Cerebral Cortex, Vol 7, 405-421, Copyright © 1997 by Oxford University Press
A Peters, JM Cifuentes and C Sethares
The aim of this study was to investigate the vertical organization of axons
and pyramidal cells in area 18, and to compare it with that in area 17. In
area 18 there are regularly spaced vertical bundles of myelinated axons
that have an average center-to-center spacing of 21 microns. This
arrangement of axons resembles that in area 17. Pyramidal cells in area 18
and their apical dendrites are less regularly arranged. The apical
dendrites of the pyramidal cells of layer 6A aggregate with those from
layer 5 pyramids to form swathes of apical dendrites that pass into layer
4. There they are joined by the apical dendrites of the small layer 4
pyramids, so that much of the neuropil of layer 4 is occupied by apical
dendrites. Most of these apical dendrites form their terminal tufts in
layer 3. Very few of them reach layer 1, which is dominated by the apical
dendrites of layer 2/3 pyramids. Thus, there are two tiers of apical
dendrites and their apical tufts, a deep one formed by the layer 4, 5 and 6
apical dendrites that terminate in layer 3, and a second one formed by the
apical dendrites of layer 2/3 pyramids that terminate in layer 1. In
contrast, in area 17 the apical dendrites of layer 5 pyramids form discrete
clusters that have a center-to-center spacing of 23 microns. These clusters
are joined by the apical dendrites of the layer 2/3 pyramids and all of
these apical dendrites form their apical tufts in layer 1. Based upon the
dispositions of the apical dendrites of the pyramidal cells in area 17 and
18, we speculate that the influences of, and the interactions between, the
feed-forward and feed-back signals in the two areas are quite different,
because in the two areas different postsynaptic targets are available to
these afferents.
ARTICLES
The organization of pyramidal cells in area 18 of the rhesus monkey
Department of Anatomy and Neurobiology, Boston University School of Medicine, MA 02118, USA.
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