Central Terminations of Carotid Body Chemoreceptor Afferents

Abstract

Despite a number of attempts to delineate them, it is only recently that the areas of termination of afferents innervating the carotid body chemorecptors have been localised. Early axonal degeneration studies, the most complete by Cottle (1964), described the regions containing glossopharyngeal nerve terminals. These were within the rostal two-thirds of the nucleus tractus solitarius (NTS) in its lateral (Slt), ventrolateral (Svl) and dorsomedial (Sdm) subnuclei. This type of anterograde degeneration study has limitations since there is only partial transganglionic degeneration following section of nerve branches distal to the sensory ganglion. More recently, however, neural tracer substances which are transported transganglionically have been used to selectively label afferent nerves from particular organs. In respect of the carotid sinus nerve (CSN), using anterograde transport of the tracer horseradish peroxidase, there is general agreement that the ipsilateral Sdm, Slt, Svl, medial (Sm) and commissural (Com) subnuclei are the most likely to receive CSN afferents but the extent of inputs to the area postrema and contralateral NTS remains controversial (Berger, 1979 Panneton and Loewy, 1980 Ciriello, Hrycyshyn and Calaresu, 1981 Davies and Kalia, 1981 Nomura and Mizuno, 1982 Seiders and Stuesse, 1984). Outside the region of the NTS there are also claims of a direct CSN input to the dorsal reticular formation and nucleus ambiguus (Davies and Kalia, 1981), the medial reticular formation, spinal nucleus V and external cuneate nucleus (Ciriello et al. 1981).