Synaptic relationships between the ascending trigeminal fibers and the third order lingual relay neurons of the rat thalamus

Abstract

Electrophysiologically monitored lesions were placed in the chief sensory trigeminal nucleus of rats to study the organization of fibers ascending into the thalamus. The Nauta-Laidlaw method for silver impregnation of degenerated fibers was used. Results indicated that almost all of the ascending trigeminal fibers cross at the level of the pons to form the ventral trigeminothalamic tract. The remaining, uncrossed fibers scatter themselves diffusely throughout the mesencephalic reticular formation. Upon entering the thalamus, terminals of the tract synapse on the third order neurons for lingual relay of gustatory, thermal, and tactile modalities. Some terminals of the tract enter also the thalamic SI and SII snout projection areas and the two nuclei, centrum medianum and parafascicularis. Homolateral distribution of fibers is sparse. Their projection is similar to the observations reported for the contralateral side, except for a lack of degeneration in SI. As no evoked field potentials can be recorded from the thalamic gustatory neurons with electrical stimulation of the lingual nerve, the mechanism serving to influence gustatory neurons with thermal stimulation must reside in the modulation of the spontaneous activity of the gustatory system. It is likely that such a modulation is accomplished by presynaptically depolarizing those gustatory fibers which enter the gustatory nucleus.