Studies of the principal sensory and spinal trigeminal nuclei of the rat: Projections to the superior colliculus,inferior olive,and cerebellum

We have analyzed the connections between the sensory trigeminal nuclei and two major sensorimotor areas (i.e., the superior colliculus and crura I and II of the cerebellar cortex) in which tactile input from peri-oral and other facial regions is a prominent feature. Following injections of horseradish peroxidase into the superior colliculus, retrogradely labeled cells occupy the ventral one chird of the contralateral principal sensory and spinal trigeminal nucleus; trigeminocollicular neurons are especially numerous within, the sub-nucleus interpolaris (Svi). Injections of either 3H-proline or horseradish per-oxidase (HRP) into the Svi reveal that trigeminocollicular axons reach the rostral two-thirds to three-quarters of the contralateral superior colliculus, where they distribute in a nonuniform, patchy manner within layers IV-VI. In addition to demonstrating the trigeminocollicular projection, antero-grade and retrograde transport studies of the Svi also reveal a trigeminooli-vary projection which terminates primarily within the contralateral rostral dorsal accessory (DAO) and adjacent principal (PO) olives; some of the Svi neurons innervate both the superior colliculus and the DAO-PO via axon collaterals. Data from a final set of retrograde tracing experiments show that the trigeminorecipient zone of the DAO-PO contains neurons which project to crura I and/or II of the cerebellar cortex. Of the various submodalities conveyed by the trigeminal system, it is likely that the trigeminal connections we have demonstrated are carrying tactile information. This is indicated by the fact that responses to tactile stimulation of the face have been reported for cells in (1) the deeper collicular layers, (2) the trigeminorecipient zone of the DAO-PO, and (3) cerebellar targets of this zone, crura I and II. All data are discussed in the context of the anatomical and physiological literature.