Ultrastructure of the cell group X of Onuf in the cat sacral spinal cord.

The cell group X of Onuf, originally described in humans, is identifiable in the cat at the ventrolateral margin of the ventral horn in the S1-S2 cord level. This nucleus is composed of medium-sized, oval neurons which were arranged longitudinally. Ultrastructurally, the most unique feature of this nucleus are the extremely well-developed longitudinal dendritic bundles in which more than 1500 dendritic profiles were counted in a single plane of the transverse section. In the dendritic bundles there are dendro-dendritic direct appositions without specialized structures and with desmosome-like contacts. However, gap junctions were not found. The group X neurons possessed many axo-dendritic synapses whereas axo-somatic were only infrequently encountered. In addition, dendritic crest synapses were rarely observed. The significance of the large dendritic bundles and the possible role of the cell group X are discussed in relation to somatic as well as autonomic motoneurons.