The organization of the pulvinar in the grey squirrel (Sciurus carolinensis). II. Synaptic organization and comparisons with the dorsal lateral geniculate nucleus

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Abstract
The purpose of these experiments was to compare the synaptic organization of the subdivisions of the pulvinar defined in the preceding paper (Robson and Hall, '77) with each other and with the organization present in the dorsal lateral geniculate nucleus. The electron microscope was used to analyze normal synaptic arrangements and degenerating axonal terminals resulting from lesions. The dorsal lateral geniculate nucleus in the grey squirrel contains synaptic clusters similar to those described previously for other species. These clusters are characterized by large optic tract terminals which form multiple contacts onto large dendritic processes and other processes containing flat or pleomorphic vesicles. The geniculate lamina adjacent to the optic tract receives projections from the superior colliculus as well are from the retina. The terminals of the superior colliculus axons are small and medium sized and lie outside of the synaptic clusters. The retinal terminals are in the clusters. In the pulvinar, the rostro-medial subdivision contains synaptic clusters which resemble those in the lateral geniculate nucleus. These clusters contain large axon terminals which make multiple contacts onto large dendrites. However, these terminals are not contributed by an ascending sensory pathway but by axons from strait cortex. The rostro-lateral and caudal subdivisions of the pulvinar also contain synaptic clusters, but these clusters consist of a segment of a large dendrite which is ensheathed by medium-sized terminals. Since only a few of these medium sized terminals in any one cluster degenerate after tectal lesions, and none degenerate after cortical lesions, it is suggested that the morphological arrangement of these clusters may permit the convergence of axons from several sources, some of which are unidentified, onto the same dendritic segment.