Cells of origin of pathways descending to the spinal cord in some quadrupedal reptiles.
The cells of origin of pathways descending to the spinal cord have been determined in several quadrupedal reptiles, viz., the turtle Pseudemys scripta elegans and Testudo hermanni and the lizards Tupinambis nigropunctatus and Varanus exanthematicus, following a technique introduced by Kuypers and Maisky ('75). This technique was very effective in producing retrograde transport of HRP to a great many neurons in the hypothalamus and in the brain stem. Projections from the hypothalamus (the nucleus paraventricularis and the nucleus periventricularis hypothalami), the interstitial nucleus of the film, the nucleus ruber, the nucleus of Edinger-Westphal, the locus coeruleus, the subcoeruleus area, a conspicuous cell group comparable to Kuypers and Maisky's (75, '77) lateral pontine area, the magnocellular reticular formation, the ventrolateral, ventromedial, and descending vestibular nuclei, the dorsal motor nucleus of the vagus, and the nucleus of the solitary tract, reach at least as far as the lumbar intumescence. Projections from two somatosensory nuclei, i.e., the nucleus descendens nervi trigemini and the nucleus funiculi dorsalis, as well as the laminar nucleus of the torus semicircularis, have been demonstrated to at least the ninth spinal segment. The two deep cerebellar nuclei, particularly the medial cerebellar nucleus, were found to project contralaterally to the spinal cord, in the lizard Varanus exanthematicus at least as far as the seventh segment; in the turtles studied so far, only projections as far caudal as the fourth spinal segment could be demonstrated. Data on the funicular trajectory of various descending pathways could also be obtained. It seems likely that in the reptiles studied, in addition to rubrospinal and reticulospinal pathways, projections from the hypothalamus, the nucleus of Edinger-Westphal, the cell group comparable to the mammalian lateral pontine area, the locus coeruleus, and subcoeruleus area, and the nucleus of the solitary tract pass via the lateral funiculus. The pathways descending from the hypothalamus and brain stem to the spinal cord in the quadrupedal reptiles studied appear to show remarkable similarities to pathway in mammals as regards their cells of origin as well as their funicular trajectory. It seems likely that some of the projections demonstrated, viz., from the locus coeruleus, at least part of the cell group comparable to the lateral pontine area, as well as the cells in and around the dorsal motor nucleus of the vagus, are noradrenergic pathways.