Distribution of inhibitory synapses on the somata of pyramidal neurons in cat motor cortex

The mechanisms by which cortical neurons perform spatial and temporal integ ration of synaptic inputs are dependent, in large part, on the numbers, typ es, and distributions of their synapses. To further our understanding of th ese integrative mechanisms, we examined the distribution of synapses on ide ntified classes of cortical neurons. Pyramidal cells in the car motor corte x projecting either to the ipsilateral somatosensory cortex or to the spina l cord were labeled by the retrograde transport of horseradish peroxidase. Entire soma of selected corticocortical and corticospinal cells were examin ed using serial-section electron microscopy. The profiles of these somata a nd the synapses formed with each of these profiles were reconstructed from each thin section with a computer-aided morphometry system. All somatic synapses were of the symmetrical, presumably inhibitory type. F or both cell types, these synapses were not homogeneously distributed over the somatic membrane, but were clustered at several discrete zones. The number and density of synapses on the somata of different corticocortic al and corticospinal neurons were not significantly different. However, the density of these synapses was inversely correlated with the size of their postsynaptic somata. We discuss the significance of these findings to the i ntegrative properties of cortical neurons.