The morphology of neurons in trigeminal nucleus oralis projecting to the medullary dorsal horn (trigeminal nucleus caudalis): a retrograde horseradish peroxidase and Golgi study.

This study demonstrates that trigeminal nucleus oralis, the most rostral subdivision of the spinal trigeminal nucleus, contains four morphologically distinct types of small neurons which project to the medullary dorsal horn (trigeminal nucleus caudalis) via descending intratrigeminal pathways. Using the retrograde transport of horseradish peroxidase following injections in the medullary dorsal horn, labeled small neurons with cell bodies ranging from 8-15 microns in diameter are found principally in the ventrolateral portion of the trigeminal nucleus oralis. Most neurons are labeled ipsilaterally throughout the entire rostrocaudal extent of the ventrolateral portion of the trigeminal nucleus oralis, but a few cells are also labeled contralaterally. From this aspect of the present study it can be concluded that a specific portion of the trigeminal nucleus oralis, i.e. the ventrolateral part, contains numerous small neurons which send descending projections to the medullary dorsal horn that could affect synaptic activity there. Utilizing both the methods of Golgi and retrograde horseradish peroxidase labeling four distinct types of small descending medullary dorsal horn projection neurons can be distinguished in the ventrolateral portion of the trigeminal nucleus oralis on the basis of their morphology and the distribution of their axons and dendrites. All four neuronal cell types are present throughout the entire rostrocaudal extent of the trigeminal nucleus oralis. Type I neurons are the most frequently labeled descending medullary dorsal horn projection neurons. They are concentrated in the medial 500-550 microns of the ventrolateral portion of the trigeminal nucleus oralis and display dendritic trees which occupy spherical domains approaching 300 microns in diameter. The unmyelinated axons of many of these cells arise either directly from the cell body or a primary dendrite and give rise to a single collateral within 50 microns of their site of origin. This collateral generates a fine axonal plexus within a portion of the dendritic arbor of the parent cell while the parent axon, without branching further, travels a short distance in the ventrolateral portion of the trigeminal nucleus oralis and enters a deep axon bundle. Type II neurons are the second most frequently labeled descending medullary dorsal horn projection neuron. They generate medial and lateral dendritic arbors which together span nearly the entire medial 500-550 microns of the ventrolateral portion of the trigeminal nucleus oralis. An unmyelinated axon emerges from the cell body and within 10-30 microns of its origin gives rise to two collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)