The Paraganglia of the Rat Superior Laryngeal Nerve
The chemoreceptor function of the aortic bodies (AB) was discovered before that of the carotid bodies (CB) in the dog (see Daly, 1997 for excellent historical overview). Whilst the reflex respiratory and cardiovascular effects of CB stimulation are well described in many vertebrates including humans, the reflex effects of AB stimulation have been less well studied (Marshall, 1994). This is partly explained by the technical difficulties associated with selective perfusion of the AB chemoreceptors. In one of the few studies of this type Daly and Ungar (1966) succeeded in selective perfusion of the two groups of chemoreceptors in the anaesthetized dog. It was found that the respiratory action of the CB was much greater (sevenfold on average) than the AB, whilst the potency of reflex vasoconstriction of hindlimb was equal. This data is particularly valuable because the comparison was made using the natural stimulus of hypoxaemic blood. More recently, experiments in the cat by Daly and Jones (1998) showed that in many animals AB stimulation using cyanide was without respiratory effect but produced a potent constriction of the femoral vasculature. Furthermore, Jones and Daly (1997) observed that whilst the primary cardiac chronotropic action of the AB was weaker than the CB, the dromotropic actions were of equal magnitude. Clearly the study of reflex actions of chemoreceptors other than the CB should include recording of parameters other than respiration. This fundamental point has been missed in many studies of chemoreceptor paraganglia conducted by respiratory physiologists (see Brophy et al. 1999 for elaboration). Whilst there have been studies of the reflex action of rat CB stimulation on the cardiovascular system (Marshall, 1994) there appear to be none related to the reflex action of chemoreceptors other than the CB. As we will show here paraganglia of the superior laryngeal nerve (SLN) may be an important group of peripheral arterial chemoreceptors in the rat. One of the few studies which examined cardiovascular changes evoked by isocapnic hypoxia in the rat, unfortunately utilised a procedure for sinoaortic denervation which included cutting both SLNs (Hirakawa et al. 1997).
KeywordsRecurrent Laryngeal Nerve Carotid Body Superior Laryngeal Nerve Arterial Chemoreceptor Isocapnic Hypoxia
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