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 investig ate what factors might determine the proportion of somatic membrane covered by inhibitory type 2 synapses. In a sample of 4654 excitatory neurons, syn apse cover did not correlate consistently with cell variety (pyramid or spi ny stellate), soma size, synaptic apposition length or thalamic input. Ther e 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 appl icable determinant of amount of inhibitory synapse cover. The pattern of co ver for cells in different layers was, however, similar between the two hem ispheres of an individual monkey. Measures of inhibitory synapse cover on f our sets of pyramidal neurons in layers 5 and 6, each with different effere nt projection targets, showed that the sets differed significantly from oth er cells in their respective layers, and differed significantly from each o ther. These findings demonstrate that there is unique circuitry for differe nt subsystems within single layers of cortex and provide a rationale for th e rich variety of cortical GABAergic interneurons within single layers.