In rodents, these patterns of activity develop during the first two postnatal weeks and are not complete until adulthood (Zhang, 2004). With respect to morphology, two general classes of pyramidal neurons have been proposed, with Type I neurons having thick apical dendrites that terminate in tufts in superficial cortical layers. These neurons project axonally to primarily sub-cortical structures. Type II neurons have less apical branching and project intracortically as well as to the striatum. Generally, Type II neurons are regular spiking whereas Type I neurons show bursting patterns of action potentials (Molnar and Cheung, 2006). All cortical pyramidal neurons show complex dendritic branching patterns and asymmetrical synapses characterized by the dendritic spine, the specialized structure that receives excitatory input. In general, PFC pyramidal neurons show more branching and greater density of spines compared with those in visual cortex and other sensory cortices (Elston, 2003).
