L2/3 interneuron groups defined by multiparameter analysis of axonal projection, dendritic geometry, and electrical excitability.

For a detailed description of the circuitry of cortical columns at the level of single neurons, it is essential to define the identities of the cell types that constitute these columns. For interneurons (INs), we described 4 "types of axonal projection patterns" in layer 2/3 (L2/3) with reference to the outlines of a cortical column (Helmstaedter et al. 2008a). In addition we quantified the dendritic geometry and electrical excitability of 3 types of the L2/3 INs: "local," "lateral," and "translaminar" inhibitors (Helmstaedter et al. 2008b). Here, we used an iterated cluster analysis (iCA) that combines axonal projection patterns with dendritic geometry and electrical excitability parameters to identify "groups" of INs, from a sample of 39 cells. The iCA defined 9 groups of INs. We propose a hierarchical scheme for identifying L2/3 INs. First, L2/3 INs can be classified as 4 types of axonal projections. Second, L2/3 INs can be subclassified as 9 groups with a high within-group similarity of dendritic, axonal, and electrical parameters. This scheme of identifying L2/3 INs may help to quantitatively describe inhibitory effects on sensory stimulus representations in L2/3 of cortical columns.