E rises when the concentration of 02, (02). in the inspired air becomes 12% Or lessi. at sea Level Mechanism Fall of PI02 (and thus of Pa 02) causes

at sea Level Mechanism Fall of PI02 (and thus of Pa 02) causes stimulation of 'peripheral chimoreceptors'(PC')' neural impulses arising fromthese PCa cells go to the RC stimulation of -respiration What causes stimulation of PC? Low Pa O2 is the most important example. Fall of blood pH is another. Rise of Pa C02 also stimulates. Some anatomical (and histological details) of PC can now be discusseld there are two sets of PCs, (l) the caotided body chemreeeptors (fig. 4.3.6) and (ii) The aortic body chemoreceptores. Between them, the carolid body chemoreceptors havt between studied much more extentsively,, no doubt, because of thir easy accessibility But it is safe to presume that the behaviores of and functions of the carotid body (CB) are more or lass same as those of aortic body chemoreceptores. CB At bffurcation of each common carotid artery (fig 4. 3.5), withen the vascular andothelium, in direct contact with the blood, lise the CB. The CB can analyze the Pa 02 f Pa Co2 / H+ concentration if there is )ow Pa 02 or high Pa C02 or low blood pH the CB cells an stimulated nauiral impulses art art up medulary RC stimulated increased repiration homeostasis regained the aortic bodies also behave similar ways (i.e. they also analyze the above mentained chemicals send signals achive homeostasis). The neural from the CB also called glomus caroticum ) travel via nerve Of Hering (a branch Of glossophryngeall or the IXh nerve) the IXlh nerve (fig. 4. 3. 5) 4 medullary RC. Whereas, from the aortic bodies, impulses travel up via (afferent) vagal fiberes medullary R.C There are altogether two CB. one On each side (fig 4.3.5.) There are two types Of Cells in the CB.' type 1 and (ii) Type II cells. Type I cells contain large vesicles and sensory nerve fibers (which eventually constitute the nerve ofHering) emerge from the type I cell Probably the type I cells themsellves or the begining of these nnerves are the detectlores (= receptors ) of the chemical sensation (e.q. anoxial). Three neurotransmitter chemicals are involved in the above mention detection of chemical senses as well as their' tranimiision. They are, (i) dopamine (DA), (ii) subttance P, and (iii) acetyl choline Ach) Of them DA and substance P are present in the vesicles of type I cells Proba- bly DA causes blunting of the sensivity of the chemoresepter cells. whereas substance P increases it. ACh might probably be the neurotransmitter involved in the transmission between the type I cell and the emeging nerve fiber. Thai it, stimutation of lypt I call releaser (locally) ofACh stimulation of the afferent nerves. Molecular level mechanism(s) of action of tha CB (glomius caraticum)
is uncertain and contraversial. Some very popular (and mutually comphmentary) ideas are given below .Any one or all of them may be correct. I
. The CB cells (glomus cells) are metabolically axtremely active Therefore, inspite oft the fact that the CB it tremendously vascular. there.are normally. areas. where there is some hypoxia Of these
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