PRIMARY AND SECONDARY SOMATOSENSORY PROJECTIONS IN DIRECT-DEVELOPING PLETHODONTID SALAMANDERS

In three species of plethodontid salamanders (Plethodon jordani, Hydro mantes italicus, and Bolitoglossa subpalmata), primary and secondary s omatosensory pathways were investigated by means of tract-tracing in v ivo and in vitro using biocytin, horseradish peroxidase, and neurobiot in. Afferent sensory fibers of cranial nerves V, VII, and X and the br achial nerve run in the dorsal funiculus of the medulla oblongata and spinal cord. Fibers ascend to the level of, but do not enter, the cere bellum. In the caudal medulla oblongata, sensory tracts of the cranial nerves descend in a dorsal and a dorsolateral bundle and reach the le vel of the fourth spinal nerve. Two bundles are likewise formed by spi nal afferent fibers, which descend to the level of the seventh spinal nerve. Secondary somatosensory projections ascend in contralateral ven tral, contralateral lateral, and ipsilateral lateral tracts, the latte r two corresponding to the spinal lemniscal tracts of Herrick. These t racts reach the cerebellum, mesencephalic, and diencephalic targets (t egmentum, torus, tectum, tuberculum posterius, pretectum, and ventral thalamus) ipsi- and contra-laterally. The projection to the tectum is confined to fiber layer 4. Fibers of the ascending tracts cross in the cerebellar and tectal commissure. Our study demonstrates that the asc ending secondary somatosensory pathways of plethodontid salamanders di ffer remarkably from those of other amphibians. J. Morphol. 238:307-32 6, 1998. (C) 1998 Wiley-Liss, Inc.