Descending supraspinal pathways in amphibians. I. A dextran amine tracing study of their cells of origin

The present study is the first of a series on descending supraspinal pathwa ys in amphibians in which hodologic and developmental aspects are studied. Representative species of anurans (the green frog, Rana perezi, and the cla wed toad, Xenopus laevis), urodeles (the Iberian ribbed newt, Pleurodeles w altl), and gymnophionans (the Mexican caecilian, Dermophis mexicanus) have been used. By means of retrograde tracing with dextran amines, previous dat a in anurans were largely confirmed and extended, but the studies in P. wal tl and D. mexicanus present the first detailed data on descending pathways to the spinal cord in urodeles and gymnophionans. In all three orders, exte nsive brainstem-spinal pathways were present with only minor representation of spinal projections originating in forebrain regions. In the rhombenceph alon, spinal projections arise from the reticular formation, several parts of the octavolateral area, the locus coeruleus, the laterodorsal tegmental nucleus, the raphe nucleus, sensory nuclei (trigeminal sensory nuclei and t he dorsal column nucleus), and the nucleus of the solitary tract. In all sp ecies studied, the cerebellar nucleus and scattered cerebellar cells innerv ate the spinal cord, predominantly contralaterally. Mesencephalic projectio ns include modest tectospinal projections, toro-spinal projections, and ext ensive tegmentospinal projections. The tegmentospinal projections include p rojections from the nucleus of Edinger-Westphal, the red nucleus, and from anterodorsal, anteroventral, and posteroventral tegmental nuclei. In the fo rebrain, diencephalospinal projections originate in the ventral thalamus, p osterior tubercle, the pretectal region, and the interstitial nucleus of th e fasciculus longitudinalis medialis. The most rostrally located cells of o rigin of descending spinal pathways were found in the suprachiasmatic nucle us, the preoptic area and a subpallial region in the caudal telencephalic h emisphere, probably belonging to the amygdaloid complex. Our data are discu ssed in an evolutionary perspective. (C) 2001 Wiley-Liss, Inc.