Tangential preferential orientations of neuron processes in the rat neocortex

The orientations in the tangential plane, i.e., parallel to the pial surface, of dendrites and axons in several functionally different areas of the rat neocortex were investigated. Using a large data base of over 30,000 measurements and the t-test and chi 2 test for significance, we found an overall preference of both dendrites and axons for the anteroposterior and adjacent orientations. The degree of preference varied from area to area and among different cortical laminae. Other variations were correlated with neuron type and dendritic rank (in a tree). The pyramidal cell dendritic fields show little orientation in the tangential plane, whereas a large fraction of the stellate cells are oriented. The overall preferential orientations are apparently related to brain axes and growth lines, whereas the variability is probably due to the influence of the functional topography.