In the neocortex, glutamatergic pyramidal neurons represent approximately 80% of the neuronal population and have been best studied in sensory cortices, especially the visual cortex. Neocortical pyramidal neurons are arranged in layers (I–VI in most parts of cortex, rodent PFC lacks a well-defined layer IV) and make connections with both glutamatergic and γ-amino-butyric acid (GABA) neurons within the cortex and those in subcortical areas. Although less well characterized than interneurons (see below), cortical pyramidal neurons also show diversity with respect to size, dendritic morphology, and firing properties. In general, cortical pyramidal neurons are grouped into two general classes, regular spiking (RS) neurons that show little or slow adaptation during repetitive firing and intrinsically bursting (IB) neurons that display more complex periods of firing. In the prefrontal cortex of both rodents and primates, most RS neurons (RS1 subtype) respond to current injection by smooth increases in action potential (AP) frequency with little change in AP threshold or amplitude (Zhang, 2004; Chang and Luebke, 2007). RS2 neurons show decreasing action-potential height and an increase in AP firing threshold whereas fast-accommodating (FA) RS
