The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. II. Terminations upon neuronal perikarya and dendritic shafts.

The forms of dendrites in layer IV receiving degenerating thalamocortical axon terminals directly on their shafts were examined in serial thin sections. Reconstructions showed these dendrites varied in thickness between 2.5 and 0.5 mum. They had essentially smooth contours and rarely showed evidence of protrusions or spines. They were further characterized by the presence of many synapses along their shafts. Only about one in 12 of these synapses was formed by degenerating thalamocortical axon terminals. These smooth dendrites emerged from neuronal perikarya that also received degenerating axon terminals which formed asymmetric synaptic junctions. Such cell bodies bore both symmetric and asymmetric synaptic junctions, and not all of the latter were caused to degenerated after a thalamic lesion. These postsynaptic neurons appeared to be of two kinds, ones with thin dendrites that often contained closely packed microtubules, and others with thicker dendrites that emerged from the poles of oval perikarya.