SPINAL ASCENDING PATHWAYS IN AMPHIBIANS - CELLS OF ORIGIN AND MAIN TARGETS

As part of a research program on the evolution of somatosensory system s in vertebrates, the various components of ascending spinal projectio ns were studied with in vivo and in vitro tract-tracing techniques in representative species of amphibians (the large green frog, Rana perez i, the clawed toad, Xenopus laevis and the ribbed newt, Pleurodeles wa ltl). Three main ascending sensory channels, each with largely separat e targets, were demonstrated: 1. Ascending projections via the dorsal funiculus include primary and nonprimary projections that ascend to te rminate mainly in the dorsal column nucleus at obex levels. A small co mponent ascends farther rostralwards to terminate in the reticular for mation, the octavolateral area, the trigeminal nuclear complex, and in the granular layer of the cerebellum. 2. Projections ascending via th e dorsolateral funiculus reach other spinal and supraspinal targets th an the dorsal funicular fibers, mainly ipsilaterally. At upper cervica l cord and obex levels, many fibers innervate a region considered the amphibian homologue of the lateral cervical nucleus of mammals. In the medulla, these fibers ascend ventral to the descending trigeminal tra ct to terminate in the dorsal column and the solitary tract nuclei, an d more rostrally, in the reticular formation, the descending trigemina l nucleus and the medial aspect of the ventral octaval nucleus. Major projections reach the area between the facial motor nucleus and the ve ntral octaval nucleus, and a mediolateral subcerebellar band. These pr ojections arise in neurons located mainly in the ipsilateral deep dors al and lateral fields throughout the spinal cord. 3. Ascending spinal projections via the ventral quadrant of the spinal cord (the ventral a nd ventrolateral funiculi) ascend throughout the brainstem up to the d iencephalon. Along its course, this component innervates various parts of the reticular formation, the octavolateral area, the granular laye r of the cerebellum, the region ventromedial and ventrolateral to the isthmic nucleus, and the subcerebellar region. Ln the mesencephalon, t he torus semicircularis, the midbrain tegmentum and, sparsely, the tec tum mesencephali are innervated. Beyond the midbrain, various dorsal a nd particularly ventral thalamic nuclei and the posterior tubercle are innervated by this ascending sensory channel. The cells of origin of some of these projections were observed in the dorsal, and to a lesser extent, in the lateral and ventral spinal fields of the spinal cord. Evidence for the presence of these three main ascending sensory channe ls throughout vertebrates will be discussed. The presence of such chan nels appears to be a shared character in the brain of both amniotes an d anamniotes. (C) 1997 Wiley-Liss, Inc.