Editing Sensory neuron (section)

=== External receptors===
{{See also|Perception#Types of perception}}
External receptors that respond to stimuli from outside the body are called '''exteroreceptors'''.<ref name="Campbell">{{cite book |last1=Campbell |first1=Neil |title=Biology |url=https://archive.org/details/biology4ewithint00neil |url-access=registration |date=1996 |publisher=Benjamin/Cummings Pub. Co |isbn=0805319409 |page=[https://archive.org/details/biology4ewithint00neil/page/1028 1028] |edition=4th}}</ref> Exteroreceptors include [[chemoreceptor]]s such as [[olfactory receptor]]s ([[sense of smell|smell]]) and [[taste receptor]]s, [[photoreceptor cell|photoreceptor]]s ([[visual perception|vision]]), [[thermoreceptors]] ([[thermoception|temperature]]), [[nociceptor]]s ([[pain]]), [[hair cell]]s ([[hearing]] and [[sense of balance|balance]]), and a number of other different [[mechanoreceptor]]s for [[touch]] and [[proprioception]] ([[Stretching|stretch]], [[distortion]] and [[Stress (mechanics)|stress]]).

==== Smell ====
The sensory neurons involved in [[olfaction|smell]] are called [[olfactory receptor neuron|olfactory sensory neuron]]s. These neurons contain [[Receptor (biochemistry)|receptor]]s, called [[olfactory receptor]]s, that are activated by [[odor]] molecules in the air. The molecules in the air are detected by enlarged [[cilia]] and [[microvilli]].<ref>Breed, Michael D., and Moore, Janice. [https://books.google.com/books?id=O5lnDwAAQBAJ&q=%22sensory+neuron%22 Encyclopedia of Animal Behavior] . London: Elsevier, 2010. Print.</ref> These sensory neurons produce action potentials. Their axons form the [[olfactory nerve]], and they synapse directly onto neurons in the cerebral cortex ([[olfactory bulb]]). They do not use the same route as other sensory systems, bypassing the brain stem and the thalamus. The neurons in the olfactory bulb that receive direct sensory nerve input, have connections to other parts of the olfactory system and many parts of the [[limbic system]]. 9.

==== Taste ====
Taste sensation is facilitated by specialized sensory neurons located in the taste buds of the tongue and other parts of the mouth and throat. These sensory neurons are responsible for detecting different taste qualities, such as sweet, sour, salty, bitter, and savory. When you eat or drink something, chemicals in the food or liquid interact with receptors on these sensory neurons, triggering signals that are sent to the brain. The brain then processes these signals and interprets them as specific taste sensations, allowing you to perceive and enjoy the flavors of the foods you consume. <ref>{{cite book |vauthors=Vincis R, Fontanini A |chapter=Central taste anatomy and physiology |title=Smell and Taste |journal=Handb Clin Neurol |series=Handbook of Clinical Neurology |volume=164 |issue= |pages=187–204 |date=2019 |pmid=31604547 |pmc=6989094 |doi=10.1016/B978-0-444-63855-7.00012-5 |isbn=978-0-444-63855-7 }}</ref> When taste receptor cells are stimulated by the binding of these chemical compounds (tastants), it can lead to changes in the flow of ions, such as sodium (Na+), calcium (Ca2+), and potassium (K+), across the cell membrane. <ref>{{cite journal |vauthors=Taruno A, Nomura K, Kusakizako T, Ma Z, Nureki O, Foskett JK |title=Taste transduction and channel synapses in taste buds |journal=Pflugers Arch |volume=473 |issue=1 |pages=3–13 |date=January 2021 |pmid=32936320 |pmc=9386877 |doi=10.1007/s00424-020-02464-4 }}</ref> In response to tastant binding, ion channels on the taste receptor cell membrane can open or close. This can lead to depolarization of the cell membrane, creating an electrical signal.

Similar to [[olfactory receptor]]s, [[taste receptor]]s (gustatory receptors) in [[taste bud]]s interact with chemicals in food to produce an [[action potential]].

==== Vision ====
[[Photoreceptor cell]]s are capable of [[phototransduction]], a process which converts light ([[electromagnetic radiation]]) into electrical signals. These signals are refined and controlled by the interactions with other types of neurons in the retina. The five basic classes of neurons within the retina are [[photoreceptor cell]]s, [[bipolar cells]], [[Retinal ganglion cell|ganglion cells]], [[horizontal cells]], and [[amacrine cells]]. The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor (either a [[Rod cell|rod]] or [[Cone cell|cone]]), bipolar cell, and the ganglion cell. The first action potential occurs in the retinal ganglion cell. This pathway is the most direct way for transmitting visual information to the brain. There are three primary types of photoreceptors: [[Cone cell|Cones]] are photoreceptors that 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 that 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]]s are involved in the [[sympathetic response]]. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.<ref>{{cite journal |vauthors=Foster RG, Provencio I, Hudson D, Fiske S, De Grip W, Menaker M |title=Circadian photoreception in the retinally degenerate mouse (rd/rd) |journal=J Comp Physiol A |volume=169 |issue=1 |pages=39–50 |date=July 1991 |pmid=1941717 |doi=10.1007/BF00198171 }}</ref>

Issues and decay of sensory neurons associated with vision lead to disorders such as:
#  [[Macular degeneration]] – degeneration of the central visual field due to either cellular debris or blood vessels accumulating between the retina and the choroid, thereby disturbing and/or destroying the complex interplay of neurons that are present there.<ref>{{Cite journal|last=de Jong|first=Paulus T.V.M.|date=2006-10-05|title=Age-Related Macular Degeneration|journal=New England Journal of Medicine|volume=355|issue=14|pages=1474–1485|doi=10.1056/NEJMra062326|issn=0028-4793|pmid=17021323}}</ref>
#  [[Glaucoma]] – loss of retinal ganglion cells which causes some loss of vision to blindness.<ref>{{Cite book|title=Clinical methods : the history, physical, and laboratory examinations|last1=Alguire|first1=Patrick|last2=Dallas|first2=Wilbur|last3=Willis|first3=John|last4=Kenneth|first4=Henry|publisher=Butterworths|year=1990|isbn=978-0409900774|edition=3rd|chapter=Ch. 118 Tonometry|oclc=15695765}}</ref>
#  [[Diabetic retinopathy]] – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.<ref>{{Cite web|url=https://nihseniorhealth.gov/diabeticretinopathy/causesandriskfactors/01.html|title=NIHSeniorHealth: Diabetic Retinopathy - Causes and Risk Factors|website=nihseniorhealth.gov|access-date=2016-12-19|archive-url=https://web.archive.org/web/20170114062500/https://nihseniorhealth.gov/diabeticretinopathy/causesandriskfactors/01.html|archive-date=2017-01-14|url-status=dead}}</ref>

==== Auditory ====
The [[auditory system]] is responsible for converting pressure waves generated by vibrating air molecules or [[sound]] into signals that can be interpreted by the brain.

This mechanoelectrical transduction is mediated with [[hair cells]] within the ear. Depending on the movement, the hair cell can either hyperpolarize or depolarize. When the movement is towards the tallest [[stereocilia]], the Na<sup>+</sup> cation channels open allowing Na<sup>+</sup> to flow into cell and the resulting depolarization causes the Ca<sup>++</sup> channels to open, thus releasing its neurotransmitter into the afferent auditory nerve. There are two types of hair cells: inner and outer. The inner hair cells are the sensory receptors <!-- the sentence about outer hair cells being from efferent neurons is incorrect -->.<ref>{{Cite book|title=Neuroscience|url=https://archive.org/details/neuroscienceissu00purv|url-access=limited|last1=Purves|first1=Dale|last2=Augustine|first2=George|last3=Fitzpatrick|first3=David|last4=Hall|first4=William|last5=LaMantia|first5=Anthony-Samuel|last6=McNamara|first6=James|last7=White|first7=Leonard|publisher=Sinauer Associates, Inc.|year=2008|isbn=978-0878936977|edition=4th|pages=[https://archive.org/details/neuroscienceissu00purv/page/n352 327]–330}}</ref>

Problems with sensory neurons associated with the auditory system leads to disorders such as:
# [[Auditory processing disorder]] – Auditory information in the brain is processed in an abnormal way. Patients with auditory processing disorder can usually gain the information normally, but their brain cannot process it properly, leading to hearing disability.<ref>{{Cite web|url=http://www.chimehealth.co.uk/web/data/apd-mrc-booklet-6.pdf|title=Auditory Processing Disorder (APD)|publisher=British Society of Audiology APD Special Interest Group MRC Institute of Hearing Research|access-date=2016-12-19|archive-date=2016-04-02|archive-url=https://web.archive.org/web/20160402132302/https://www.chimehealth.co.uk/web/data/apd-mrc-booklet-6.pdf|url-status=dead}}</ref>
# [[Auditory verbal agnosia]] – Comprehension of speech is lost but hearing, speaking, reading, and writing ability is retained. This is caused by damage to the posterior superior [[temporal lobes]], again not allowing the brain to process auditory input correctly.<ref>{{Cite journal|last1=Stefanatos|first1=Gerry A.|last2=Gershkoff|first2=Arthur|last3=Madigan|first3=Sean|date=2005-07-01|title=On pure word deafness, temporal processing, and the left hemisphere|journal=Journal of the International Neuropsychological Society|volume=11|issue=4|pages=456–470; discussion 455|doi=10.1017/S1355617705050538|issn=1355-6177|pmid=16209426|s2cid=25584363}}</ref>

==== Temperature ====
[[Thermoreceptor]]s 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 name="krantz">Krantz, John. ''[http://www.saylor.org/content/krantz_sensation/Experiencing_Sensation_and_Perception.pdf Experiencing Sensation and Perception] {{Webarchive|url=https://web.archive.org/web/20171117002814/https://www.saylor.org/content/krantz_sensation/Experiencing_Sensation_and_Perception.pdf |date=2017-11-17 }}''. Pearson Education, Limited, 2009. p. 12.3</ref>
The [[bulboid corpuscle]], is a [[cutaneous receptor]] a ''cold-sensitive'' receptor, that detects cold temperatures. The other type is a warmth-sensitive receptor.

==== Mechanoreceptors ====
{{main|Mechanoreceptor}}
{{further|Mechanosensation}}

Mechanoreceptors are sensory receptors which respond to mechanical forces, such as [[pressure]] or [[distortion]].<ref>{{cite journal |vauthors=Winter R, Harrar V, Gozdzik M, Harris LR |title=The relative timing of active and passive touch |journal=Brain Res |volume=1242 |issue= |pages=54–8 |date=November 2008 |pmid=18634764 |doi=10.1016/j.brainres.2008.06.090 }}</ref>

Specialized sensory receptor cells called mechanoreceptors often encapsulate afferent fibers to help tune the afferent fibers to the different types of somatic stimulation. Mechanoreceptors also help lower thresholds for action potential generation in afferent fibers and thus make them more likely to fire in the presence of sensory stimulation.<ref>{{Cite book|title=Neuroscience|url=https://archive.org/details/neuroscienceissu00purv|url-access=limited|last1=Purves|first1=Dale|last2=Augustine|first2=George|last3=Fitzpatrick|first3=David|last4=Hall|first4=William|last5=LaMantia|first5=Anthony-Samuel|last6=McNamara|first6=James|last7=White|first7=Leonard|publisher=Sinauer Associates, Inc.|year=2008|isbn=978-0878936977|edition=4th|pages=[https://archive.org/details/neuroscienceissu00purv/page/n234 209]}}</ref>

Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.

[[Proprioceptors]] are another type of mechanoreceptors which literally means "receptors for self". These receptors provide spatial information about limbs and other body parts.<ref>{{Cite book|title=Neuroscience|url=https://archive.org/details/neuroscienceissu00purv|url-access=limited|last1=Purves|first1=Dale|last2=Augustine|first2=George|last3=Fitzpatrick|first3=David|last4=Hall|first4=William|last5=LaMantia|first5=Anthony-Samuel|last6=McNamara|first6=James|last7=White|first7=Leonard|publisher=Sinauer Associates|year=2008|isbn=978-0878936977|edition=4th|pages=[https://archive.org/details/neuroscienceissu00purv/page/n240 215]–216}}</ref>

[[Nociceptors]] are responsible for processing pain and temperature changes. The burning pain and irritation experienced after eating a chili pepper (due to its main ingredient, capsaicin), the cold sensation experienced after ingesting a chemical such as menthol or icillin, as well as the common sensation of pain are all a result of neurons with these receptors.<ref name=":0">{{Cite journal|last1=Lee|first1=Y|last2=Lee|first2=C|last3=Oh|first3=U|year=2005|title=Painful channels in sensory neurons|journal=Molecules and Cells|volume=20|issue=3|pages=315–324|doi=10.1016/S1016-8478(23)25242-5|pmid=16404144|doi-access=free}}</ref>

Problems with mechanoreceptors lead to disorders such as:
# [[Neuropathic pain]] - a severe pain condition resulting from a damaged sensory nerve <ref name=":0" />
# [[Hyperalgesia]] - an increased sensitivity to pain caused by sensory ion channel, [[TRPM8]], which is typically responds to temperatures between 23 and 26 degrees, and provides the cooling sensation associated with menthol and icillin <ref name=":0" />
# [[Phantom limb syndrome]] - a sensory system disorder where pain or movement is experienced in a limb that does not exist <ref>{{Cite journal|last1=Halligan|first1=Peter W|last2=Zeman|first2=Adam|last3=Berger|first3=Abi|date=1999-09-04|title=Phantoms in the brain|journal=BMJ: British Medical Journal|volume=319|issue=7210|pages=587–588|doi=10.1136/bmj.319.7210.587|issn=0959-8138|pmc=1116476|pmid=10473458}}</ref>