Figure 1.

This is a schematic of the classical microcircuit adapted from Douglas and Martin (1991). This minimal circuitry comprises superficial (layers 2 and 3) and deep (layers, 5 and 6) pyramidal cells and a population of smooth inhibitory cells. Feedforward inputs – from the thalamus – target all cell populations, but with an emphasis on inhibitory interneurons and superficial and granular layers. Note the symmetrical deployment of inhibitory and excitatory intrinsic connections that maintain a balance of excitation and inhibition.