<P> Carotid and aortic bodies are clusters of cells located on the common carotid artery and the aortic arch, respectively . Each of these peripheral chemoreceptors is composed of type I glomus cells and glia - like type II cells . The type - I cells transduce the signals from the bloodstream and are innervated by afferent nerve fibers leading back to (in the carotid body) the carotid sinus nerve and then on to the glossopharyngeal nerve and medulla of the brainstem . The aortic body, by contrast, is connected to the medulla via the vagus nerve . </P> <P> They also receive input from efferent nerve fibers leading back to the same set of nerves . The entire cluster of cells is infiltrated with capillaries to provide access to the bloodstream; the high capillary density makes this one of the areas of the body with the greatest blood flow . Type I cells are densely packed with vesicles containing various neurotransmitters, including dopamine, ATP, serotonin, catecholamine, released during transduction . Type I cells are often connected via gap junctions, which might allow for quick communication between cells when transducing signals . </P> <P> Type II cells occur in a ratio of about 1 to 4 with type I cells . Their long bodies usually occur in close association with type I cells, though they do not entirely encase type I cells . They lack the vesicles of type I cells used in neurotransmitter communication, but studies indicate they function as chemoreceptor stem cells and can respond to prolonged exposure to hypoxia by proliferating into type I cells themselves . They may also bolster rapid communication among type I cells by amplifying release of one of the primary neurotransmitters in chemoreceptive signaling, ATP . </P> <P> Sensitivity and physiology of the peripheral chemoreceptors changes throughout the lifespan . </P>

Where are the chemo-receptors that monitor blood levels of o2 co2 & h+ located