Investigations of microcircuitry in the rat barrel cortex using an experimentally constrained layer V pyramidal neuron model

The mammalian neocortex consists of repeated structural and functional units known as columns, spanning the six layers of the isocortex. A column encompasses different types of neurons sharing common response properties to afferent inputs. The coding and processing of information in local microcircuits comprised of interconnected neurons from different populations define the functional properties of the columns, and can be regarded as the basis of information processing in the cortex. The barrel cortex of the rat, which receives sensory information from the facial whiskers of the animal, provides a useful model system for investigating the properties of microcircuits, since the columnar structure is well defined with each large barrel-related column receiving its primary input from one facial whisker. In this thesis, the detailed distribution of connections formed by neuronal populations in the local microcircuitry onto the dendrites of large layer V intrinsically burst firing (IB) pyramidal cells of the rat barrel cortex was investigated. These neurons have large dendritic trees consisting of subdomains with different intrinsic properties, suggesting that the specific position of individual synapses on the dendrites could be critical. Layer V IB pyramidal neurons have been shown to play a critical role in learning processes as well as the modulation of ongoing behaviour in the awake animal. Simulations and analysis of extensive datasets from mapping of functional connections between layer V IB neurons and local neuronal populations were carried out using an experimentally constrained single neuron model. Simulation results suggest that projections from neurons located in the local and neighbouring columns of the rat barrel cortex selectively target the basal and proximal apical dendritic domains of the layer V IB neurons. Feedback connections from higher cortical areas critical in learning processes are known to selectively target the distal apical tufts of layer V pyramidal cells, thus the findings in this thesis suggest the existence of a functional segregation of the inputs to different dendritic subdomains correlated with the intrinsic properties of the dendritic compartments that they target.