Dual termination modes of corticothalamic fibers originating from pyramids of layers 5 and 6 in cat visual cortical area 17.

Terminal morphology of corticothalamic fibers originating from cat area 17 was examined. Injections of an anterograde axonal tracer, phaseolus vulgaris leucoagglutinin (PHA-L), in area 17 resulted in labeling of small boutons in the dorsal lateral geniculate, perigeniculate, and thalamic reticular nuclei and in labeling of large boutons in the lateral nucleus of lateral posterior-pulvinar, ventral lateral geniculate, and pulvinar nuclei. Since it is well known that the dorsal geniculate nucleus is a major corticothalamic target for layer 6 pyramids and the lateral posterior-pulvinar complex is that for layer 5 pyramids, the findings indicate that layer 5 pyramids in cat area 17 project axons ending with large boutons, while layer 6 pyramids project those ending with small boutons.