Figure 7. Schematic illustration of the excitatory circuitry in L6a infrabarrels and surrounding septa.

(Left, Input) CT cells in upper L6 are densest in the infrabarrels (green shadows), which are aligned with L4 barrels (gray). In contrast, CC projection cells are more prevalent in the surrounding septa. CC cells receive strong excitatory input from both VPm and POm thalamic nuclei, whereas CT cells receive weaker thalamocortical input, exclusively from the VPm. (Right, Output) Previous work has shown that CT neurons in upper L6 project their axons to the thalamus and vertically within their home cortical column, whereas CC neurons have horizontally projecting axons that target infragranular layers of neighboring cortical columns and other cortical areas (Zhang and Deschenes, 1997). These two distinct excitatory circuits are also partially segregated from each other; local synaptic connections between CT and CC neurons are generally sparse and unidirectional. and most likely to go from CC cells to CT cells.