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Peripheral chemoreceptors
Peripheral chemoreceptors (or carotid and aortic bodies) are so named because they are sensory extensions of the peripheral nervous system into blood vessels where they detect changes in chemical concentrations.〔Gonzalez, C., Almaraz, L., Obeso, A., and Rigual, R. (1994). Carotid body chemoreceptors: from natural stimuli to sensory discharges. Phys Rev, 74(4), 829-898〕 As transducers of patterns of variability in the surrounding environment, carotid and aortic bodies count as ‘sensors’ in a similar way as taste buds and photoreceptors.〔COGS 211 lecture, K. R. Livingston, September 11, 2013〕 However, because carotid and aortic bodies detect variation within the body’s internal organs, they are considered interoceptors.〔http://sinoemedicalassociation.org/AP/peripheralnervoussystem.pdf〕 Taste buds, olfactory bulbs, photoreceptors, and other receptors associated with the five traditional sensory modalities, by contrast, are exteroceptors in that they respond to stimuli outside the body.〔 The body also contains proprioceptors, which respond to the amount of stretch within the organ, usually muscle, that they occupy.〔
As for their particular function, peripheral chemoreceptors help maintain homeostasis in the cardiorespiratory system by monitoring concentrations of blood borne chemicals.〔Peers, C., Wyatt, C. N., and Evans, A. M. (2010). Mechanisms for acute oxygen sensing in the carotid body. Resp Physiol Neurobio, 174, 292-298〕 These polymodal sensors respond to variations in a number of blood properties, including low oxygen (hypoxia), high carbon dioxide (hypercapnia), and low glucose (hypoglycemia).〔 Hypoxia and hypercapnia are the most heavily studied and understood conditions d etected by the peripheral chemoreceptors. Glucose is discussed in a later section. Afferent nerves carry signals back from the carotid and aortic bodies to the brainstem, which responds accordingly (e.g. increasing ventilation).〔
== Structure ==
Both carotid and aortic bodies increase sensory discharge during hypoxia.〔Prabhakar, N. R. and Peng, Y. (2004). Peripheral chemoreceptors in health and disease. J Appl Physiol, 96, 359-356〕 Carotid bodies are considered the primary peripheral chemoreceptor and have been shown to contribute more to a hypoxic response. However, in the chronic absence of the carotid body, the aortic body is able to perform a similar respiratory regulatory role, suggesting that it possesses efficacious mechanisms of signal transduction as well.〔 The differing locations of the two bodies ideally position them to take advantage of different information; the carotid bodies, located on one of the main arteries of the neck, monitor partial pressure within arterial vessels while aortic body, located on the aortic arch, monitors oxygen concentration closer to the heart.〔 Each of these bodies is composed of a similar collection of cells, and it is the post-transduction signal processing that differentiates their responses. However, little is known about the specifics of either of these signaling mechanisms.〔Nurse, C. A. and Piskuric, N. A. (2013). Signal processing at mammalian carotid body chemoreceptors. Sem Cell Dev Bio, 24, 22-30〕
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