Editing Reflex arc

{{Short description|Neural pathway which controls a reflex}}
[[File:Reflex Arc.svg|thumb|In a reflex arc, an action potential can bypass the brain for processing and uses dedicated neural pathways for faster processing. When a stimulus (A) is encountered, the signal from that stimulus will travel up the sensory neuron (B, in green) to the spinal column (C). There it will likely pass through a short interneuron (D, in purple) before continuing down a motor neuron (E, in blue) to the origin of the signal. Then, a contraction of the muscles (F, in red) is triggered, moving the bone (G).]]

A '''reflex arc''' is a [[neural pathway]] that controls a [[reflex]]. In [[vertebrate]]s, most [[sensory neuron]]s [[synapse]] in the [[spinal cord]] and the signal then travels through it into the [[brain]]. This allows for faster reflex actions to occur by activating spinal [[motor neuron]]s without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

There are two types: [[autonomic nervous system|autonomic]] reflex arc (affecting inner organs) and [[somatic nervous system|somatic]] reflex arc (affecting muscles). Autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.<ref name=":0">{{Cite book|title=Anatomy & Physiology: The Unity of Form and Function|last=Saladin|first=Kenneth|publisher=McGraw-Hill|year=2015|isbn=978-0073403717|location=New York|pages=496–497}}</ref>

During a somatic reflex, nerve signals travel along the following pathway:<ref name=":0" />
# ''Somatic receptors'' in the skin, muscles and tendons
# ''Afferent nerve fibers'' carry signals from the somatic receptors to the [[Posterior horn of spinal cord|posterior horn]] of the spinal cord or to the brainstem
# An ''integrating center'', the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse
# ''Efferent nerve fibers'' carry motor nerve signals from the [[Anterior horn of spinal cord|anterior horn]] to the muscles
# ''Effector'' muscle innervated by the efferent nerve fiber carries out the response.  
A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b.) carry the response generated by the spinal cord to effector organs during a reflex action.
The pathway taken by the nerve impulse to accomplish a reflex action is called the reflex arc.

==Monosynaptic vs. polysynaptic==
[[File:Imgnotraçat arc reflex eng.svg |thumb|280px |Reflex arc demonstrated]]
When a reflex arc in an animal consists of only one [[sensory neuron]] and one [[motor neuron]], it is defined as '''monosynaptic''', referring to the presence of a single [[chemical synapse]]. In the case of peripheral muscle reflexes ([[patellar reflex]], [[achilles reflex]]), brief stimulation to the [[muscle spindle]] results in contraction of the agonist or effector muscle.
By contrast, in '''polysynaptic''' reflex pathways, one or more interneurons connect [[afferent nerve|afferent]] ([[sensory system|sensory]]) and [[efferent nerve|efferent]] ([[motor system|motor]]) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain.{{Clarify | date=August 2019 | reason=This sentence is ambiguous: doesn't being 'monosynaptic' allow the brain to inhibit reflexes?  Does the brain always process polysynaptic reflexes? }}

== 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]].

==See also==
* [[Lazarus sign]]

==References==
{{Reflist}}

==External links==
* {{Cite thesis
 | author =  Ole Kæseler Andersen
 | url = http://person.hst.aau.dk/oka/oka_phd_thesis.pdf
 | title = Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex
 | type = PhD Thesis
 | publisher = Center for Sansory-Motor Interaction, Aalborg University
 | date = 1996
}}
* [http://www.wisc-online.com/objects/index_tj.asp?objid=AP11704 Tutorial at wisc-online.com]

[[Category:Reflexes]]