A Golgi and electron microscopical study of nerve cells in layer I of the cat auditory cortex

The nerve cells of the plexiform layer of the cat auditory cortex were studied in Golgi rapid preparations, and by electron microscopy. In the Golgi material 178 cells were found. Thirty were horizontal cells (Cajal's cells) with spiny dendrites and a long axon with horizontal trajectory. Seventy-two small cells had smooth dendrites and a short axon branching profusely around the perikaryon, within the plexiform layer. Twenty-eight were slightly larger cells, morphologically similar to the small ones, but their axons and dendrites were longer, extending to layer II. Twenty-five cells had large bottle-shaped perikarya, spiny dendrites reaching layer II, and a vertically oriented axon branching in layer II, and sometimes reaching layer III. In addition, 23 small modified pyramidal cells, similar to those of layer II, were found within the boundaries of layer I. In the most superficial 50 mum of the cortex horizontal cells were found, but the other categories were very rare. The 3 categories of short axon cells were preferentially localized between 50 and 150 mum from the pial surface. By electron microscopy two classes of cells were found. The first had abundant mitochondria, well defined Nissl bodies and few axosomatic synapses. The second had few mitochondria, undefined Nissl bodies and 6-8 axosomatic synapses per profile. Desmosome-like membrane specializations were found between perikarya of the second class and pyramidal dendrites or their spines. It is suggested that the first class of cells found by electron microscopy may correspond to the horizontal cells while the second class may correspond to the 3 categories of short axon cells.