Hierarchical Organization of Neocortical Neuron Types

2 Abstract The neocortex consists of many diverse neuron populations distributed across cortical layers having specialized connectivity and projection patterns. Glutamatergic pyramidal cells, which are cortical projection neurons, reside in all layers except layer 1, while GABAergic nonpyramidal cells are ubiquitous throughout all cortical layers. These broad classes of excitatory and inhibitory neurons comprise specialized neuron subtypes that have specific morphological, physiological, and chemical properties. However, while much is now known about the types in the cortex, less is known regarding the rules governing their selective connectivity into cortical and extracortical circuits. In layer 5 of the rat frontal cortex, several distinct populations of pyramidal cells are identifiable based on their distinct extracortical projections, firing characteristics, morphologies, and positions within layer 5. We have characterized highly selective synaptic connectivity among and between these pyramidal cell populations, which likely contributes to their establishing and maintaining functional loops between the frontal cortex, basal ganglia, and thalamus. However, less is known about how GABAergic neuron subpopulations are selectively incorporated into cortical circuits or how they might differentially regulate cortical output to subcortical targets.