Intracortical axonal projections of lamina VI cells of the primary somatosensory cortex in the rat: a single-cell labeling study.

A sample of 84 neurons in lamina VIa of rat somatosensory cortex (S1) was juxtacellularly labeled with biocytin, and the axons of the neurons were traced. Three classes of cells were identified as corticothalamic, corticocortical, and local circuit neurons. Corticothalamic cells (46%) are small, short pyramids projecting either to the ventral posteromedial nucleus alone or to the posterior group as well. The former are in upper lamina VI, have apical dendrites terminating in layer IV, and have intracortical collaterals ascending to layer IV as a narrow column about the size of a barrel. The latter are in the lower half of lamina VI, have apical dendrites terminating in layer V, and have a more extensive network of collaterals terminating in the upper part of lamina V. Corticothalamic cells do not project to distant cortical targets through branching axons. Corticocortical cells (44%) are small, short pyramids, inverted or modified pyramids, or bipolar spiny neurons. They send collaterals principally to infragranular layers of S1 and branches to the second somatosensory cortex, the motor cortex, or the corpus callosum. Local circuit neurons (10%) are basket cells, concentrated in upper lamina VI, having smooth, beaded dendrites and a rich collateral network densely covered with varicosities in layers V and VI. We conclude that (1) dendritic morphology and axonal arborizations of corticothalamic cells relate to the projection target; (2) many apparently diverse layer VI cells project to other cortical fields; and (3) lamina VI is a network for corticothalamic and corticocortical communication.