Morphological Features of Identified Trigeminocerebellar Projection Neurons in the Border Zone of Rat Trigeminal Nucleus Oralis

Abstract

The retrograde transport of horseradish peroxidase (HRP) was used to assess the overall morphology of neurons in the dorsal half of the border zone (BZ) of rat trigeminal nucleus oralis (Vo) that project to ipsilateral orofacial portions of four major tactile areas (crura I and II, the paramedian lobule, and the uvula) of the cerebellar cortex. We wished to answer two important questions: (1) Does this group of cells consist of one or more morphologically distinct types, each projecting to different cerebellar tactile areas? (2) Does the overall morphology or morphologies of these BZ neurons resemble one or more of the five types of identified trigeminocerebellar neurons in the dorsomedial (DM) subdivision of Vo, or do these trigeminocerebellar cells represent an additional morphologically distinct type or types restricted to BZ? The morphology of BZ neurons innervating the orofacial portions of all four cerebellar tactile areas was similar and did not resemble that of any of the five types of DM cells. They were characterized by a pyramidal- to fusiform-shaped cell body measuring 10-13 × 20-25 μm, which emitted three or four primary dendrites; one was directed dorsally, while the others took a more ventral trajectory. The primary dendrites generated a dendritic arbor arranged as a flattened disk oriented parallel to the spinal V tract. The dendritic field was largely restricted to BZ; it measured up to 150 μm in width, and spanned up to 450 μm dorsoventrally and rostrocaudally. An axon arose from the dorsal aspect of the cell body and gave rise to a single short collateral within 10 μm of its origin. This collateral remained unbranched and generated several boutons within BZ, while the parent axon, without branching further, traveled dorsolaterally toward the inferior cerebellar peduncle. Frequently, a second axon arose ventrally from the soma, and after a short unbranched course entered a deep axon bundle, where it assumed a descending trajectory. The intranuclear portion of this second axon was characterized by several boutons en passant