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 neuron s were traced, Three classes of cells were identified as corticothalam ic, corticocortical, and local circuit neurons, Corticothalamic cells (46%) are small, short pyramids projecting either to the ventral poste romedial nucleus alone or to the posterior group as well. The former a re in upper lamina Vl, have apical dendrites terminating in layer IV, and have intracortical collaterals ascending to layer IV as a narrow c olumn 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 mo re extensive network of collaterals terminating in the upper part of l amina V. Corticothalamic cells do not project to distant cortical targ ets through branching axons, Corticocortical cells (44%) are small, sh ort pyramids, inverted or modified pyramids, or bipolar spiny neurons. They send collaterals principally to infragranular layers of S1 and b ranches to the second somatosensory cortex, the motor cortex, or the c orpus callosum. Local circuit neurons (10%) are basket cells, concentr ated in upper lamina VI, having smooth, beaded dendrites and a rich co llateral network densely covered with varicosities in layers V and V1. We conclude that (1) dendritic morphology and axonal arborizations of corticothalamic cells relate to the projection target; (2) many appar ently diverse layer VI cells project to other cortical fields; and (3) lamina VI is a network for corticothalamic and corticocortical commun ication.