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

Abstract

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 μm 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 μm of the ventrolateral portion of the trigeminal nucleus oralis and display dendritic trees which occupy spherical domains approaching 300 μm 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 μm 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 μm of the ventrolateral portion of the trigeminal nucleus oralis. An unmyelinated axon emerges from the cell body and within 10–30 μm of its origin gives rise to two collaterals. One collateral enters the space occupied by the medial dendritic arbor of the parent cell while the other enters the lateral dendritic zone. Within each dendritic arbor the collateral divides repeatedly to form an extensive axonal plexus. The parent axon, after giving rise to the collaterals, continues its trajectory through the ventrolateral neuropil without additional branching and eventually enters a deep axon bundle. On the basis of axonal morphology Type I and Type II neurons would appear to be capable of not only affecting synaptic activity in the medullary dorsal horn, but of also exerting effects within and near their own dendritic fields. Type III and Type IV cells are the least frequently labeled descending medullary dorsal horn projection neurons. The cell bodies of Type III neurons are usually found in the ventrolateral portion of the trigeminal nucleus oralis on a line approximately 100 μm from the medial border of the spinal V tract where they give rise to medially directed (up to 100 μm) cone-shaped dendritic trees. These dendritic arbors spread out over 400 μm dorsoventrally. The axon emerges primarily from the lateral aspect of the cell body and almost immediately enters a deep axon bundle without giving rise to collateral branches. Type IV neurons are located in the extreme lateral portion of the ventrolateral portion of the trigeminal nucleus oralis where they are observed to be the predominant cell type. Their flattened disk-like dendritic fields are confined within 100 μm of the spinal V tract and extend up to 700 μm dorsoventrally as well as rostrocaudally. The axon arises from the lateral aspect of the cell body and immediately enters the medial portion of the spinal V tract without further branching. On the basis of their axonal morphology Type III and Type IV cells would appear to have little influence on synaptic activity in the trigeminal nucleus oralis, but would seem to be designed to maximally affect synaptic activity below the trigeminal nucleus oralis in the medullary dorsal horn.