Fine structural survey of the rat's brainstem sensory trigeminal complex

Publisher Website
Google Scholar
Abstract
The fine structural organization of the principal sensory trigeminal nucleus was compared with that of the spinal trigeminal nucleus (subnuclei oralis, interpolaris, and the deep layers of caudalis) in adult albino rats. Direct comparisons indicate similarities between all of the subdivisions of the brainstem trigeminal complex both in the major morphological classes of neurons present and in basic patterns of synaptic connections. Major differences between the several subdivisions occur in the relative numbers and distribution of the different cell types. The spinal trigeminal nucleus is distinguished by more numerous large (22–40 μm) polygonal neurons which give rise to long straight primary dendrites. Both the perikaryal surface and the thick primary dendrites of many of these cells are densely innervated by synaptic terminals. Especially large cells of this type are a prominent feature of subnucleus oralis. By contrast, the principal sensory nucleus is distinguished by its high density of small to medium-sized (8–20 μm) round or ovoid neurons. These smaller neurons tend to receive a sparse axosomatic innervation. In addition to these differences the spinal trigeminal neuropil is distinguished by the striking manner in which it is broken up by large rostrocaudally oriented bundles of myelinated axons. Proximal dendrites of polygonal and fusiform neurons often wrap around these large axon bundles. Morphologically heterogeneous populations of synaptic terminals with round vesicles (R terminals) and terminals with predominantly flattened vesicles (F terminals) occur in all of the subdivisions of the trigeminal complex. Both types of terminal make primarily axodendritic synapses, but both also make axosomatic synapses, and axospinous synapses with somatic as well as dendritic spines. In addition, axoaxonic synaptic contacts from F terminals onto large R terminals are seen in all subdivisions. Convincing examples of presynaptic dendrites were not observed in any of the brainstem subdivisions. Synaptic glomeruli, characteristic groupings of dendrites and synaptic terminals, are found throughout the brainstem trigeminal complex. The dendritic elements in these glomeruli tend to be small-diameter dendrites, spines, and large, spinelike appendages. Within the glomerulus these elements are postsynaptic to a single large R terminal and may also be postsynaptic to smaller F terminals. In addition, axoaxonic synaptic contacts from the F terminals onto the R terminal are a consistent feature of trigeminal synaptic glomeruli. Serial reconstructions of glomeruli indicate that both the R and the F terminals commonly occur as boutons en passant, with single axons innervating multiple glomeruli. Within the trigeminal complex synaptic glomeruli are often grouped together into clusters, which we have termed “glomerular nests.” We suggest that the peripherally related afferent segmentation characteristic of the rat's brainstem trigeminal complex is reflected in the distribution of these nests.