Carotid body
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2008-07-16T16:55:04Z
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{{Infobox Anatomy |
Name = {{PAGENAME}} |
Latin = glomus caroticum |
GraySubject = 277 |
GrayPage = 1281 |
Image = Gray1186.png |
Caption = Section of part of human glomus caroticum. Highly magnified. Numerous bloodvessels are seen in section among the gland cells. |
Image2 = Gray474.png |
Caption2 = Diagram showing the origins of the main branches of the carotid arteries. |
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Nerve = |
Lymph = |
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DorlandsPre = g_07 |
DorlandsSuf = 12394794 |
}}
The '''carotid body''' ([[carotid glomus]] or [[glomus caroticum]]) is a small cluster of [[chemoreceptor]]s and supporting cells located near the fork ([[bifurcation]]) of the [[carotid artery]] (which runs along both sides of the throat).
The carotid body detects changes in the composition of arterial blood flowing through it, mainly the partial pressure of [[oxygen]], but also of [[carbon dioxide]]. Furthermore, it is also sensitive to changes in [[pH]] and [[temperature]].
==Composition==
The carotid body is made up of two types of cell: type I (glomus) cells, and type II (sustentacular) cells. Glomus cells are derived from [[neural crest]],<ref name="pmid7938227">{{cite journal |author=Gonzalez C, Almaraz L, Obeso A, Rigual R |title=Carotid body chemoreceptors: from natural stimuli to sensory discharges |journal=Physiol. Rev. |volume=74 |issue=4 |pages=829–98 |year=1994 |pmid=7938227 |doi=}}</ref> which, in turn are derived from [[neuroectoderm]]. They release a variety of [[neurotransmitter]]s, including [[acetylcholine]], [[adenosine triphosphate|ATP]], and [[dopamine]] that trigger [[EPSP]]s in synapsed neurons leading to the respiratory center.
Type II cells resemble [[Glial cell|glia]], express the glial marker S100 and act as supporting cells.
==Function==
The carotid body functions as a sensor, it responds to a stimulus, primarily O<sub>2</sub> partial pressure, which is detected by the type I (glomus) cells, and triggers an [[action potential]] in an afferent nerve fiber, the carotid sinus nerve, which relays the information to the central nervous system.
===Stimulus===
While the central chemoreceptors in the brainstem are highly sensitive to CO<sub>2</sub> the carotid body is a peripheral chemoreceptor that mainly provides afferent input to the respiratory center that is highly O<sub>2</sub> dependent. However, the carotid body also senses increases in CO<sub>2</sub> partial pressure and decreases in arterial pH, but to a lesser degree than for O<sub>2</sub>
The output of the carotid bodies is low at an oxygen [[partial pressure]] above about 100 mmHg ([[torr]]) (at normal physiological pH), but below this the activity of the type I (glomus) cells increases rapidly.
===Detection===
The mechanism for detecting reductions in P<small>O<sub>2</sub></small> is not well understood. There may be a [[heme]]-containing protein in the glomus cell which responds to the loss of [[Complex (chemistry)|complexed]] oxygen by reducing the probability of potassium channels being open. Another possibility is that low P<small>O<sub>2</sub></small> inhibits [[NADPH oxidase]] in mitochondria. This would increase the ratio of reduced glutathione to oxidised glutathione, which blocks potassium channels.
An increased P<small>CO<sub>2</sub></small> is detected because the CO<sub>2</sub> [[diffusion|diffuses]] into the cell, where it increase the concentration of [[carbonic acid]] and thus [[protons]]. These protons displace calcium from [[BK channel|high-conductance calcium channels]], reducing potassium current.
Arterial [[acidosis]] (either [[metabolic acidosis|metabolic]] or from altered [[respiratory acidosis|P<small>CO<sub>2</sub></small>]]) inhibits acid-base transporters (e.g. Na<sup>+</sup>-H<sup>+</sup>) which raise intracellular pH, and activates transporters (e.g. Cl<sup>-</sup>-HCO<sub>3</sub><sup>-</sup>) which decrease it. Changes in proton concentration caused by acidosis (or the opposite from [[alkalosis]]) inside the cell stimulates the same pathways involved in P<small>CO<sub>2</sub></small> sensing.
===Action potential===
The type 1 (glomus) cells in the carotid (and aortic bodies) are derived from neuroectoderm and are thus electrically excitable. A decrease in oxygen partial pressure, an increase in carbon dioxide partial pressure, and a decrease in arterial pH can all cause [[depolarization]] of the [[cell membrane]], and they effect this by blocking [[potassium]] currents. This reduction in the [[membrane potential]] opens [[voltage-gated]] [[calcium]] channels, which causes a rise in intracellular calcium concentration. This causes [[exocytosis]] of [[vesicles]] containing a variety of [[neurotransmitters]], including [[acetylcholine]], [[noradrenaline]], [[dopamine]], [[adenosine]], [[ATP]], [[substance P]], and [[met-enkephalin]]. These act on [[Receptor (biochemistry)|receptors]] on the afferent nerve fibres which lie in apposition to the glomus cell to cause an action potential.
===Relay===
The feedback from the carotid body is sent to the cardiorespiratory centers in the [[medulla oblongata]] via the afferent branches of the [[glossopharyngeal nerve]] . The cell bodies of this nerve are located in the petrosal ganglion. The afferent fibres of the aortic body chemoreceptors are relayed by the [[vagus nerve]]. These centers, in turn, regulate breathing and blood pressure.
==Disorders==
A [[paraganglioma]] is a tumor that may involve the carotid body.
==References==
<references/>
==See also==
* [[Aortic body]]
* [[Peripheral chemoreceptors]]
==External links==
*[http://www.ursa.kcom.edu/Department/LectureNotes/Summer/ContRespiration.doc Respiratory physiology notes] at [[Kirksville College of Osteopathic Medicine]]
* {{eMedicineDictionary|Carotid+body}}
[[Category:Cardiovascular system]]
[[Category:Respiration]]
{{respiratory-stub}}
[[de:Glomus caroticum]]
[[mk:Каротидно тело]]
[[ja:頚動脈小体]]
[[pl:Kłębek szyjny]]