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{{Short description|Automatic, involuntary response to a stimulus}}
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{{Other uses|Reflex (disambiguation)}}
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{{human centric|date=September 2022}}
In [[biology]], a '''reflex''', or '''reflex action''', is an involuntary, unplanned sequence or action<ref>{{cite web |
[[File:Improved General Reflex Diagram.png|thumb|429x429px|The simplest reflex is initiated by a stimulus, which activates an afferent nerve. The signal is then passed to a response neuron, which generates a response.]]
Reflexes are found with varying levels of complexity in organisms with a [[nervous system]]. A reflex occurs via [[
Many reflexes are fine-tuned to increase organism survival and self-defense.<ref>{{Cite journal |last=Price |first=Joseph L. |date=2005-12-05 |title=Free will versus survival: Brain systems that underlie intrinsic constraints on behavior
==Types of human reflexes==
===
''Reflex'' is an anatomical concept and it refers to a
loop consisting, in its simplest form, of a sensory
nerve, the input, and a motor nerve, the output.
Autonomic does not mean automatic. The term ''autonomic'' is an anatomical term and it refers
to a type of nervous system in animals and humans
that is very primitive. ''Skeletal'' or ''somatic'' are,
similarly, anatomical terms that refer to a type of
nervous system that is more recent in terms of
evolutionary development. There are autonomic
reflexes and skeletal, somatic reflexes.<ref>Nikoletseas Michael M. (2010) Behavioral and Neural Plasticity. {{ISBN|978-1-4537-8945-2}}</ref>
===Myotatic reflexes===
The myotatic or muscle [[stretch reflex]]es (sometimes known as ''deep tendon reflexes'') provide information on the integrity of the [[central nervous system]] and [[peripheral nervous system]]. This information can be detected using [[Electromyography|electromyography (EMG)]].<ref name=":1">{{Cite journal |last1=Tsuji |first1=Hironori |last2=Misawa |first2=Haruo |last3=Takigawa |first3=Tomoyuki |last4=Tetsunaga |first4=Tomoko |last5=Yamane |first5=Kentaro |last6=Oda |first6=Yoshiaki |last7=Ozaki |first7=Toshifumi |date=2021-01-27 |title=Quantification of patellar tendon reflex using portable mechanomyography and electromyography devices |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=2284 |doi=10.1038/s41598-021-81874-5 |issn=2045-2322 |pmc=7840930 |pmid=33504836}}</ref> Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one.<ref name=":1" /> A stretch reflex is the contraction of a muscle in response to its lengthwise stretch.▼
▲The myotatic or muscle [[stretch reflex]]es (sometimes known as ''deep tendon reflexes'')
* [[Biceps reflex]] ([[cervical spinal nerve 5|C5]], [[cervical spinal nerve 6|C6]])
* [[Brachioradialis reflex]] (C5, C6, [[cervical spinal nerve 7|C7]])
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===Tendon reflex===
A [[tendon reflex]] is the contraction of a muscle in response to striking its [[tendon]]. The [[Golgi tendon reflex]] is the inverse of a stretch reflex.
===Reflexes involving cranial nerves===
{| class="wikitable"
| '''Name''' || '''Sensory''' || '''Motor'''
|-
| [[Pupillary light reflex]] ||
|-
| [[Accommodation reflex]] || II || III
|- ▼
|-
| [[
| [[Corneal reflex]], also known as the [[blink]] reflex || V || [[Facial nerve|VII]]
|-
| [[Glabellar reflex]] || V || VII
|-
| [[Vestibulo-ocular reflex]] ||
|-
| [[Gag reflex]] ||
|}
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[[Image:Greifreflex.JPG|thumb|Grasp reflex]]<!-- Deleted image removed: [[Image:Flexor Reflex.jpg|thumb|spinal cord]] -->
[[Newborn babies]] have a number of other reflexes which are not seen in adults, referred to as '''primitive reflexes'''. These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:
▲* [[Asymmetrical tonic neck reflex]] (ATNR)
* [[Palmomental reflex]]
* [[Moro reflex]], also known as the startle reflex
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* [[Primitive reflexes#Rooting reflex|Rooting reflex]]
* [[Primitive reflexes#Sucking reflex|Sucking reflex]]
* [[Symmetrical tonic neck reflex]]
* [[Tonic labyrinthine reflex]]
===Other kinds of reflexes===
Other reflexes found in the central nervous system include:
* [[Abdominal reflex]]es (T6-L1)
* [[Gastrocolic reflex]]
* [[Anal wink|Anocutaneous reflex]] (S2-S4)
* [[Baroreflex]]
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* [[Startle reflex]]
* [[Withdrawal reflex]]
** [[Crossed extensor reflex]]
Many of these reflexes are quite complex, requiring a number of synapses in a number of different nuclei in the
Processes such as [[
===Grading===
{{Unreferenced section|date=April 2023}}
In [[medicine]], reflexes are often used to assess the health of the [[nervous system]]. [[physician|Doctors]] will typically grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.
{|class="wikitable"
|'''Grade
|-
| 0 || Absent ("mute")
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| 3+ or +++ || Hyperactive ''without'' [[clonus]], with spread to adjacent muscle groups
|-
| 4+ or ++++ || Hyperactive
|}
Depending on where you are, another way of grading is from –4 (absent) to +4 (clonus), where 0 is "normal".
== Reflex modulation ==
[[File:Demonstration of reflex reversal.svg|thumb|An example of reflex reversal is depicted. Activating the same spinal reflex pathway can cause limb flexion while standing, and extension while walking.]]
A good example of reflex modulation is the [[stretch reflex]].<ref>{{Cite journal |vauthors=Bässler U |date=March 1976 |title=Reversal of a reflex to a single motoneuron in the stick insect Çarausius morosus |url=https://link.springer.com/article/10.1007/BF00365594 |journal=[[Biological Cybernetics]] |language=en |volume=24 |issue=1 |pages=47–49 |doi=10.1007/BF00365594 |issn=1432-0770 |s2cid=12007820}}</ref><ref>{{cite journal |vauthors=Forssberg H, Grillner S, Rossignol S |date=August 1977 |title=Phasic gain control of reflexes from the dorsum of the paw during spinal locomotion |url=https://www.sciencedirect.com/science/article/abs/pii/0006899377907107 |journal=[[Brain Research]] |volume=132 |issue=1 |pages=121–39 |doi=10.1016/0006-8993(77)90710-7 |pmid=890471 |s2cid=32578292}}</ref><ref>{{cite journal |
The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through [[presynaptic inhibition]].<ref>{{cite journal |
==Other reflexes==
Breathing can also be considered both involuntary and voluntary, since breath can be held through [[internal intercostal muscles]].<ref>{{Cite journal |last1=Mitchell |first1=R. A. |last2=Berger |first2=A. J. |date=February 1975 |title=Neural regulation of respiration |url=https://pubmed.ncbi.nlm.nih.gov/1089375/ |journal=[[The American Review of Respiratory Disease]] |publisher=[[American Thoracic Society]] |volume=111 |issue=2 |pages=206–224 |doi=10.1164/arrd.1975.111.2.206 |doi-broken-date=31
== See also ==
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* [[Instinct]]
* [[Jumping Frenchmen of Maine]]
* [[List of reflexes
* [[Preflexes]]
* [[Voluntary action]]
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