Carotid body

The carotid body (carotid glomus or glomus caroticum) is a small cluster of chemoreceptors and supporting cells located in the adventitia, near the fork (bifurcation) of the carotid artery (which runs along both sides of the throat). The carotid body (carotid glomus or glomus caroticum) is a small cluster of chemoreceptors and supporting cells located in the adventitia, 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 arterial oxygen, but also of carbon dioxide. Furthermore, it is also sensitive to changes in pH and temperature. The carotid body is made up of two types of cells, called glomus cells: glomus type I cells are peripheral chemoreceptors, and glomus type II cells are sustentacular supportive cells. The carotid body contains the most vascularized tissue in the human body. The thyroid gland is very vascular, but not quite as much as the carotid body. The carotid body functions as a sensor: it responds to a stimulus, primarily O2 partial pressure, which is detected by the type I (glomus) cells, and triggers an action potential through the afferent fibers of the glossopharyngeal nerve, which relays the information to the central nervous system. The carotid body peripheral chemoreceptors are primarily sensitive to decreases in the partial pressure of oxygen (PO2). This is in contrast to the central chemoreceptors in the medulla oblongata that are primarily sensitive to changes in pH and PCO2 (a decrease in pH and an increase in PCO2). The carotid body chemoreceptors are also sensitive to pH and PCO2, but only secondarily. More specifically, the sensitivity of carotid body chemoreceptors to decreased PO2 is greater when pH is decreased and PCO2 is increased. The output of the carotid bodies is low at an oxygen partial pressure above about 100mmHg (13,3 kPa) (at normal physiological pH), but below 60mmHg the activity of the type I (glomus) cells increases rapidly due to a decrease in hemoglobin-oxygen saturation below 90%. The mechanism for detecting reductions in PO2 has yet to be identified, there may be multiple mechanisms and could vary between species. Hypoxia detection has been shown to depend upon increased hydrogen sulfide generation produced by cystathionine gamma-lyase as hypoxia detection is reduced in mice in which this enzyme is knocked out or pharmacologically inhibited. The process of detection involves the interaction of cystathionine gamma-lyase with hemeoxygenase-2 and the production of carbon monoxide. Yet, some studies show that physiologic concentration of hydrogen sulfide may not be strong enough to trigger such responses. Other theories suggest it may involve mitochondrial oxygen sensors and the haem-containing cytochromes that undergo reversible one-electron reduction during oxidative-phosphorylation. Haem reversibly binds O2 with an affinity similar to that of the carotid body, suggesting that haem containing proteins may have a role in O2, potentially this could be one of the complexes involved in oxidative-phosphorylation. This leads to increases in reactive oxygen species and rises in intracellular Ca2+. However, whether hypoxia leads to an increase or decrease in reactive oxygen species is unknown. The role of reactive oxygen species in hypoxia sensing is also under question.