[Electron microscopy of the neural and synaptic organization of the regio cingularis in the rat].

The neuronal structure and synaptology of the regio cingularis of the rat was examined with electron microscopical methods. On the basis of GOLGI-impregnation studies of the cingulate region ultrastructural characteristics of pyramidal and nonpyramidal neurons were investigated concerning the variations in size and content of intracytoplasmatic organelles, morphology of the nucleus and the number, site and kind of synaptic contacts. There are mainly small and medium sized pyramidal cells. The axo-somatic synapses appear as symmetrical contacts (type GRAY II). The number and distribution of small and medium sized nonpyramidal neurons (interneurons) are different in various cortical layers. Both types of synapses, asymmetrical as well as symmetrical contacts (type GRAY I and type GRAY II) were found on the pericarya of these nonpyramidal cells. As has been established for the neocortex, the dendrites of cingulate pyramidal neurons are bundled. The numerous spines along these dendrites are of different size and shape. The asymmetric axo-spinodendritic contacts (type GRAY I) are the most frequent type of synapses compared with axo-dendritic junctions. The dendrites of nonpyramidal neurons are of the spiny type or of the sparsely spined type with dilations of "beads" (varicosities) that are present along them. The ultrastructural investigations and the results following terminal degeneration (electrolytical stereotaxic lesions in the nucleus mediodorsalis and in the nucleus anteromedialis thalami) cause the suggestion that the specific thalamic afferents reach monosynaptically the layer III or V pyramids and specific interneurons. Recurrent axon collaterals of the pyramids and inhibitory interneurons represent the morphological correlates for the recurrent inhibition of the pyramids. These results demonstrate a similarity between the cingulate cortex and the neocortex concerning the synaptic organization made up by excitatory and inhibitory neurons.