The excitatory neuronal network of rat layer 4 barrel cortex

Sensory whiskers are mapped to rodent layer 4 somatosensory cortex as discr ete units termed barrels, which can be visualized at high resolution in liv ing brain slices. Both anatomical and physiological properties of the layer 4 neuronal network can thus be investigated in the context of the function al boundaries of this sensory map. Large-scale confinement of neuronal arbo rs to single barrels was suggested by restricted lateral diffusion of Dil a cross septa between barrels. Morphological analysis of dendritic and axonal arborizations of individual excitatory neurons showed that neuronal proces ses remain within the barrel of origin through polarization toward the cent er of the barrel. Functionally, the large-scale properties of the neuronal network were investigated through mapping the spatial extent of field EPSPs , which were found to attenuate at barrel borders. This ensemble property o f a layer 4 barrel was further investigated by analyzing the connectivity o f pairs of excitatory neurons with respect to the locations of the somata. Approximately one-third of the excitatory neurons within the same barrel we re synaptically coupled. At the septum between adjacent barrels the connect ivity dropped rapidly, and very few connections were found between neurons located in adjacent barrels. Each layer 4 barrel is thus composed of an exc itatory neuronal network, which to a first order approximation, acts indepe ndently of its neighbors.