Sensory neuron

Sensory neurons in vertebrates are predominantly pseudounipolar or bipolar, and different types of sensory neurons have different sensory receptors that respond to different kinds of stimuli. There are at least six external and two internal sensory receptors:

{{See also|Perception#Types of perception}}

External receptors that respond to stimuli from outside the body are called exteroreceptors.{{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}} Exteroreceptors include chemoreceptors such as olfactory receptors (smell) and taste receptors, photoreceptors (vision), thermoreceptors (temperature), nociceptors (pain), hair cells (hearing and balance), and a number of other different mechanoreceptors for touch and proprioception (stretch, distortion and stress).

The sensory neurons involved in smell are called olfactory sensory neurons. These neurons contain receptors, called olfactory receptors, that are activated by odor molecules in the air. The molecules in the air are detected by enlarged cilia and microvilli.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. 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 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. {{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 }} 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. {{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 }} 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 receptors, taste receptors (gustatory receptors) in taste buds interact with chemicals in food to produce an action potential.

Photoreceptor cells 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 cells, bipolar cells, 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 or 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: 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)."eye, human." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010. 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 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. Retinal ganglion cells are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive.{{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 }}

Issues and decay of sensory neurons associated with vision lead to disorders such as:

1. 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.{{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}}
2. Glaucoma – loss of retinal ganglion cells which causes some loss of vision to blindness.{{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}}
3. Diabetic retinopathy – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina.{{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}}

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⁺ cation channels open allowing Na⁺ to flow into cell and the resulting depolarization causes the Ca⁺⁺ 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 .{{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}}

Problems with sensory neurons associated with the auditory system leads to disorders such as:

1. 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.{{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}}
2. 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.{{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}}

Thermoreceptors are sensory receptors, which respond to varying temperatures. While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that mammals have at least two distinct types of thermoreceptors.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

The bulboid corpuscle, is a cutaneous receptor a cold-sensitive receptor, that detects cold temperatures. The other type is a warmth-sensitive receptor.

{{main|Mechanoreceptor}}

{{further|Mechanosensation}}

Mechanoreceptors are sensory receptors which respond to mechanical forces, such as pressure or distortion.{{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 }}

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.{{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]}}

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.{{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}}

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.{{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}}

Problems with mechanoreceptors lead to disorders such as:

1. Neuropathic pain - a severe pain condition resulting from a damaged sensory nerve
2. 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
3. Phantom limb syndrome - a sensory system disorder where pain or movement is experienced in a limb that does not exist {{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}}

Internal receptors that respond to changes inside the body are known as interoceptors.

The aortic bodies and carotid bodies contain clusters of glomus cells – peripheral chemoreceptors that detect changes in chemical properties in the blood such as oxygen concentration.{{cite journal |vauthors=Satir P, Christensen ST |title=Structure and function of mammalian cilia |journal=Histochem Cell Biol |volume=129 |issue=6 |pages=687–93 |date=June 2008 |pmid=18365235 |pmc=2386530 |doi=10.1007/s00418-008-0416-9 }} These receptors are polymodal responding to a number of different stimuli.

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.Sherrington C. The Integrative Action of the Nervous System. Oxford: Oxford University Press; 1906.{{cite journal | last=St. John Smith | first=Ewan | title=Advances in understanding nociception and neuropathic pain | journal=Journal of Neurology| volume=265 | issue=2 | date=2017-10-14 | issn=0340-5354 | pmid=29032407 | pmc=5808094 | doi=10.1007/s00415-017-8641-6 | pages=231–238}} They are found in internal organs as well as on the surface of the body to "detect and protect". Nociceptors detect different kinds of noxious stimuli indicating potential for damage, then initiate neural responses to withdraw from the stimulus.

1. Thermal nociceptors are activated by noxious heat or cold at various temperatures.
2. Mechanical nociceptors respond to excess pressure or mechanical deformation, such as a pinch.
3. Chemical nociceptors respond to a wide variety of chemicals, some of which signal a response. They are involved in the detection of some spices in food, such as the pungent ingredients in Brassica and Allium plants, which target the sensory neural receptor to produce acute pain and subsequent pain hypersensitivity.{{Cite journal|last1=Zhao|first1=Jianhua|last2=Lin King|first2=John V.|last3=Paulsen|first3=Candice E.|last4=Cheng|first4=Yifan|last5=Julius|first5=David|date=2020-07-08|title=Irritant-evoked activation and calcium modulation of the TRPA1 receptor|journal=Nature|volume=585|issue=7823|pages=141–145|doi=10.1038/s41586-020-2480-9|issn=1476-4687|pmid=32641835|pmc=7483980|bibcode=2020Natur.585..141Z}}

Information coming from the sensory neurons in the head enters the central nervous system (CNS) through cranial nerves. Information from the sensory neurons below the head enters the spinal cord and passes towards the brain through the 31 spinal nerves.{{cite book|title=Biological Psychology|date=2013|publisher=Wadsworth |isbn=978-1-111-83100-4|edition=11th|last1=Kalat|first1=James W.}} The sensory information traveling through the spinal cord follows well-defined pathways. The nervous system codes the differences among the sensations in terms of which cells are active.

A sensory receptor's adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus. Adequate stimulus can be used to classify sensory receptors:

1. Baroreceptors respond to pressure in blood vessels
2. Chemoreceptors respond to chemical stimuli
3. Electromagnetic radiation receptors respond to electromagnetic radiation{{cite web|url=http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/sensory%20systems/sensory.htm|title=Sensory Systems|publisher=Clinton Community College|access-date=2013-06-06|author=Michael J. Gregory|url-status=dead|archive-url=https://archive.today/20130625101331/http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/sensory%20systems/sensory.htm|archive-date=2013-06-25}}
4. Infrared receptors respond to infrared radiation
5. Photoreceptors respond to visible light
6. Ultraviolet receptors respond to ultraviolet radiation {{Citation needed|date=March 2017}}
7. Electroreceptors respond to electric fields
8. Ampullae of Lorenzini respond to electric fields, salinity, and to temperature, but function primarily as electroreceptors
9. Hydroreceptors respond to changes in humidity
10. Magnetoreceptors respond to magnetic fields
11. Mechanoreceptors respond to mechanical stress or mechanical strain
12. Nociceptors respond to damage, or threat of damage, to body tissues, leading (often but not always) to pain perception
13. Osmoreceptors respond to the osmolarity of fluids (such as in the hypothalamus)
14. Proprioceptors provide the sense of position
15. Thermoreceptors respond to temperature, either heat, cold or both

Sensory receptors can be classified by location:

1. Cutaneous receptors are sensory receptors found in the dermis or epidermis.{{Cite web|url=http://medical-dictionary.thefreedictionary.com/Cutaneous+receptor|title=Cutaneous receptor}}
2. Muscle spindles contain mechanoreceptors that detect stretch in muscles.

Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:

1. Free nerve endings characterize the nociceptors and thermoreceptors and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout the dermis and epidermis.
2. Encapsulated receptors consist of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning.

1. A tonic receptor is a sensory receptor that adapts slowly to a stimulus{{cite book|last1=Binder|first1=Marc D.|last2=Hirokawa|first2=Nobutaka|last3=Windhorst|first3=Uwe|title=Encyclopedia of Neuroscience|date=2009|publisher=Springer|isbn=978-3-540-29678-2}} and continues to produce action potentials over the duration of the stimulus.mentor.lscf.ucsb.edu/course/fall/eemb157/lecture/Lectures%2016,%2017%2018.ppt {{Dead link|date=July 2014}} In this way it conveys information about the duration of the stimulus. Some tonic receptors are permanently active and indicate a background level. Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle.{{cite web |url=http://frank.mtsu.edu/~jshardo/bly2010/nervous/receptor.html |title= Sensory Receptor Function|website=frank.mtsu.edu |archive-url=https://web.archive.org/web/20080803215836/http://frank.mtsu.edu/~jshardo/bly2010/nervous/receptor.html |archive-date=August 3, 2008}}
2. A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. The response of the cell diminishes very quickly and then stops.{{cite book|last1=Sherwood|first1=Lauralee|last2=Klandorf|first2=Hillar|last3=Yancey|first3=Paul|title=Animal Physiology: From Genes to Organisms|date=2012|publisher=Cengage Learning|isbn=978-0-8400-6865-1|url=https://books.google.com/books?id=I6X8G8YPdv4C&q=A+phasic+receptor+is+a+sensory+receptor+that+adapts+rapidly+to+a+stimulus.+The+response+of+the+cell+diminishes+very+quickly+and+then+stops.&pg=PA213|access-date=13 December 2017|language=en}} It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate. An example of a phasic receptor is the Pacinian corpuscle.

There are many drugs currently on the market that are used to manipulate or treat sensory system disorders. For instance, gabapentin is a drug that is used to treat neuropathic pain by interacting with one of the voltage-dependent calcium channels present on non-receptive neurons. Some drugs may be used to combat other health problems, but can have unintended side effects on the sensory system. Dysfunction in the hair cell mechanotransduction complex, along with the potential loss of specialized ribbon synapses, can lead to hair cell death, often caused by ototoxic drugs like aminoglycoside antibiotics poisoning the cochlea.{{cite journal |vauthors=Wagner EL, Shin JB |title=Mechanisms of Hair Cell Damage and Repair |journal=Trends Neurosci |volume=42 |issue=6 |pages=414–424 |date=June 2019 |pmid=30992136 |pmc=6556399 |doi=10.1016/j.tins.2019.03.006 }} Through the use of these toxins, the K+ pumping hair cells cease their function. Thus, the energy generated by the endocochlear potential which drives the auditory signal transduction process is lost, leading to hearing loss.{{Cite journal|last1=Priuska|first1=E.M.|last2=Schacht|first2=J.|year=1997|title=Mechanism and prevention of aminoglycoside ototoxicity: Outer hair cells as targets and tools|journal=Ear, Nose, & Throat Journal|volume=76|issue=3|pages=164–171|doi=10.1177/014556139707600310|pmid=9086645|s2cid=8216716}}

Ever since scientists observed cortical remapping in the brain of Taub's Silver Spring monkeys, there has been a large amount of research into sensory system plasticity. Huge strides have been made in treating disorders of the sensory system. Techniques such as constraint-induced movement therapy developed by Taub have helped patients with paralyzed limbs regain use of their limbs by forcing the sensory system to grow new neural pathways.Schwartz and Begley 2002, p. 160; "Constraint-Induced Movement Therapy", excerpted from "A Rehab Revolution," Stroke Connection Magazine, September/October 2004. Print. Phantom limb syndrome is a sensory system disorder in which amputees perceive that their amputated limb still exists and they may still be experiencing pain in it. The mirror box developed by V.S. Ramachandran, has enabled patients with phantom limb syndrome to relieve the perception of paralyzed or painful phantom limbs. It is a simple device which uses a mirror in a box to create an illusion in which the sensory system perceives that it is seeing two hands instead of one, therefore allowing the sensory system to control the "phantom limb". By doing this, the sensory system can gradually get acclimated to the amputated limb, and thus alleviate this syndrome.{{Cite book|title=Phantoms in the brain : probing the mysteries of the human mind|last1=Blakeslee|first1=Sandra|last2=Ramachandran|first2=V. S.|publisher=William Morrow & Company|year=1998|isbn=978-0-688-15247-5|oclc=43344396|url-access=registration|url=https://archive.org/details/phantomsinbrain00vsra}}

Hydrodynamic reception is a form of mechanoreception used in a range of animal species.

File:Blausen 0809 Skin TactileReceptors.png|Illustration of tactile receptors in the skin

File:Blausen 0804 Skin LamellatedCorpuscle.png|Illustration of lamellated corpuscle

File:Blausen 0807 Skin RuffiniCorpuscle.png|Illustration of Ruffini corpuscle

File:Blausen 0805 Skin MerkelCell.png|Illustration of skin Merkel cell

File:Blausen 0808 Skin TactileCorpuscle.png|Illustration of tactile corpuscle

File:Blausen 0806 Skin RootHairPlexus.png|Illustration of root hair plexus

File:Blausen 0803 Skin FreeNerveEndings.png|Illustration of free nerve endings

• Pseudounipolar neuron
• Neural coding
• Posterior column
• Receptive field
• Sensory system
• List of distinct cell types in the adult human body
• Sensory nerve
• Motor nerve
• Afferent nerve fiber
• Efferent nerve fiber
• Motor neuron

{{Reflist}}

• {{Commons category-inline}}
• {{cite book |chapter=Table 9.1 The major classes of somatic sensory receptors |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK11162/table/A611/?report=objectonly |veditors=Purves D, Augustine GJ, Fitzpatrick D, et al |title=Neuroscience |edition=2nd |publisher=Sinauer Associates |location=Sunderland MA |date=2001 |isbn=0-87893-742-0 |url=https://www.ncbi.nlm.nih.gov/books/NBK10799/}}

{{Nervous tissue}}

{{Somatosensory system}}

Category:Afferent neurons

Category:Human cells

Category:Sensory receptors

Category:Receptor cells