Substantia gelatinosa Rolandi neurons: ultrastructure of the cellular somata, synapses and intercellular contacts

The central cells of substantia gelatinosa are small spindle-shaped neurons with an average perimeter of 26.8 μm and an average cross-sectional area of 44.9 μm2. No difference has been found between neurons of C1 and those of the lower cervical segments. It was calculated that axo-somatic synapses are about 20 per cell and generally occur in small clusters of 3-4 synapses each. Despite their low number they are probably of functional relevance when compared to the small size of the somata of substantia gelatinosa neurons. Most of the synaptic boutons contain pleomorphic electronlucent vesicles and have symmetrical synaptic junctions. Large granular synaptic vesicles also occur intermingled with the electronlucent ones. Sites of somato-dendritic and somato-somatic membrane apposition are fairly extensive: they cover about 8% of the cellular perimeter. The possibility of cellular interactions or material interchanges at these sites of direct membrane apposition is suggested. Somatodendritic synapses were occasionally observed. Although the neuropil is indeed the main target of the different inputs to the substantia gelatinosa and the site of most of the local circuits, the somata may too be involved in synaptic transmission and possibly in intercellular coupling.