[Morphologic correlates of the electrical interactions between neurons in the limbic system]

Electron microscopical results on "special structural relations" between neighbouring neuronal elements (pericarya, dendrites) are demonstrated in different regions of the rat limbic system: field CA3 and field CA1 of the hippocamp, dentate gyrus, area entorhinalis (L V) and area retrosplenialis granularis (L III). The "special structural relations" are to be characterized as follows: direct apposition of membranes of the neighbouring neurons including an extracellular space of relatively constant width over a larger distance without neuroglia intervening this space. Areas of membrane appositions have been observed between neuronal somata ("soma-somatic appositions"), between dendrites ("dendro-dendritic appositions") and between both, somata and dendrites ("dendro-somatic appositions"). In the elmigraphs their length varied between 1 and 6 microns. In the entorhinal cortex and in the retrosplenial cortex soma-somatic appositions were in the majority, whereas in the hippocampal regions dendrodendritic appositions, in the dentate gyrus all types seem to be predominant. In the literature such direct appositions of neuronal membrane often have been described in the CNS of different species. They are considered as a possible morphological correlate of the ephaptic interaction, which is caused by field effects between neighbouring neurons. Following the literature the results of the ephaptic interactions vary between a weak facilitation and synchronous discharge of an unstimulated neuron by a stimulated one. Although such direct appositions are present in different groups of neurons in the limbic system, especially in the hippocamp, up to now their functional meaning is unclear. No correlation seem to exist between neuronal membrane appositions/ephaptic interactions and such a phenomenon like the hippocampal longterm potentiation; mechanisms of cooperativity included in the LTP like coactivation effects of pre- or postsynaptic elements still remain to be clarified.