The study of golgi stained cells and of experimental degeneration under the electron microscope: A direct method for the identification in the visual cortex of three successive links in a neuron chain

Abstract A direct method is presented which makes it possible to identify, from synapse to synapse, three successive links of a neuron chain. The potentialities of the method are shown by examples of the termination of specific afferents in the visual cortex. Following unilateral lesion of the lateral geniculate nucleus in the rat, the distribution of degenerating geniculocortical boutons was studied on two Golgi-stained cells in layer IV of the primary visual cortex. One of the cells was definitely a small pyramidal cell; the other was identified as a spiny stellate (although the possibility that it too was a small pyramidal cell was not rigorously excluded). Both cells received monosynaptic input from the specific afferents as proved by the existence of degenerating boutons synapsing on their dendritic spines. The axonal arborizations of both Golgi-stained cells were traced at the electron microscopic level in thin section series in order to identify the postsynaptic structures contacted by their boutons. All boutons studied established asymmetrical contacts and about 50% of the synapses given by the impregnated boutons were found on smooth dendritic shafts of stellate cells, the rest on spines. The results obtained suggest a neuron circuit involving, successively, the visual afferents, spiny interneurons or monosynaptic visual target pyramidal cells and nonspiny stellate cells. It is suggested that a similar approach might provide direct information about the connectivity in neuron networks in many other parts of the central nervous system hitherto defying elucidation with conventional methods.