Ultrastructural features of the synapses of the periaqueductal gray matter (PAG) of the cat.

The periaqueductal gray matter (PAG) of cat has a very complex cytoarchitecture. Given the considerable functional role of connection and modulation of this region, we investigated the ultrastructure of the synapses completing the analysis with morphoquantitative evaluations of some characteristics of the synaptic boutons. The quantitative data revealed that the somatic synapses are scarce, 93% are of the symmetrical type and the pleomorphic vesicles are predominant. The synapses of the neuropil appear more numerous and complex; only 50% are of the symmetrical type. The vesicular contents are of the pleomorphic type as in the somatic synapses. A significant difference was found between the dimensions of the somatic vesicles and those of the neuropil, which may indicate the presence of two distinct populations of vesicles. These findings may be correlated with the variety of substances (neuro-transmitters, neuropeptides etc.) shown to be in the PAG.