Canonical Cortical Circuit

The canonical cortical circuit, with emphasis on the elements that underlie the evoked potential (EP). The two primary cell populations responsible for the EP are supragranular and infragranular pyramidal cells, shown here with apical dendrites that extend to the pial surface. Thalamic input is shown in green and includes projections from the specific and nonspecific nuclei to layers IV and I, respectively. Projections to middle lamina preferentially contact basilar dendrites of supragranular pyramidal cells and also excitatory stellate cells (star-shaped glyph) which in turn relay excitation to supragranular cells. This combination of excitatory drive underlies the P1 wave of the EP, as has been determined by current source density analysis. Supragranular cells project to other cortical regions, but also give off collaterals that target the apical dendrites of local infragranular cells in superficial lamina, thereby generating the N1 wave of the evoked potential. Infragranular pyramidal cells in turn give off long range projections primarily to subcortical regions. Inhibitory interneurons (presumably GABAergic) are depicted as filled circles. These of course exist in all lamina and exhibit a variety of morphology; in this diagram only two groups are suggested and are shown paired with accompanying pyramidal cells. This simplification is meant to emphasize the general bauplan of cortex, with monosynaptic excitation accompanied by disynaptic inhibition or, put more generally, excitation of order m usually accompanied by inhibition of order m+1.