[Principles of synaptic organization in the central nervous system of vertebrates. Regional structural differences...

The large variety of neuromorphological and neurophysiological data makes it necessary to look for possible principles of intrinsic organization and for common features and essential differences in the structure of neuronal networks in the CNS. Comparing the references on these networks one finds different termini technici related to interneuronal synapses such as "diads", "triads" complex synapses and glomeruli. AKERT and STEIGER (1967) discussed "glomeruli" in a considerable review. The ultrastructural analysis of the neuronal processes in these synaptic complexes has contributed considerably to our understanding of their synaptic connections. We differentiate "conventional" synapses such as simple axo-dendritic, axo-somatic, axo-spinous and axo-initialsegment synapses from the so-called "unconventional" synapses such as axo-axonic, dendro-dendritic (= serial) and reciprocal synapses. In the prepiriform, hippocampal, and visual cortices--even in the glomeruli of the turtle's cortex and in the glomeruli of the cerebellar cortex there are no serial but only simple synapses, whereas in the glomeruli of the olfactory bulb and in the glomeruli of the lateral geniculate body there are besides simple synapses--serial synapses with presynaptic dendrites. Similar complex synaptic arrangements are present in the pulvinar, colliculus superior, thalamus, globus pallidus and medulla, but not in the neostriatum. That means that the--many cortical neuronal types--are directly connected by simple synapses, but other--less numerous neuronal types--in the brain stem (with exception of the neostriatum) are coupled together in a very complex manner, where serial synapses, triads and reciprocal synapses (retina) play a very important role. The functional relevance of these synapses and the importance of an exact nomenclature for interdisciplinary cooperation are discussed.