The cells of origin of cat trigeminothalamic projections: especially in the caudal medulla.

Thalamic projections from the caudal medulla of the cat were examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). Injections were made unilaterally in various thalamic regions. Large injections labeled cells in the subnuclei: zonalis (Vcz), gelatinosus (Vcg), magnocellularis (Vcm), reticularis dorsalis (Vcrd) and ventralis (Vcv) medullae oblongatae. The largest number of labeled cells were in Vcz, Vcrd and Vcrv. Most of the labeled cells in Vcz and Vcrd were contralateral to the injection site, although the labeled cells in the Vcrv were bilateral. Small injections were made into the medial, lateral and dorsal regions of the nucleus ventralis posteromedialis (VPM), rostral regions of the posterior nuclei (POm and PO1), caudal POm, the nucleus centralis lateralis (CL) and the center median-parafascicular nuclear complex (CM-Pf). Most of the neurons in Vcz were found to project to the medial VPM and some to the caudal POm. A small number of cells in the Vcrd project to the medial VPM, but a large number project to the caudal POm and CM-Pf complex. The largest number of neurons projecting to the CM-Pf complex was present in Vcrv, where the labeled cells were bilateral. The types of trigeminothalamic projecting cells and the sizes of their somata were observed for different subnuclei and a considerable difference was found to exist among the subnuclei. This anatomical differentiation of the trigeminothalamic projections probably reflects a functional specialization of neuronal location since the functional properties of neurons vary according to their locations.