Terminal patterns of single, physiologically characterized optic tract fibers in the cat's lateral geniculate nucleus

We have examined the patterns of termination of single, physiologically identified optic tract fibers in the dorsal lateral geniculate nucleus (LGNd) of the cat. The axons were impaled with a recording micropipette and characterized by their responses to light and by their conduction latencies to electrical stimulation of the optic nerve. Horseradish peroxidase was then iontophoretically injected from the pipette into the single fibers. Subsequent histological processing and serial reconstruction revealed the fibers' destinations, patterns of arborization, and the spatial distributions of their terminal boutons. Axons classified as type X invariably projected to layer A or A1 of the LGNd and some sent a collateral into the medial interlaminar nucleus (MIN). Axons of the Y type terminated in layer A or A1 and the MIN. Those from the contralateral eye nearly always had an additional termination in the magnocellular C layer, whereas those from the ipsilateral eye occasionally terminated in layer C1. In the A layers axons distributed their terminal boutons in columns that spanned the widths of the layers. The columns differed in size, shape, and density depending on whether they were from X, Y-on, or Y-off axons. The columns from both X-on and X-off axons were narrow cylinders averaging about 150 micron in width. The columns usually contained fewer than a thousand boutons and about 60% of the boutons were concentrated in the upper halves of the layers. The Y columns were wider, averaging about 375 micron in width, and contained around 1500 boutons. About 70% of the Y boutons were concentrated in the lower halves of the layers. Y-on columns were often broader at the tops and the bottoms of the layers than near the centers, giving them an hourglass shape. The Y-off columns were broader at the bottoms of the layers than at the tops, giving them the shape of a cone or a truncated cone. These distinctive bouton distributions suggest the presence of a sublaminar organization in the A layers based on differences in the density and lateral spread of the terminations of X, Y-on, and Y-off afferents.