MORPHOLOGICAL-CHARACTERISTICS OF LAYER-II PROJECTION NEURONS IN THE RAT MEDIAL ENTORHINAL CORTEX

The entorhinal cortex receives inputs from a variety of neocortical re gions. Neurons in layer II of the entorhinal cortex originate one comp onent of the perforant path which conveys this information to the dent ate gyrus and hippocampus. The current study extends our previous work on the electroresponsive properties of layer II neurons of the medial entorhinal cortex in which we distinguished two categories of layer I I neurons based on their electrophysiological attributes (Alonso and K link [1993] J Neurophysiol 70:128-143). Here we report on the morpholo gical features of layer II projection neurons, as revealed by in vitro intracellular injection of biocytin. We now report that the two elect rophysiologically distinct types of neurons correspond to morphologica lly distinct types of cells. All neurons (65% of the total cells recor ded) that developed sustained, subthreshold, sinusoidal membrane poten tial oscillations were found to have a stellate appearance. Neurons th at did not exhibit oscillatory behavior had either a pyramidal-like (3 2%) or a horizontal cell morphology (3%). Stellate cells had multiple, thick, primary dendrites. Their widely diverging upper dendritic doma in expanded mediolaterally over a distance of around 500 mu m close to the pial surface. This mediolateral extent was more than double that of the pyramidal-like cells. Dendrites of stellate cells demonstrated long dendritic appendages, and their dendritic spines had a more compl ex morphology than those of nonstellates. The stellate cell axons emer ged from a primary dendrite and were more than double the thickness (a pproximately 1.4 mu m) of the axons of nonstellate cells. Recurrent ax onal collaterization appeared more extensive in axons arising from ste llate cells than from pyramidal-like cells. (C) 1997 Wiley-Liss, Inc.