Morphology of pontomedullary raphe and reticular formation neurons in the brainstem of the cat: an intracellular HRP study.

In order to understand better the anatomical substrates underlying processing of sensory information, the cytoarchitecture of neurons in the pontomedullary raphe and reticular formation was investigated following intracellular injections of horseradish peroxidase in the cat. Raphe cells studied were located in the nucleus raphe magnus, nucleus raphe obscurus, and nucleus raphe pallidus. The most prominent type had a smooth, oval cell body and oval dendritic tree with dendrites extending laterally into the adjacent reticular formation. Two other raphe cell types, large cells with a dorsoventral orientation of both cell body and dendritic tree, and very small cells, were rarer. The primary dendritic orientation lay in the coronal plane for all three raphe cell types. Wispy, straight, or clublike spines were located on more distal regions of dendrites, although we also found spineless dendrites. Raphe cells lying near longitudinal fiber pathways exhibited bundling of dendrites around the passing axon fascicles. Reticular formation cells studied were located in the nucleus gigantocellularis, nucleus magnocellularis, nucleus paragigantocellularis dorsalis, and nucleus reticularis paramedianus. Two morphological types were found on the basis of dendritic branching patterns: sparsely branched and densely branched. Most reticular formation cells had round dendritic trees as viewed in the coronal plane and polygonal cell bodies that were medium to large in size. There was no correlation between reticular formation cell morphology and nuclear location. Spines were more common on the densely branched cells, but for both reticular cell types they were usually absent from cell bodies and proximal dendrites. Thus, by using the criteria of dendritic branching and arbor shape along with distance from the midline it was possible to identify raphe cells as distinct from reticular formation cells. In contrast, no morphological characteristics were found that would differentiate cells in the two major median reticular formation nuclei, gigantocellularis and magnocellularis.