Observations on synaptic patterns in the dorsal lateral geniculate nucleus of the cat: The C laminae and the perikaryal synapses.

The laminae near the optic tract which have previously been described as C, C1 and C2, have been studied electron microscopically and compared ] to the A laminae. In the A laminae most of the presumed retinogeniculate axons end in encapsulated synaptic zones that contain multiple interconnected synaptic profiles. Only a few of these axons form simple axodendritic synapses upon proximal dendritic segments of the largest cells. In the C laminae the situation is reversed. Most of the retinogeniculate axons end as simple synapses upon medium or small dendrites and few end in encapsulated synaptic zones. Further, these zones are simpler in the C laminae than in the A laminae. The neuronal profiles are smallest and the synaptic interconnections appear to be least complex in lamina C2, where there are only a few, small, presumed retinogeniculate terminals. Of the three C laminae, lamina C, furthest from the optic tract, shows the largest profiles and the most complex interconnections. Bundles of very fine axons occur in all the laminae, but they dominate the regions closest to the optic tract (lamina C2) where they tend to run parallel to the lamina. Some of the largest cells in laminae A, A1 and C are contacted by very fine perisomatic axons which contain round vesicles. These axons have not been described before and are likely to represent an input too fine to have been demonstrable by light microscopical methods. [ABSTRACT FROM AUTHOR]