Patellar reflex

Source: Wikipedia, the free encyclopedia.
Schematic representation of patellar tendon reflex (knee jerk) pathway

The patellar reflex, also called the knee reflex or knee-jerk, is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species. [1]

Mechanism

Representation of the patellar reflex pathway.

Striking of the

posture and 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 reflex arc.[3] There is no 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.[3] However, there is 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 the 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

cerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found in upper 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 in infants to test the nervous system.[7]

History

Wilhelm Heinrich Erb (1840–1921) and Carl Friedrich Westphal (1833–1890) simultaneously reported the patellar tendon or knee reflex in 1875.[8] The term knee-jerk was recorded 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."[9]

In popular culture

The term began to be used figuratively from the early 20th century onwards.

The Coshocton Tribune, October 1921, wrote: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk."[9]

See also

References

Sources