View source for Sensory nervous system

{{Short description|Part of the nervous system}}
[[File:restingStateModels.jpg|thumb|The [[visual system]] and the [[somatosensory system]] are active even during [[resting state fMRI]]]]
[[File:1212 Sensory Neuron Test Water.jpg|thumb|280px|Activation and response in the sensory nervous system]]
The '''sensory nervous system''' is a part of the [[nervous system]] responsible for processing [[sense|sensory]]  information.  A '''sensory system''' consists of [[sensory neuron]]s (including the sensory receptor cells), [[neural pathway]]s, and parts of the [[brain]] involved in sensory [[perception]] and [[interoception]].  Commonly recognized sensory systems are those for [[Vision (sense)|vision]], [[Auditory system|hearing]], [[somatic sensation|touch]], [[taste]], [[olfaction|smell]], [[Vestibular system|balance]] and visceral sensation. Sense organs are [[transducers]] that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their [[perception]] of the world around them.<ref>{{cite book |last1=Krantz |first1=John |chapter=What is Sensation and Perception? |page=1.6 |title=Experiencing Sensation and Perception |date=2012 |publisher=Pearson Education, Limited |isbn=978-0-13-097793-9 |chapter-url=http://psych.hanover.edu/classes/sensation/chapters/Chapter%201.pdf }}</ref>

The [[receptive field]] is the area of the body or environment to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field; the light that each [[Rod cell|rod]] or [[Cone cell|cone]] can see, is its receptive field.<ref>{{Cite book |last=Kolb |first=Bryan |title=Fundamentals of human neuropsychology |date=2003 |publisher=Worth Publishers |isbn=0-7167-5300-6 |editor-last=Winshaw |editor-first=Ian Q. |edition=5th |location=New York, NY |oclc=55617319}}{{pn|date=November 2024}}</ref> [[Receptive field]]s have been identified for the [[visual system]], [[auditory system]] and [[somatosensory system]].

==Senses and receptors==

While debate exists among neurologists as to the specific number of senses due to differing definitions of what constitutes a [[sense]], [[Gautama Buddha]] and [[Aristotle]] classified five 'traditional' human senses which have become universally accepted: [[touch]], [[taste]], [[olfaction|smell]], [[visual system|vision]], and [[hearing (sense)|hearing]].  Other senses that have been well-accepted in most mammals, including humans, include [[nociception|pain]], [[equilibrioception|balance]], [[kinaesthesia]], and [[thermoception|temperature]].  Furthermore, some nonhuman animals have been shown to possess alternate senses, including [[magnetoception|magnetoreception]] and [[electroreception]].<ref>{{cite journal | last1 = Hofle | first1 = M. | last2 = Hauck | first2 = M. | last3 = Engel | first3 = A. K. | last4 = Senkowski | first4 = D. | year = 2010 | title = Pain processing in multisensory environments | journal = Neuroforum | volume = 16 | issue = 2| page = 172 | doi = 10.1007/s13295-010-0004-z | s2cid = 20865665 | doi-access = free }}</ref>

===Receptors===
{{main|sensory receptor}}

The initialization of sensation stems from the response of a specific receptor to a physical stimulus. The receptors which react to the stimulus and initiate the process of sensation are commonly characterized in four distinct categories: [[chemoreceptors]], [[photoreceptor cell|photoreceptors]], [[mechanoreceptors]], and [[thermoreceptors]]. All receptors receive distinct physical stimuli and transduce the signal into an electrical [[action potential]]. This action potential then travels along [[afferent nerve fiber|afferent neurons]] to specific brain regions where it is processed and interpreted.<ref>[http://www.encyclopedia.com/doc/1O87-sensoryreceptor.html] {{webarchive|url=https://web.archive.org/web/20090112034609/http://www.encyclopedia.com/doc/1O87-sensoryreceptor.html|date=January 12, 2009}}</ref>

===Chemoreceptors===
{{main|Chemoreceptor}}
Chemoreceptors, or chemosensors, detect certain chemical stimuli and transduce that signal into an electrical action potential. The two primary types of chemoreceptors are:
# Distance chemoreceptors are integral to receiving stimuli in [[gases]] in the [[olfactory system]] through both olfactory receptor neurons and neurons in the [[vomeronasal organ]].
# Direct chemoreceptors that detect stimuli in [[liquids]] include the [[taste buds]] in the [[gustatory system]] as well as receptors in the [[aortic bodies]] which detect changes in [[oxygen]] concentration.<ref>{{cite journal | last1 = Satir | first1 = P | last2 = Christensen | first2 = ST | year = 2008 | title = Structure and function of mammalian cilia | journal = Histochemistry and Cell Biology | volume = 129 | issue = 6 | pages = 687–693 | pmc = 2386530 | pmid = 18365235 | doi = 10.1007/s00418-008-0416-9 }}</ref>

===Photoreceptors===
{{main|Photoreceptor cell}}
Photoreceptors are neuron cells and are specialized units that play the main role in initiating vision function. Photoreceptors are light-sensitive cells that capture different wavelengths of light. Different types of photoreceptors are able to respond to the varying light wavelengths in relation to color, and transduce them into electrical signals.<ref>{{cite journal |last1=Molday |first1=Robert S. |last2=Moritz |first2=Orson L. |title=Photoreceptors at a glance |journal=Journal of Cell Science |date=15 November 2015 |volume=128 |issue=22 |pages=4039–4045 |doi=10.1242/jcs.175687 |pmc=4712787 |pmid=26574505 }}</ref> Photoreceptors are capable of [[phototransduction]], a process which converts light ([[electromagnetic radiation]]) into, among other types of [[energy]], a [[membrane potential]]. There are five compartments that are present in these cells. Each compartment corresponds to differences in function and structure. The first compartment is the outer segment (OS), where it is responsible for capturing light and transducing it. The second compartment is the inner segment (IS), which includes the necessary organelles that function in cellular metabolism and biosynthesis. Mainly, these organelles include mitochondria, Golgi apparatus and endoplasmic reticulum as well as among others. The third compartment is the connecting cilium (CC). As its name suggests, CC works to connect the OS and the IS regions together for the purpose of essential protein trafficking. The fourth compartment contains the nucleus and is a continuation of the IS region, known as the nuclear region. Finally, the fifth compartment is the synaptic region, where it acts as a final terminal for the signal, consisting of synaptic vesicles. In this region, glutamate neurotransmitter is transmitted from the cell to secondary neuron cells.<ref>{{Cite journal |last1=Vinberg |first1=Frans |last2=Chen |first2=Jeannie |last3=Kefalov |first3=Vladimir J. |date=November 2018 |title=Regulation of calcium homeostasis in the outer segments of rod and cone photoreceptors |journal=Progress in Retinal and Eye Research |volume=67 |pages=87–101 |doi=10.1016/j.preteyeres.2018.06.001 |pmc=6235702 |pmid=29883715 }}</ref><ref>{{Cite journal |last1=Malhotra |first1=Himanshu |last2=Barnes |first2=Cassandra L. |last3=Calvert |first3=Peter D. |date=September 2021 |title=Functional Compartmentalization of Photoreceptor Neurons |journal=Pflügers Archiv: European Journal of Physiology |volume=473 |issue=9 |pages=1493–1516 |doi=10.1007/s00424-021-02558-7 |pmc=8690575 |pmid=33880652 }}</ref> The three primary types of photoreceptors are:
[[Cone cell|cones]] are photoreceptors which respond significantly to [[color]].  In humans, the three different types of cones correspond with a primary response to short wavelength (blue), medium wavelength (green), and long wavelength (yellow/red).<ref name="Encyclopædia Britannica 2010">"eye, human." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010.</ref>
[[Rod cell|Rods]] are photoreceptors which are very sensitive to the intensity of light, allowing for vision in dim lighting.  The concentrations and ratio of rods to cones is strongly correlated with whether an animal is [[diurnality|diurnal]] or [[nocturnal]]. In humans, rods outnumber cones by approximately 20:1, while in nocturnal animals, such as the [[tawny owl]], the ratio is closer to 1000:1.<ref name="Encyclopædia Britannica 2010"/>
[[Retinal ganglion cell|Ganglion cell]]s reside in the [[adrenal medulla]] and [[retina]] where they are involved in the [[sympathetic response]]. Of the ~1.3 million ganglion cells present in the retina, 1–2% are believed to be [[intrinsically photosensitive retinal ganglion cells|photosensitive ganglia]].<ref>{{cite journal | last1 = Foster | first1 = R. G. | last2 = Provencio | first2 = I. | last3 = Hudson | first3 = D. | last4 = Fiske | first4 = S. | last5 = Grip | first5 = W. | last6 = Menaker | first6 = M. | year = 1991 | title = Circadian photoreception in the retinally degenerate mouse (rd/rd) | journal = Journal of Comparative Physiology A | volume =  169| issue = 1| pages =  39–50| doi = 10.1007/BF00198171 | pmid = 1941717 | s2cid = 1124159 }}</ref> These photosensitive ganglia play a role in conscious vision for some animals,<ref name=Ecker>{{cite journal | title = Melanopsin-Expressing Retinal Ganglion-Cell Photoreceptors: Cellular Diversity and Role in Pattern Vision |author=Jennifer L. Ecker |author2=Olivia N. Dumitrescu |author3=Kwoon Y. Wong |author4=Nazia M. Alam |author5=Shih-Kuo Chen |author6=Tara LeGates |author7=Jordan M. Renna |author8=Glen T. Prusky |author9=David M. Berson |author10=Samer Hattar | volume = 67 | issue = 1 | journal = Neuron | doi=10.1016/j.neuron.2010.05.023 | pmid = 20624591 | pmc = 2904318 | year=2010 | pages=49–60}}</ref> and are believed to do the same in humans.<ref name="HoriguchiWinawer2012">{{cite journal|last1=Horiguchi|first1=H.|last2=Winawer|first2=J.|last3=Dougherty|first3=R. F.|last4=Wandell|first4=B. A.|title=Human trichromacy revisited|journal=Proceedings of the National Academy of Sciences|volume=110|issue=3|year=2012|pages=E260–E269 |doi=10.1073/pnas.1214240110|pmid=23256158|pmc=3549098|doi-access=free}}</ref>

===Mechanoreceptors===
{{main|Mechanoreceptor}}
Mechanoreceptors are sensory receptors which respond to mechanical forces, such as [[pressure]] or [[distortion]].<ref>{{cite journal | last1 = Winter | first1 = R. | last2 = Harrar | first2 = V. | last3 = Gozdzik | first3 = M. | last4 = Harris | first4 = L. R. | year = 2008 | title = The relative timing of active and passive touch. [Proceedings Paper] | journal = Brain Research | volume = 1242 | pages = 54–58 | doi = 10.1016/j.brainres.2008.06.090 | pmid = 18634764 | s2cid = 11179917 }}</ref>  While mechanoreceptors are present in [[hair cells]] and play an integral role in the [[vestibular system|vestibular]] and [[auditory system]]s, the majority of mechanoreceptors are [[cutaneous]] and are grouped into four categories:
# ''Slowly adapting type 1 receptors'' have small receptive fields and respond to static stimulation. These receptors are primarily used in the sensations of [[Shape|form]] and [[Surface roughness|roughness]].
# ''Slowly adapting type 2 receptors'' have large receptive fields and respond to stretch. Similarly to type 1, they produce sustained responses to a continued stimuli.
# ''Rapidly adapting receptors'' have small receptive fields and underlie the perception of slip.
# ''Pacinian receptors'' have large receptive fields and are the predominant receptors for high-frequency vibration.

=== Thermoreceptors ===
{{main|Thermoreceptor}}
Thermoreceptors are sensory receptors which respond to varying [[temperature]]s. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that [[mammal]]s have at least two distinct types of thermoreceptors:<ref>{{cite journal |last1=Lezama-García |first1=Karina |last2=Mota-Rojas |first2=Daniel |last3=Pereira |first3=Alfredo M. F. |last4=Martínez-Burnes |first4=Julio |last5=Ghezzi |first5=Marcelo |last6=Domínguez |first6=Adriana |last7=Gómez |first7=Jocelyn |last8=de Mira Geraldo |first8=Ana |last9=Lendez |first9=Pamela |last10=Hernández-Ávalos |first10=Ismael |last11=Falcón |first11=Isabel |last12=Olmos-Hernández |first12=Adriana |last13=Wang |first13=Dehua |title=Transient Receptor Potential (TRP) and Thermoregulation in Animals: Structural Biology and Neurophysiological Aspects |journal=Animals |date=2 January 2022 |volume=12 |issue=1 |pages=106 |doi=10.3390/ani12010106 |doi-access=free |pmid=35011212 |pmc=8749608 |hdl=10174/34240 |hdl-access=free }}</ref>
# The end-bulb of Krause or [[bulboid corpuscle]] detects temperatures above body temperature.
# [[Ruffini corpuscle|Ruffini's end organ]] detects temperatures below body temperature.

TRPV1 is a heat-activated channel that acts as a small heat detecting thermometer in the membrane which begins the polarization of the neural fiber when exposed to changes in temperature. Ultimately, this allows us to detect ambient temperature in the warm/hot range. Similarly, the molecular cousin to TRPV1, TRPM8, is a cold-activated ion channel that responds to cold. Both cold and hot receptors are segregated by distinct subpopulations of sensory nerve fibers, which shows us that the information coming into the spinal cord is originally separate. Each sensory receptor has its own "labeled line" to convey a simple sensation experienced by the recipient. Ultimately, TRP channels act as thermosensors, channels that help us to detect changes in ambient temperatures.<ref name="Thermosensor">{{cite web|title=How peppers & peppermint identified sensory receptors for temperature and pain|url=https://www.ibiology.org/neuroscience/sensory-receptors/|last1=Julius|first1=David|website=iBiology|access-date=12 May 2020}}</ref>

=== Nociceptors ===
{{main|Nociceptor}}
Nociceptors respond to potentially damaging stimuli by sending signals to the spinal cord and brain. This process, called [[nociception]], usually causes the perception of [[pain]].<ref>Sherrington C. ''The Integrative Action of the Nervous System.'' Oxford: Oxford University Press; 1906.</ref> They are found in internal organs, as well as on the surface of the body. Nociceptors detect different kinds of damaging stimuli or actual damage. Those that only respond when tissues are damaged are known as "sleeping" or "silent" nociceptors.
# Thermal nociceptors are activated by noxious heat or cold at various temperatures.
# Mechanical nociceptors respond to excess pressure or mechanical deformation.
# Chemical nociceptors respond to a wide variety of chemicals, some of which are signs of tissue damage. They are involved in the detection of some spices in food.

== Sensory cortex ==

All [[stimulus (physiology)|stimuli]] received by the [[Receptor (biochemistry)|receptors]] listed above are [[signal transduction|transduced]] to an [[action potential]], which is carried along one or more [[afferent nerve fiber|afferent neurons]] towards a specific area of the brain. While the term [[sensory cortex]] is often used informally to refer to the [[somatosensory cortex]], the term more formally refers to the multiple areas of the brain at which [[sense]]s are received to be processed.  For the five [[Sense#Traditional senses|traditional senses]] in humans, this includes the primary and secondary [[sensory cortex|cortices of the different senses]]: the somatosensory cortex, the [[visual cortex]], the [[auditory cortex]], the [[primary olfactory cortex]], and the [[gustatory cortex]].<ref name="Brynie, F.H. 2009">{{Cite book |last=Brynie |first=Faith Hickman |title=Brain sense: the science of the senses and how we process the world around us |date=2009 |publisher=American Management Association |isbn=978-0-8144-1326-5 |location=New York |oclc=457057287}}</ref> Other [[stimulus modality|modalities]] have corresponding sensory cortex areas as well, including the [[vestibular cortex]] for the sense of balance.<ref>{{Citation |last=Brandt |first=Thomas |title=Vestibular cortex: its locations, functions, and disorders |date=2003 |work=Vertigo |pages=219–231 |place=New York, NY |publisher=Springer New York |language=en |doi=10.1007/978-1-4757-3801-8_13 |isbn=978-0-387-40500-1|doi-access=free }}</ref>

The human sensory system consists of the following subsystems:
* [[Visual system]] (Vision)
* [[Auditory system]] (Hearing)
* [[Somatosensory system]] (Touch/Temperature/Kinesthesia/Pain)
* [[Gustatory system]] (Taste)
* [[Olfactory system]] (Smell)
* [[Vestibular system]] (Balance)

===Somatosensory cortex===
Located in the [[parietal lobe]], the [[primary somatosensory cortex]] is the primary receptive area for the sense of touch and [[proprioception]] in the [[somatosensory system]]. This cortex is further divided into [[Brodmann areas]] 1, 2, and 3.  [[Brodmann area 3]] is considered the primary processing center of the somatosensory cortex as it receives significantly more input from the [[thalamus]], has neurons highly responsive to somatosensory stimuli, and can evoke somatic sensations through electrical [[stimulation]].  Areas 1 and 2 receive most of their input from area 3. There are also pathways for [[proprioception]] (via the [[cerebellum]]), and [[motor neuron|motor]] control (via [[Brodmann area 4]]). See also: S2 [[Secondary somatosensory cortex]].
[[File:Eye iris.jpg|thumb|100px|The [[human eye]] is the first element of a '''sensory system''': in this case, [[Visual perception|vision]], for the [[visual system]].]]

===Visual cortex===
The visual cortex is the part of the cerebral cortex that processes visual information. Processing in the visual cortex starts in the primary visual cortex, labeled V1 or [[Brodmann area 17]], as well as the extrastriate visual cortical areas V2-V5.<ref>{{cite journal | last1 = McKeeff | first1 = T. J. | last2 = Tong | first2 = F. | year = 2007 | title = The timing of perceptual decisions for ambiguous face stimuli in the human ventral visual cortex. [Article] | journal = Cerebral Cortex | volume = 17 | issue = 3| pages = 669–678 | doi = 10.1093/cercor/bhk015 | pmid = 16648454 | doi-access = free }}</ref>  Located in the [[occipital lobe]], V1 acts as the primary relay station for visual input, transmitting information to two primary pathways labeled the [[dorsal stream|dorsal]] and [[ventral stream]]s. The dorsal stream includes areas V2 and V5, and is used in interpreting visual 'where' and 'how.' The ventral stream includes areas V2 and V4, and is used in interpreting 'what.'<ref>{{cite journal | last1 = Hickey | first1 = C. | last2 = Chelazzi | first2 = L. | last3 = Theeuwes | first3 = J. | year = 2010 | title = Reward Changes Salience in Human Vision via the Anterior Cingulate. [Article] | journal = Journal of Neuroscience | volume = 30 | issue = 33| pages = 11096–11103 | doi = 10.1523/jneurosci.1026-10.2010 | pmid = 20720117 | pmc = 6633486 | doi-access = free }}</ref> Increases in [[task-negative]] activity are observed in the ventral attention network, after abrupt changes in sensory stimuli,<ref>{{cite journal | last1 = Downar | first1 = J. | last2 = Crawley | first2 = A. P. | last3 = Mikulis | first3 = D. J. | last4 = Dav | year = 2000 | title =  multimodal cortical network for the detection of changes in the sensory environment | doi = 10.1038/72991 | pmid = 10700261 | journal = Nature Neuroscience | volume = 3 | issue = 3| pages = 277–283 | s2cid = 8807081 }}</ref> at the onset and offset of task blocks,<ref>{{cite journal | last1 = Fox | first1 = M. D. | last2 = Snyder | first2 = A. Z. | last3 = Barch | first3 = D. M. | last4 = Gusnard | first4 = D. A. | last5 = Raichle | first5 = M. E. | year = 2005 | title =  Transient BOLD responses at block transitions| journal = NeuroImage | volume = 28 | issue = 4| pages = 956–966 | doi=10.1016/j.neuroimage.2005.06.025| pmid = 16043368 | s2cid = 8552739 }}</ref> and at the end of a completed trial.<ref>{{cite journal | last1 = Shulman | first1 = G. I. | last2 = Tansy | first2 = A. P. | last3 = Kincade | first3 = M. | last4 = Petersen | first4 = S. E. | last5 = McAvoy | first5 = M. P. | last6 = Corbetta | first6 = M. | year = 2002 | title =  Reactivation of Networks Involved in Preparatory States| journal = Cerebral Cortex | volume = 12 | issue = 6| pages = 590–600 | doi=10.1093/cercor/12.6.590| pmid = 12003859 | doi-access = free }}</ref>{{Relevance inline | date = May 2019 | reason = This article seems to be an overview of different sensory systems, this statement starting from 'Increases' seems to be too technical and too detailed for the purpose. Suggest removal or clarification.}}
[[File:Earcov.JPG|thumb|100px|Human [[ear]]]]

===Auditory cortex===
Located in the [[temporal lobe]], the auditory cortex is the primary receptive area for sound information. The auditory cortex is composed of Brodmann areas 41 and 42, also known as the [[anterior transverse temporal area 41]] and the [[posterior transverse temporal area 42]], respectively. Both areas act similarly and are integral in receiving and processing the signals transmitted from [[hair cell|auditory receptors]].
[[File:Hump nose2.jpg|thumb|100px|[[Human nose]]]]

===Primary olfactory cortex===
Located in the temporal lobe, the [[primary olfactory cortex]] is the primary receptive area for [[olfaction]], or smell.  Unique to the olfactory and gustatory systems, at least in [[mammals]], is the implementation of both [[peripheral nervous system|peripheral]] and [[central nervous system|central]] mechanisms of action.{{Clarify | date = May 2019 | reason = It is known that both CNS and PNS are required for all the sensory systems to work.  What's the difference here?  No elaboration anywhere.}}  The peripheral mechanisms involve [[olfactory receptor neurons]] which [[signal transduction|transduce]] a chemical signal along the [[olfactory nerve]], which terminates in the [[olfactory bulb]]. The [[chemoreceptors]] in the receptor neurons that start the [[signal cascade]] are [[G protein-coupled receptors]]. The central mechanisms include the convergence of olfactory nerve [[axon]]s into [[Glomerulus (olfaction)|glomeruli]] in the olfactory bulb, where the signal is then transmitted to the [[anterior olfactory nucleus]], the [[piriform cortex]], the medial [[amygdala]], and the [[entorhinal cortex]], all of which make up the primary olfactory cortex.

In contrast to vision and hearing, the [[olfactory bulb]]s are not cross-hemispheric; the right bulb connects to the right hemisphere and the left bulb connects to the left hemisphere.
[[File:Tongue.agr.jpg|thumb|100px|Human [[tongue]]]]

===Gustatory cortex===
The [[gustatory cortex]] is the primary receptive area for [[taste]]. The word ''taste'' is used in a technical sense to refer specifically to sensations coming from taste buds on the tongue. The five qualities of taste detected by the tongue include sourness, bitterness, sweetness, saltiness, and the protein taste quality, called [[umami]]. In contrast, the term ''flavor'' refers to the experience generated through integration of taste with smell and tactile information. The gustatory cortex consists of two primary structures: the [[insular cortex|anterior insula]], located on the [[insular cortex|insular lobe]], and the frontal [[operculum (brain)|operculum]], located on the [[frontal lobe]]. Similarly to the olfactory cortex, the gustatory pathway operates through both peripheral and central mechanisms.{{Clarify | date = May 2019 | reason = It is known that both CNS and PNS are required for all the sensory systems to work.  What's the difference here?  No elaboration anywhere.}} Peripheral [[taste receptor]]s, located on the [[tongue]], [[soft palate]], [[pharynx]], and [[esophagus]], transmit the received signal to primary sensory axons, where the signal is projected to the [[nucleus of the solitary tract]] in the [[medulla oblongata|medulla]], or the gustatory nucleus of the solitary tract complex. The signal is then transmitted to the [[thalamus]], which in turn projects the signal to several regions of the [[neocortex]], including the gustatory cortex.<ref>{{cite book |editor1-first=Dale |editor1-last=Purves |editor2-first=George J |editor2-last=Augustine |editor3-first=David |editor3-last=Fitzpatrick |editor4-first=Lawrence C |editor4-last=Katz |editor5-first=Anthony-Samuel |editor5-last=LaMantia |editor6-first=James O |editor6-last=McNamara |editor7-first=S Mark |editor7-last=Williams |title=Neuroscience |date=2001 |publisher=Sinauer Associates |isbn=978-0-87893-742-4 |edition=2nd |url=https://www.ncbi.nlm.nih.gov/books/NBK10799/ }}{{pn|date=November 2024}}</ref>

The neural processing of taste is affected at nearly every stage of processing by concurrent somatosensory information from the tongue, that is, [[mouthfeel]]. Scent, in contrast, is not combined with taste to create flavor until higher cortical processing regions, such as the insula and orbitofrontal cortex.<ref>{{cite book |last1=Small |first1=Dana M. |last2=Green |first2=Barry G. |title=The Neural Bases of Multisensory Processes |date=2012 |publisher=CRC Press/Taylor & Francis |isbn=978-1-4398-1217-4 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK92876/ |chapter=A Proposed Model of a Flavor Modality |pmid=22593893 |doi=10.1201/9781439812174 |editor-last1=Murray |editor-last2=Wallace |editor-first1=Micah M. |editor-first2=Mark T. }}</ref>

== Quiescent state ==
Most sensory systems have a quiescent state, that is, the state that a sensory system converges to when there is no input.{{fact|date=November 2024}}

This is well-defined for a [[linear time-invariant system]], whose input space is a vector space, and thus by definition has a point of zero. It is also well-defined for any passive sensory system, that is, a system that operates without needing input power. The quiescent state is the state the system converges to when there is no input power.{{fact|date=November 2024}}

It is not always well-defined for nonlinear, nonpassive sensory organs, since they can't function without input energy. For example, a cochlea is not a passive organ, but actively vibrates its own sensory hairs to improve its sensitivity. This manifests as [[otoacoustic emission]]s in healthy ears, and [[tinnitus]] in pathological ears.<ref>{{cite journal |last1=Dallos |first1=P |title=The active cochlea |journal=The Journal of Neuroscience |date=December 1992 |volume=12 |issue=12 |pages=4575–4585 |doi=10.1523/jneurosci.12-12-04575.1992 |pmc=6575778 |pmid=1464757 }}</ref> There is still a quiescent state for the cochlea, since there is a well-defined mode of power input that it receives (vibratory energy on the eardrum), which provides an unambiguous definition of "zero input power".{{fact|date=November 2024}}

Some sensory systems can have multiple quiescent states depending on its history, like [[Flip-flop (electronics)|flip-flops]], and magnetic material with [[hysteresis]]. It can also adapt to different quiescent states. In complete darkness, the retinal cells become extremely sensitive, and there is noticeable "[[visual snow]]" caused by the retinal cells firing randomly without any light input. In brighter light, the retinal cells become much less sensitive, consequently decreasing visual noise.<ref>{{cite journal |last1=Laughlin |first1=S. B. |title=The role of sensory adaptation in the retina |journal=Journal of Experimental Biology |date=September 1989 |volume=146 |issue=1 |pages=39–62 |doi=10.1242/jeb.146.1.39 |pmid=2689569 }}</ref>

Quiescent state is less well-defined when the sensory organ can be controlled by other systems, like a dog's ears that turn towards the front or the sides as the brain commands. Some spiders can use their nets as a large touch-organ, like weaving a skin for themselves. Even in the absence of anything falling on the net, hungry spiders may increase web thread tension, so as to respond promptly even to usually less noticeable, and less profitable prey, such as small fruit flies, creating two different "quiescent states" for the net.<ref>{{cite journal |last1=Japyassú |first1=Hilton F. |last2=Laland |first2=Kevin N. |title=Extended spider cognition |journal=Animal Cognition |date=May 2017 |volume=20 |issue=3 |pages=375–395 |doi=10.1007/s10071-017-1069-7 |pmc=5394149 |pmid=28176133 }}</ref>

Things become completely ill-defined for a system which connects its output to its own input, thus ever-moving without any external input. The prime example is the brain, with its [[default mode network]].{{fact|date=November 2024}}

==Diseases==
[[File:Sense organ diseases world map - DALY - WHO2002.svg|thumb|upright=1.2|[[Disability-adjusted life year]] for sense organ diseases per 100,000&nbsp;inhabitants in 2002.<ref>{{cite web |url=https://www.who.int/entity/healthinfo/statistics/bodgbddeathdalyestimates.xls |title=Mortality and Burden of Disease Estimates for WHO Member States in 2002|format=xls |work=World Health Organization|year=2002 }}</ref>{{Div col|small=yes|colwidth=10em}}
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* [[Amblyopia]]
* [[Anacusis]]
* [[Color blindness]]
* [[Deafness]]

== See also ==
* [[Multisensory integration]]
* [[Neural adaptation]]
* [[Neural coding]]
* [[Sensor]]
* [[Sensory augmentation]]
* [[Sensory neuroscience]]
* [[Sensory systems in fish]]

== References ==
{{reflist|30em}}

==External links==
{{wikibooks|Sensory Systems}}
{{commons category|Sensory system}}

{{Human system and organs}}
{{Sensation and perception}}
{{Eye anatomy|state=collapsed}}
{{Auditory and vestibular systems}}
{{Gustatory system}}
{{Olfactory system}}
{{Somatosensory system}}
{{Systems}}

{{Authority control}}

{{DEFAULTSORT:Sensory System}}
[[Category:Nervous system]]
[[Category:Sensory systems]]
[[Category:Sensory organs]]