Although GABAergic neurons within neocortex represent only 20% of the neuronal population, there is a great deal of diversity with respect to their anatomical, biochemical, and functional properties. The Petilla Interneuron Nomenclature Group (named in honor of Ramon Y Cajal’s hometown) has recently suggested a set of common terms to classify the various subtypes of interneurons (Ascoli et al., 2008). Although beyond the scope of the present review, the conclusions of this group are summarized below. GABAergic neurons within the neocortex (interneurons) usually have short axons that bifurcate within a cortical column but can project across columns. Despite this general scheme, there are long-axon GABA neurons that project to distal subcortical structures although less is known about these types of neurons. Cortical GABAergic interneurons show a wide variety of axonal and dendritic branching patterns that have historically been grouped into several categories, including basket cells, chandelier cells, bipolar cells, double bouquet cells, bitufted cells, and neurogliaform cells (Markram et al., 2004). These different cell types specialize in their location of axon innervation of target neurons, including soma and proximal dendrites
