Connections between pyramidal neurons in layer 5 of cat visual cortex (area 17)

The structural features of two physiologically-characterised pyramidal neurons (PC1 and PC2) closely situated in layer 5b in the visual cortex (area 17) of a single cat were studied using a combination of electrophysiological and anatomical techniques. Both PC1 and PC2 had exceptionally large somata (30-40 microns in diameter). On the basis of this and other morphological features cell PC1 was classified as a Meynert cell. PC1 possessed a very large (2.75 degrees X 4.50 degrees) binocularly driven standard complex receptive field. PC2 was also binocularly driven with a small, B-type receptive field. Both cells had the same preference for the direction and orientation of visual stimuli. PC1 and PC2 could be antidromically activated from stimulating electrodes positioned above the dorsal lateral geniculate nucleus with a response latency indicating that these cells probably innervated the visual tectum or pretectum. In addition to corticoefferent axons, the two neurons possessed extensive intracortical axon arbors that ramified extensively in layers 5 and 6 of the medial and lateral banks of the lateral gyrus in area 17. Axon collaterals from both PC1 and PC2 also innervated a small common target region in area 18. A total of 313 boutons from the axonal arbors of PC1 and PC2 were examined in the electron microscope. All of the identified synaptic junctions were found to establish Gray type 1 asymmetrical contacts. The combined ultrastructural data for both neurons indicated that 80% of boutons were onto dendritic spine heads, with 14%, 6%, and 1% onto small-, medium-, and large-calibre dendritic shafts, respectively. The spectrum of postsynaptic targets showed little variation with respect to lamina, distance from somata, or cortical area. Other large pyramidal neurons in layer 5 and spiny neurons in layer 6 were identified as receiving synaptic input from either PC1 or PC2. Using a computer graphics system, rotations of the bouton distributions revealed the existence of a clustered innervation of layers 5 and 6 in areas 17 and 18 derived from the two identified neurons. The bouton distributions strongly resembled the tangential pattern described previously for the functional slab-like organisation of the cortex. The results provide a morphological basis for the clustered intrinsic connectivity of pyramidal cells in layers 5 and 6 of the cat visual cortex. Furthermore, the results indicate the widespread excitatory influence of large pyramidal neurons on other cells projecting subcortically to sites dealing with visually guided behavior.