Synaptology of retinal terminals in the dorsal lateral geniculate nucleus of the cat.

We have made a fine structural investigation of the synaptic patterns made by axon terminals of retinal ganglion cells in the dorsal lateral geniculate nucleus of the cat. We compared the retinal input to dendritic processes that bear clusters of large appendages with the retinal input to relatively smooth dendritic segments that have only a few isolated spines. The study was restricted to the portion of laminae A and A1 that receive central visual field input. We were able to completely reconstruct 33 individual terminal boutons from long series of consecutive thin sections. Retinal terminals that were presynaptic to dendritic appendages tended to occupy the central position in the complex synaptic zones of geniculate fine structure called glomeruli. These terminals were surrounded by significantly more profiles than retinal terminals that were presynaptic to dendritic stems and averaged twice as many synaptic contacts per terminal bouton. The retinal input to dentritic appendages was heavily involved in a specific synaptic pattern called the triadic arrangement while retinal input to dendritic stems was only lightly involved in triads. Dendritic appendages in triads received greater synaptic input from profiles with flattened vesicles than did the dendritic stems that were found in triads.