Ascending and descending internuclear projections within the trigeminal sensory nuclear complex.

The cells of origin of ascending and descending internuclear pathways in the trigeminal sensory nuclear complex were studied by the method of retrograde transport of horseradish peroxidase in the cat. The cells of origin of the ascending internuclear pathways are distributed in all laminae of the caudal part of the spinal trigeminal nucleus (Vc) except for lamina II and the caudal regions of the pars interpolaris of the spinal trigeminal nucleus (Vi). The cells arising from the Vc project to all rostral trigeminal nuclei except the caudal Vi and dorsal part of the principal trigeminal nucleus (Vpd), and neurons of the caudal Vi project to the dorsomedial (Vo.dm) and rostrodorsomedial (Vo.r) divisions of the spinal trigeminal nucleus and the ventral part of the principal trigeminal nucleus (Vpv), although the main ascending fibers from the Vc arise from laminae III-V and project to the rostral Vi and pars oralis. By contrast, the cells of origin of the descending internuclear pathways are distributed in all trigeminal nuclei, with chain-like connections between the neighboring nuclei, while the caudal regions of the Vi and laminae I-II do not receive any descending projections. The main ascending fibers from the paratrigeminal nucleus (or interstitial nucleus) at the caudal level of the Vi project to the parabrachial nucleus. These findings indicate that the internuclear pathways are differentially organized between the ascending and descending projections, and suggest that the internuclear trigeminal connections have a smaller influence on the trigeminothalamic tract cells in the Vpd, caudal Vi, and lamina I.