Correlations between morphology and electrophysiology of pyramidal neurons in slices of rat visual cortex. I. Establishment of cell classes.

The purpose of this study was to determine whether different classes of neocortical pyramidal neurons possess distinctive electrophysiological properties. Neurons were assigned to classes on the basis of their somadendritic morphology as revealed by intracellular HRP injection. The electrophysiological features of each class are compared in the following paper (Mason and Larkman, 1990). Coronal slices were prepared from the visual cortex of the albino rat and maintained in vitro. Pyramidal neurons from layers 2/3 and 5 were impaled with electrodes containing HRP. After intracellular recording, the cells were injected, reacted to visualize the enzyme, and examined in the light microscope. The 18 neurons from layer 2/3 showed an underlying similarity of somadendritic morphology and were considered to constitute a single cell class, although some features varied with the depth of the soma in the cortex. The 22 layer 5 cells were more diverse and were divided into 2 main classes. Eleven cells had single, thick apical trunks which ascended to close to the layer 1 border then branched to form a terminal arbor. These cells were termed thick layer 5 cells, and were found mainly in the upper part of the layer. Ten layer 5 cells had single apical trunks which tapered to a fine diameter and terminated, without forming an obvious terminal arbor, near the top of layer 4 (3 cells) or in layer 2/3 (7 cells). These cells were termed slender layer 5 cells and were found throughout the layer.(ABSTRACT TRUNCATED AT 250 WORDS)