Medical dictionary

  • Patellar reflex — Reflex Re flex (r? fl?ks; formerly r?*fl?ks ), n. [L. reflexus a bending back. See {Reflect}.] 1. Reflection; the light reflected from an illuminated surface to one in shade. [1913 Webster] Yon gray is not the morning s eye, Tis but the pale… …   The Collaborative International Dictionary of English

  • patellar reflex — n. Med. a normal reflex kick with extension of the leg at the knee, produced by sharply tapping the tendon below the patella * * * …   Universalium

  • patellar reflex — n. Med. a normal reflex kick with extension of the leg at the knee, produced by sharply tapping the tendon below the patella …   English World dictionary

  • patellar reflex — noun a reflex extension of the leg resulting from a sharp tap on the patellar tendon • Syn: ↑knee jerk, ↑knee jerk reflex • Hypernyms: ↑reflex, ↑reflex response, ↑reflex action, ↑instinctive reflex, ↑ …   Useful english dictionary

  • patellar reflex — knee jerk reflex contraction of the quadriceps (thigh) muscle so that the leg kicks, elicited in a patient sitting with one knee crossed over the other by sharply tapping the tendon of the muscle below the kneecap. The reflex is mediated through… …   The new mediacal dictionary

  • patellar reflex — noun The reflex which causes a bent knee to kick when the tendon below the patella (kneecap) is tapped. Syn: kneejerk reflex …   Wiktionary

  • patellar reflex. — See knee jerk. [1890 95] * * * …   Universalium

  • patellar reflex. — See knee jerk. [1890 95] …   Useful english dictionary

  • reflex arc — n the complete nervous path that is involved in a reflex * * * the nervous circuit involved in a reflex, being at its simplest a sensory nerve with a receptor, linked at a synapse in the brain or spinal cord with a motor nerve, which supplies a… …   Medical dictionary

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