Inhibitory synapse cover on the somata of excitatory neurons in macaque monkey visual cortex

Electron microscopy was used in macaque monkey cortical area V1 to investigate what factors might determine the proportion of somatic membrane covered by inhibitory type 2 synapses. In a sample of 4654 excitatory neurons, synapse cover did not correlate consistently with cell variety (pyramid or spiny stellate), soma size, synaptic apposition length or thalamic input. There were significant differences in somatic synapse cover per layer, but the pattern of differences in cover among layers differed significantly between animals, suggesting that laminar environment alone is not a generally applicable determinant of amount of inhibitory synapse cover. The pattern of cover for cells in different layers was, however, similar between the two hemispheres of an individual monkey. Measures of inhibitory synapse cover on four sets of pyramidal neurons in layers 5 and 6, each with different efferent projection targets, showed that the sets differed significantly from other cells in their respective layers, and differed significantly from each other. These findings demonstrate that there is unique circuitry for different subsystems within single layers of cortex and provide a rationale for the rich variety of cortical GABAergic interneurons within single layers.