The Dorsomedial Portion of Trigeminal Nucleus Oralis (Vo) in the Rat: Cytology and Projections to the Cerebellum

Abstract

Electrophysiological studies have described four major tactile areas in the rat cerebellar cortex. These areas are in crus I, crus II, the paramedian lobule (PML), and the uvula, and a major portion of each is related to the ipsilateral orofacial region. This study demonstrates that neurons in trigeminal nucleus oralis (Vo) that project to the orofacial portions of these four major tactile areas are localized in the dorsomedial (DM) subdivision of the nucleus. The distribution, light-microscopic morphology, and relative densities of trigeminocerebellar neurons within DM, retrogradely labeled with horseradish peroxidase (HRP) following injections into each of the four major tactile areas, were analyzed and compared as well as correlated with the myelo- and cytoarchitecture of DM observed in Nissl sections, 1-μm sections, and Golgi material. On the basis of myelo- and cytoarchitectonic as well as trigeminocerebellar connectional criteria, three portions of DM were identified: caudal DM (CDM), middle DM (MDM), and rostral DM (RDM).

The greatest portion of DM is made up of MDM (1.3 mm long), which can be further subdivided into dorsal (MDMd) and ventral (MDMv) zones. CDM forms the caudal 800 μm of DM, while RDM makes up the rostral 280 μm of the subdivision. Longitudinally running deep axon bundles permeate CDM, MDMv, and RDM, but are conspicuously absent from MDMd. The majority of neurons found throughout CDM, MDMv, and RDM have medium-sized (15- to 30-μm) somata and can be divided into two types on the basis of their somatodendritic morphology. CDM, MDMv, and RDM also contain a small neuronal cell type (5- to 15-μm cell body) that is encountered less frequently than either one of the two types of medium-sized cells. A fourth type of neuron with a large (25- to 50-μum) fusiform- to pyramidal-shaped cell body is the least frequently observed neuronal cell type and is located principally in CDM and MDMv. MDMd contains a fifth type of neuron characterized by a small (5- to 15-μm) oval soma.

Data from the retrograde HRP experiments show that all five of these neuronal cell types in their respective portions of DM project to one or more of the orofacial portions of the four major tactile areas of the cerebellar cortex. Many medium-sized neurons of both types in CDM, MDMv, and RDM project to crus I, crus II, and/or PML. Several small neurons in CDM and MDMv project to PML, while a few of the large cells in these same portions of DM project to crura I and/or II. Many of the small neurons in MDMd project specifically to PML. Some medium-sized and small cells in CDM project to the uvula On the basis of the locations of labeled neurons in DM, following injections into crus I, crus II, PML, and uvula, the CDM projection would appear to be capable of conveying orofacial tactile information to all four tactile areas of the cerebellar cortex, while the MDMd projection would seem to be the most specific, conveying orofacial tactile information only to PML. Furthermore, based on the density of labeled trigeminocerebellar neurons and their distribution within DM, PML would appear to receive the strongest tactile projection from Vo, while the uvula would receive the weakest.