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We have investigated the dendritic and axonal morphology of connected pairs of L4 spiny neurons and L2/3 pyramidal cells in rat barrel cortex. The ‘projection’ field of the axons of L4 spiny neurons in layers 2/3, 4 and 5 has a width of 400–500 μm thereby defining an anatomical barrel-column. In layer 2/3, the averaged axonal ‘projection’ field of L4 spiny neurons together with the dendritic ‘receptive’ field of the connected L2/3 pyramidal cells form a mostly column-restricted anatomical L4-to-L2/3 ‘innervation domain’ that extends 300–400 μm and includes mostly basal dendrites. In the L4-to-L2/3 innervation domain a single L4 spiny neuron contacts ∼300–400 pyramidal cells while in the L4-to-L4 innervation domain it contacts ∼200 other L4 spiny neurons. Similarly ∼300–400 L4 spiny neurons converge onto a single pyramidal cell and ∼200 L4 spiny neurons innervate another L4 spiny neuron. The L2/3 pyramidal cell axon has a vertical projection field spanning all cortical layers, and a long-range horizontal field in layers 2/3 (width 1100–1200 μm) and 5 (700–800 μm) projecting across column borders. The results suggest that the flow of excitation within a barrel-column is determined by the largely columnar confinement of the L4-to-L4 and L4-to-L2/3 innervation domains. A whisker deflection activates ∼140 L4 spiny neurons that will generate EPSPs in most barrel-related L2/3 pyramidal cells of a principal whisker column. The translaminar synaptic transmission to layer 2/3 and the axonal projection fields of L2/3 pyramidal cells are the major determinants of the dynamic, multi-columnar map in which a single whisker deflection is represented in the cortex.