Editing Chemoreceptor

{{Short description|Sensory receptor that detects chemicals}}

A '''chemoreceptor''', also known as '''chemosensor''', is a specialized [[Sensory neuron|sensory receptor]] which [[transduction (physiology)|transduces]] a [[chemical substance]] ([[Endogeny (biology)|endogenous]] or induced) to generate a biological signal.<ref name="kumar">{{cite journal |last1=Kumar |first1=Prem |last2=Prabhakar |first2=Nanduri R. |title=Peripheral Chemoreceptors: Function and Plasticity of the Carotid Body |journal=Comprehensive Physiology |date=January 2012 |volume=2 |issue=1 |pages=141–219 |doi=10.1002/cphy.c100069 |pmid=23728973 |pmc=3919066 |isbn=978-0-470-65071-4 }}</ref> This signal may be in the form of an [[action potential]], if the chemoreceptor is a [[neuron]],<ref name="Rawson 23–43">{{cite book |doi=10.1159/000093749 |chapter=Transduction and Coding |title=Taste and Smell |series=Advances in Oto-Rhino-Laryngology |year=2006 |last1=Rawson |first1=Nancy E. |last2=Yee |first2=Karen K. |volume=63 |pages=23–43 |pmid=16733331 |isbn=3-8055-8123-8 }}</ref> or in the form of a [[neurotransmitter]] that can activate a [[Axon|nerve fiber]] if the chemoreceptor is a specialized cell, such as [[taste receptor]]s,<ref>{{cite journal |last1=Saunders |first1=Cecil J. |last2=Christensen |first2=Michael |last3=Finger |first3=Thomas E. |last4=Tizzano |first4=Marco |title=Cholinergic neurotransmission links solitary chemosensory cells to nasal inflammation |journal=Proceedings of the National Academy of Sciences of the United States of America |date=22 April 2014 |volume=111 |issue=16 |pages=6075–6080 |doi=10.1073/pnas.1402251111 |pmc=4000837|pmid=24711432 |bibcode=2014PNAS..111.6075S |doi-access=free }}</ref> or an internal [[peripheral chemoreceptor]], such as the [[carotid body|carotid bodies]].<ref>{{cite journal |last1=Nurse |first1=Colin A. |last2=Piskuric |first2=Nikol A. |title=Signal processing at mammalian carotid body chemoreceptors |journal=Seminars in Cell & Developmental Biology |date=January 2013 |volume=24 |issue=1 |pages=22–30 |doi=10.1016/j.semcdb.2012.09.006 |pmid=23022231 }}</ref> In [[physiology]], a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of [[carbon dioxide]] (hypercapnia) or a decrease in blood levels of [[oxygen]] (hypoxia), and transmits that information to the [[central nervous system]] which engages body responses to restore [[homeostasis]].

In [[bacteria]], chemoreceptors are essential in the mediation of [[chemotaxis]].<ref>{{Cite journal |last1=Hazelbauer |first1=Gerald L. |last2=Falke |first2=Joseph J. |last3=Parkinson |first3=John S. |date=January 2008 |title=Bacterial chemoreceptors: high-performance signaling in networked arrays |journal=Trends in Biochemical Sciences |volume=33 |issue=1 |pages=9–19 |doi=10.1016/j.tibs.2007.09.014 |issn=0968-0004 |pmc=2890293 |pmid=18165013}}</ref><ref>{{Cite journal |last1=Bi |first1=Shuangyu |last2=Lai |first2=Luhua |date=February 2015 |title=Bacterial chemoreceptors and chemoeffectors |url=https://pubmed.ncbi.nlm.nih.gov/25374297/ |journal=Cellular and Molecular Life Sciences |volume=72 |issue=4 |pages=691–708 |doi=10.1007/s00018-014-1770-5 |issn=1420-9071 |pmid=25374297|s2cid=15976114 |pmc=11113376 }}</ref>

==Cellular chemoreceptors==

===In prokaryotes===
Bacteria utilize complex long helical proteins as chemoreceptors, permitting signals to travel long distances across the cell's membrane. Chemoreceptors allow bacteria to react to chemical stimuli in their environment and regulate their movement accordingly.<ref>{{Cite journal |last1=Samanta |first1=Dipanjan |last2=P. Borbat |first2=Peter |last3=Dzikovski |first3=Boris |last4=H. Freed |first4=Jack |last5=R. Crane |first5=Brian |date=9 February 2015 |title=Bacterial chemoreceptor dynamics correlate with activity state and are coupled over long distances |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=8 |pages=2455–2460 |doi=10.1073/pnas.1414155112 |pmid=25675479 |pmc=4345563 |bibcode=2015PNAS..112.2455S |doi-access=free }}</ref> In [[archaea]], [[Cell surface receptor|transmembrane receptors]] comprise only 57% of chemoreceptors, while in bacteria the percentage rises to 87%. This is an indicator that chemoreceptors play a heightened role in the sensing of [[cytosol]]ic signals in archaea.<ref>{{Cite journal |last=Krell |first=Tino |date=1 April 2007 |title=Exploring the (Almost) Unknown: Archaeal Two-Component Systems |journal=Journal of Bacteriology |volume=200 |issue=7 |doi=10.1128/JB.00774-17 |pmc=5847645 |pmid=29339416 }}</ref>

===In eukaryotes===
[[Primary cilia]], present in many types of mammalian [[cell (biology)|cells]], serve as ''cellular antennae''.<ref name="Satir2008">{{cite journal |doi=10.1007/s00418-008-0416-9 |pmid=18365235 |pmc=2386530 |title=Structure and function of mammalian cilia |journal=Histochemistry and Cell Biology |volume=129 |issue=6 |pages=687–93 |year=2008 |last1=Satir |first1=Peter |last2=Christensen |first2=Søren T. }}</ref> The motile function of these cilia is lost in favour of their sensory specialization.<ref>{{Cite book |last=R. Mitchell |first=David |title=Eukaryotic Membranes and Cytoskeleton |chapter=The Evolution of Eukaryotic Cilia and Flagella as Motile and Sensory Organelles |series=Advances in Experimental Medicine and Biology |date=10 April 2012 |volume=607 |pages=130–140 |doi=10.1007/978-0-387-74021-8_11 |pmc=3322410 |pmid=17977465 |isbn=978-0-387-74020-1 }}</ref>

==Plant chemoreceptors==

Plants have various mechanisms to perceive danger in their environment. Plants are able to detect pathogens and microbes through surface level receptor kinases (PRK). Additionally, receptor-like proteins (RLPs) containing [[Ligand binding domain|ligand binding receptor domain]]s capture [[pathogen-associated molecular pattern]]s (PAMPS) and [[damage-associated molecular pattern]]s (DAMPS) which consequently initiates the plant's innate immunity for a defense response.<ref>{{cite journal |last1=Zipfel |first1=Cyril |title=Plant pattern-recognition receptors |journal=Trends in Immunology |date=July 2014 |volume=35 |issue=7 |pages=345–351 |doi=10.1016/j.it.2014.05.004 |pmid=24946686 }}</ref>

Plant receptor kinases are also used for growth and hormone induction among other important biochemical processes. These reactions are triggered by a series of [[signaling pathway]]s which are initiated by plant chemically sensitive receptors.<ref>{{cite journal |last1=Haffani |first1=Yosr Z. |last2=Silva |first2=Nancy F. |last3=Goring |first3=Daphne R. |title=Receptor kinase signalling in plants |journal=Canadian Journal of Botany |date=2 February 2011 |volume=82 |pages=1–15 |doi=10.1139/b03-126 |s2cid=53062169 }}</ref> [[Plant hormone]] receptors can either be integrated in plant cells or situate outside the cell, in order to facilitate chemical structure and composition. There are 5 major categories of hormones that are unique to plants which once bound to the receptor, will trigger a response in target cells. These include [[auxin]], [[abscisic acid]], [[gibberellin]], [[cytokinin]], and [[ethylene]]. Once bound, hormones can induce, inhibit, or maintain function of the target response.<ref>{{cite journal |last1=Armitage |first1=Lynne |last2=Leyser |first2=Ottoline |title=Plant hormone receptors |journal=Access Science |date=2021 |doi=10.1036/1097-8542.900137 }}</ref>

== Classes ==
There are two main classes of chemoreceptor: direct and distance.{{Citation needed|date=July 2010}} 
* Examples of ''distance chemoreceptors'' are:
**[[olfactory receptor neuron]]s in the [[olfactory system]]: Olfaction involves the ability to detect chemicals in the gaseous state. In vertebrates, the olfactory system detects [[odor]]s and [[pheromone]]s in the nasal cavity. Within the olfactory system there are two anatomically distinct organs: the main olfactory epithelium (MOE) and the [[vomeronasal organ]] (VNO). It was initially thought that the MOE is responsible for the detection of odorants, while the VNO detects pheromones. The current view, however, is that both systems can detect odorants and pheromones.<ref>{{cite book |doi=10.1007/400_2008_4 |pmid=19145414 |chapter=Extraordinary Diversity of Chemosensory Receptor Gene Repertoires Among Vertebrates |title=Chemosensory Systems in Mammals, Fishes, and Insects |volume=47 |pages=57–75 |series=Results and Problems in Cell Differentiation |year=2009 |last1=Shi |first1=P. |last2=Zhang |first2=J. |isbn=978-3-540-69918-7 }}</ref> Olfaction in invertebrates differs from olfaction in vertebrates. For example, in insects, olfactory sensilla are present on their antennae.<ref name="Chapman RF 1998 Chemoreception ">{{cite book |last1=Chapman |first1=R. F. |chapter=Chemoreception |pages=636–654 |chapter-url=https://books.google.com/books?id=jHUCdbgW4MAC&pg=PA636 |title=The Insects: Structure and Function |date=1998 |publisher=Cambridge University Press |isbn=978-0-521-57890-5 }}</ref>
* Examples of ''direct chemoreceptors'' include: 
** [[Taste receptor]]s in the [[gustatory system]]: The primary use of gustation as a type of chemoreception is for the detection of tasteants. Aqueous chemical compounds come into contact with chemoreceptors in the mouth, such as taste buds on the tongue, and trigger responses. These chemical compounds can either trigger an appetitive response for nutrients, or a defensive response against toxins depending on which receptors fire. Fish and crustaceans, who are constantly in an aqueous environment, use their gustatory system to identify certain chemicals in the mixture for the purpose of localization and ingestion of food. 
**Insects use contact chemoreception to recognize certain chemicals such as cuticular hydrocarbons and chemicals specific to host plants. Contact chemoreception is more commonly seen in insects but is also involved in the mating behavior of some vertebrates. The contact chemoreceptor is specific to one type of chemical.<ref name="Chapman RF 1998 Chemoreception "/>

== Sensory organs ==
*Olfaction: In terrestrial vertebrates, olfaction occurs in the [[nose]]. Volatile chemical stimuli enter the nose and eventually reach the [[olfactory epithelium]] which houses the chemoreceptor cells known as [[olfactory sensory neuron]]s often referred to as OSNs. Embedded in the olfactory epithelium are three types of cells: supporting cells, basal cells, and OSNs. While all three types of cells are integral to normal function of the epithelium, only OSN serve as receptor cells, i.e. responding to the chemicals and generating an action potential that travels down the [[olfactory nerve]] to reach the brain.<ref name="Rawson 23–43"/> In insects, [[antenna (biology)|antennae]] act as distance chemoreceptors. For example, antennae on moths are made up of long feathery hairs that increase sensory surface area. Each long hair from the main antenna also has smaller sensilla that are used for volatile olfaction.<ref>{{cite book|last1=Haupt|first1=S. Shuichi|last2=Sakurai|first2=Takeshi|last3=Namiki|first3=Shigehiro|last4=Kazawa|first4=Tomoki|last5=Kanzaki|first5=Ryohei|title=The Neurobiology of Olfaction|date=2010|publisher=CRC Press/Taylor & Francis|isbn=9781420071979|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK55976/|chapter=Olfactory information processing in moths|pmid=21882429}}</ref> Since moths are mainly nocturnal animals, the development of greater olfaction aids them in navigating the night.
*Gustation: In many terrestrial vertebrates, the [[tongue]] serves as the primary gustatory sensory organ. As a muscle located in the mouth, it acts to manipulate and discern the composition of food in the initial stages of digestion. The tongue is rich in vasculature, allowing the chemoreceptors located on the top surface of the organ to transmit sensory information to the brain. Salivary glands in the mouth allow for molecules to reach chemoreceptors in an aqueous solution. The chemoreceptors of the tongue fall into two distinct superfamilies of [[G protein-coupled receptor]]s. GPCR's are intramembrane proteins than bind to an extracellular ligand- in this case chemicals from food- and begin a diverse array of signaling cascades that can result in an action potential registering as input in an organism's brain. Large quantities of chemoreceptors with discrete ligand-binding domains provide for the five basic tastes: sour, salty, bitter, sweet, and [[Umami|savory]]. The salty and sour tastes work directly through the ion channels, the sweet and bitter taste work through [[G protein-coupled receptor]]s, and the savory sensation is activated by [[glutamate]].{{citation needed|date=May 2015}}Gustatory chemosensors are not just present on the tongue but also on different cells of the gut epithelium where they communicates the sensory information to several effector systems involved in the regulation of appetite, immune responses, and gastrointestinal motility.<ref>{{cite journal |last1=Steensels |first1=S. |last2=Depoortere |first2=I. |title=Chemoreceptors in the Gut |journal=Annual Review of Physiology |date=10 February 2018 |volume=80 |issue=1 |pages=117–141 |doi=10.1146/annurev-physiol-021317-121332 |pmid=29029594 }}</ref>
*Contact Chemoreception: Contact chemoreception is dependent on the physical contact of the receptor with the stimulus. The receptors are short hairs or cones that have a single pore at, or close to the tip of the projection. They are known as uniporous receptors. Some receptors are flexible, while others are rigid and do not bend with contact. They are mostly found in the mouthparts, but can also occur on the antennae or legs of some insects. There is a collection of dendrites located near the pores of the receptors, yet the distribution of these dendrites changes depending on the organism being examined. The method of transduction of the signal from the dendrites differs depending on the organism and the chemical it is responding to.

When inputs from the environment are significant to the survival of the organism, the input must be detected. As all life processes are ultimately based on [[chemistry]] it is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals.{{citation needed|date=May 2015}}

Chemoreception is important for the detection of food, habitat, conspecifics including mates, and predators. For example, the emissions of a predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been. Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. It passes in either chemical or electrochemical form to the central processor, the [[brain]] or [[spinal cord]]. The resulting output from the CNS ([[central nervous system]]) makes body actions that will engage the food and enhance survival.{{citation needed|date=May 2015}}

==Physiology==

* [[carotid body|Carotid bodies]] and [[Aortic body|aortic bodies]] detect changes primarily in pCO<sub>2</sub> and H<sup>+</sup> ion concentration. They also sense decrease in partial pressure of O<sub>2</sub>, but to a lesser degree than for pCO<sub>2</sub> and H<sup>+</sup> ion concentration.
* The [[chemoreceptor trigger zone]] is an area of the [[medulla oblongata|medulla]] in the brain that receives inputs from [[blood]]-borne [[drug]]s or [[hormone]]s, and communicates with the [[vomiting center]] (area postrema) to induce [[vomiting]].{{Citation needed|date=July 2010}}
* Primary cilia play important roles in chemosensation. In adult tissues, these cilia regulate cell proliferation in response to external stimuli, such as tissue damage. In humans, improper functioning of primary cilia is associated with important diseases known as [[Ciliopathy|ciliopathies]].<ref name="Satir2008" />

===Control of breathing===
Particular chemoreceptors, called [[acid-sensing ion channel|ASICs]], detect the levels of [[carbon dioxide]] in the blood. To do this, they monitor the concentration of [[hydrogen ion]]s in the blood, which decrease the [[pH]] of the blood. This can be a direct consequence of an increase in carbon dioxide concentration, because aqueous carbon dioxide in the presence of [[carbonic anhydrase]] reacts to form a [[proton]] and a [[bicarbonate]] ion.{{citation needed|date=May 2015}}

The response is that the respiratory centre (in the medulla), sends [[nerve impulse|nervous impulses]] to the external [[intercostal muscles]] and the [[diaphragm (anatomy)|diaphragm]], via the [[intercostal nerve]] and the [[phrenic nerve]], respectively, to increase breathing rate and the volume of the lungs during inhalation.

Chemoreceptors that regulate the depth and rhythm of breathing are broken down into two categories.{{Citation needed|date=July 2010}}

* [[central chemoreceptors]] are located on the ventrolateral surface of [[medulla oblongata]] and detect changes in pH of cerebrospinal fluid. They have also been shown experimentally to respond to hypercapnic hypoxia (elevated {{CO2}}, decreased O2), and eventually desensitize, partly due to redistribution of bicarbonate out of the cerebrospinal fluid (CSF) and increased renal excretion of bicarbonate.<ref>{{cite book |doi=10.1016/B978-1-4377-1679-5.00025-9 |chapter=Pulmonary Physiology |title=Pharmacology and Physiology for Anesthesia |year=2013 |last1=Lumb |first1=Andrew B. |last2=Horner |first2=Deborah |pages=445–457 |isbn=9781437716795 }}</ref> These are sensitive to pH and {{CO2}}.<ref>{{Cite web|title=Central Chemoreceptors|url=http://pathwaymedicine.org/central-chemoreceptors|access-date=2021-03-16|website=pathwaymedicine.org|language=en|archive-date=2021-04-13|archive-url=https://web.archive.org/web/20210413171352/http://pathwaymedicine.org/Central-Chemoreceptors|url-status=dead}}</ref>
* [[peripheral chemoreceptors]]: consists of aortic and carotid bodies. [[Aortic body]] detects changes in blood oxygen and carbon dioxide, but not pH, while [[carotid body]] detects all three. They do not desensitize. Their effect on breathing rate is less than that of the central chemoreceptors.{{citation needed|date=May 2015}}

=== Heart rate ===
The response to stimulation of chemoreceptors on the heart rate is complicated. Chemoreceptors in the heart or nearby large arteries, as well as chemoreceptors in the lungs, can affect heart rate. Activation of these peripheral chemoreceptors from sensing decreased O<sub>2</sub>, increased CO<sub>2</sub> and a decreased pH is relayed to cardiac centers by the [[vagus nerve|vagus]] and [[glossopharyngeal nerve|glossopharyngeal nerves]] to the [medulla oblongata|medulla] of the brainstem. This increases the sympathetic nervous stimulation on the heart and a corresponding increase in heart rate and [[myocardial contractility|contractility]] in most cases.<ref>{{Cite web|url=http://www.columbia.edu/~kj3/Chapter4.htm|title=Chapter 4|website=www.columbia.edu|access-date=2017-01-29}}</ref> These factors include activation of [[stretch receptors]] due to increased ventilation and the release of circulating catecholamines.

However, if respiratory activity is arrested (e.g. in a patient with a high cervical spinal cord injury), then the primary cardiac reflex to transient [[hypercapnia]] and [[Hypoxia (medical)|hypoxia]] is a profound [[bradycardia]] and coronary [[vasodilation]] through vagal stimulation and systemic vasoconstriction by sympathetic stimulation.<ref>{{cite journal |last1=Berk |first1=James L. |last2=Levy |first2=Matthew N. |title=Profound Reflex Bradycardia Produced by Transient Hypoxia or Hypercapnia in Man |journal=European Surgical Research |date=1977 |volume=9 |issue=2 |pages=75–84 |doi=10.1159/000127928 |pmid=852474 }}</ref> In normal cases, if there is reflexive increase in respiratory activity in response to chemoreceptor activation, the increased sympathetic activity on the cardiovascular system would act to increase heart rate and contractility.

== See also ==

{{div col|colwidth=20em}}
* [[Cell surface receptor]]
* [[Chemosensory clusters]]
* [[Chemoreceptor trigger zone]]
* [[Diffuse chemosensory system]]
* [[Haptic technology]]
* [[Molecular sensor]]
* [[Odor]]
* [[Olfaction]]
* [[Solitary chemosensory cells]]
* [[Sensory receptor]]
* [[Taste]]
* [[Vomeronasal organ]]
{{div col end}}[[List of distinct cell types in the adult human body]]

== References ==
{{Reflist|30em}}

== External links ==
{{Americana Poster|Animals, Chemical Sense in|Chemoreceptor}}
* {{MeshName|Chemoreceptors}}
*Association of Chemoreception Sciences http://www.achems.org/

{{Sensation and perception}}
{{Respiratory physiology}}
{{Authority control}}

[[Category:Sensory receptors]]
[[Category:Gas sensors]]
[[Category:Analytical chemistry]]
[[Category:Respiratory physiology]]
[[Category:Vomiting]]