Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex

The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity injected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of the other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neurons and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer IVab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicles and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive field properties of this neuron, and the possible effects are discussed.