The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. IV. Terminations upon spiny dendrites.

The forms of the spiny dendrites in layer IV receiving degenerating thalamocortical axon terminals have been examined in serial thin sections. Reconstructions of segments of these dendrites show that the axon terminals synapse with both the dendritic spines and the dendritic shafts. No main shafts of apical dendrites of pyramidal neurons were found to synapse with the thalamic afferents, which are received mainly by spiny dendrites 1-2 micron in diameter, at least some of which appear to be the oblique branches of apical dendrites. The forms of these postsynaptic dendrites are so variable that is concluded they arise from more than one morphological type of neuron. The conclusion based on this and previous articles in the series is that most neuronal elements in layer IV which form asymmetric synaptic junctions are potential recipients of the thalamocortical afferents.