Editing Reflex arc (section)

== The patellar reflex (aka "knee jerk") ==
{{main article|Patellar reflex}}
[[File:Reflex arc of decapod including LG interneuron (1).svg|thumb|(A) Microscopic hairs etched along the tail of the decapod activate a somatic signal (2) in response to the presence of an environmental stimulus (1). (B) The action potential activated by the somatic interneuron (3) relays an impulse to the lateral giant (LG) interneuron (4). (C) The lateral giant interneuron executes a reflex by relaying impulses to various giant motor neurons (5) within the abdomen of the lobster. These muscular contractions result in the decapod being capable of successfully propelling itself through the water, away from the site of stimulus.]]

When the [[patellar tendon]] is tapped just below the knee, the tap initiates an [[action potential]] in a specialized structure known as a [[muscle spindle]] located within the [[quadriceps muscle|quadriceps]]. This action potential travels to the L3 and L4 nerve roots of the spinal cord,<ref name=":1">
{{Cite web
 | title= Deep Tendon Reflexes
 | date= November 28, 2016
 | website= The Precise Neurological Exam
 | publisher= New York University School of Medicine
 | url= https://informatics.med.nyu.edu/modules/pub/neurosurgery/reflexes.html
 | access-date= November 28, 2016
 }}
</ref>
via a sensory [[axon]] which chemically communicates by releasing [[glutamate]] onto a [[motor nerve]]. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate an injury of the [[central nervous system]].<ref name=":1"/>

The sensory input from the quadriceps also activates local [[interneuron]]s that release the inhibitory [[neurotransmitter]] [[glycine]] onto motor neurons of [[antagonist]] muscles, blocking their stimulation (in this case the [[hamstring muscles]]). The relaxation of the opposing muscle facilitates (by not opposing) the extension of the lower leg.

In invertebrates reflex interneurons do not necessarily reside in the spinal cord, for example as in the [[lateral giant neuron]] of [[crayfish]].