Properties of neocortical microcircuits

The mammalian neocortex is one of the most complex brain areas, integrating all sensory modalities and allowing the animal to perform high level cognitive operations. Despite some cytoarchitectural specializations, the general neocortical structure is remarkably preserved between different areas as well as between species. Thus, a profound understanding of its "canonical" cortical microcircuitry and its way of transmitting information would give great insights in the common working principles and the language our brain is using to integrate signals from all regions of the brain. In this thesis I investigated four main aspects of small cortical networks with the multi-cell patch clamp technique in acute cortical brain slices of juvenile rats. In the first part, I characterize summation properties of a recently discovered frequency-dependent disynaptic inhibitory pathway (FDSI) between layer 5 pyramidal cells (PCs). FDSI is readily recruited by few bursting PCs and is mediated by only few intermediate interneurons. It seems to be a ubiquitous pathway since it is present in all investigated cortical areas (primary somatosensory, primary motor, primary auditory, secondary visual, and medial prefrontal cortex). In the second part, limits of the information transmission abilities of the spike generating mechanism of PCs and of their synapses are estimated in an information theoretic framework. Information transmission of PC-PC synapses is only measurable for very low frequencies (~ 1 Hz). In the third part, a method to determine the connection proximity of two interneurons being part of a gap junction network is introduced. Finally, the synaptic network of layer 6 in the primary somatosensory cortex is investigated, and various synaptic dynamics between six different cell types are revealed. All these findings contribute to a more precise knowledge on structural and functional aspects of intra-cortical information processing.