The organization of pyramidal cells in area 18 of the rhesus monkey

The aim of this study was to investigate the vertical organization of axonsand pyramidal cells in area 18, and to compare it with that in area 17. Inarea 18 there are regularly spaced vertical bundles of myelinated axonsthat have an average center-to-center spacing of 21 microns. Thisarrangement of axons resembles that in area 17. Pyramidal cells in area 18and their apical dendrites are less regularly arranged. The apicaldendrites of the pyramidal cells of layer 6A aggregate with those fromlayer 5 pyramids to form swathes of apical dendrites that pass into layer4. There they are joined by the apical dendrites of the small layer 4pyramids, so that much of the neuropil of layer 4 is occupied by apicaldendrites. Most of these apical dendrites form their terminal tufts inlayer 3. Very few of them reach layer 1, which is dominated by the apicaldendrites of layer 2/3 pyramids. Thus, there are two tiers of apicaldendrites and their apical tufts, a deep one formed by the layer 4, 5 and 6apical dendrites that terminate in layer 3, and a second one formed by theapical dendrites of layer 2/3 pyramids that terminate in layer 1. Incontrast, in area 17 the apical dendrites of layer 5 pyramids form discreteclusters that have a center-to-center spacing of 23 microns. These clustersare joined by the apical dendrites of the layer 2/3 pyramids and all ofthese apical dendrites form their apical tufts in layer 1. Based upon thedispositions of the apical dendrites of the pyramidal cells in area 17 and18, we speculate that the influences of, and the interactions between, thefeed-forward and feed-back signals in the two areas are quite different,because in the two areas different postsynaptic targets are available tothese afferents.