Innervation of the Carotid Body

Abstract

Without doubt the outstanding contribution to the study of the carotid body innervation was made by de Castro (1926, 1928) in his classical studies of methylene blue and silver stain preparations. He showed (1926) that single nerve fibres branched to innervate a number of cells within the carotid body and that the nerve terminals were of variable shapes, such as boutons, calyces and large plate-like structures; in addition, he provided the first experimental demonstration that the carotid body was innervated by the ninth nerve, as most endings had degenerated 6–12 days after section of this nerve at its exit from the jugular foramen. De Castro (1928) went on to show that 5–12 days after intracranial section of the glossopharyngeal nerve there was no degeneration of nerve endings in the carotid body, and hence concluded that the carotid body endings were derived from cell bodies within the petrosal ganglion. As his previous studies of the ganglion indicated that it was composed of typical pseudo-unipolar cells, he concluded that the nerve endings were sensory terminals. The ability of neuronal cell bodies to take up tritiated amino acids and then transport them, or compounds synthesised utilising them, to their peripheral processes has also provided evidence of the sensory nature of the innervation to Type I cells (Smith and Mills 1976, 1977; Fidone et al. 1977 a, b). Both groups of workers showed labelling of Type I cell endings after injection of labelled amino acids into the petrosal ganglion; in the absence of evidence that the petrosal ganglion contains any efferent neurones, the authors concluded that the endings are therefore sensory. The problem with these tracer experiments is that by their very nature they are difficult to quantitate; hence it may be that more than one type of ending can exist on Type I cells (see below).