Journal of Neuroscience, Vol 8, 1887-1900, Copyright © 1988 by Society for Neuroscience
Intracortical connections of two functional subdivisions of the somatosensory forepaw cerebral cortex of the raccoon
GS Doetsch, GP Standage, KW Johnston and CS Lin
Department of Surgery (Section of Neurosurgery), Medical College of Georgia, Augusta 30912.
The aim of this study was to compare the intrinsic intracortical
connectivities of 2 functionally distinct subdivisions of the somatosensory
(Sml) forepaw cortex of the raccoon--the somatotopic glabrous skin
representation and the more heterogeneous, hairy skin and claw
representation of the digits. HRP was injected into one or the other
functional subdivision of a particular digit subgyrus of Sml cortex in 10
adult raccoons. The distribution of HRP-labeled neurons and axon terminals
in the cortex showed that intrinsic "horizontal" connections exist within
and between individual cortical digit zones; the labeling tended to have an
oval-shaped configuration that was longer in the mediolateral than in the
anteroposterior curvilinear plane. The 2 cortical sectors were found to
have different patterns of intracortical projections. The connections of
the glabrous skin region of each cortical digit zone were primarily local
and confined to that same digit representation. HRP-filled neurons were
concentrated near the injection site and decreased in density within the
banks and fundi demarcating the injected digit subgyrus; few labeled cells
were found in adjoining digit zones. Longer projections to the glabrous
subdivision of a particular digit area typically originated from neurons in
the heterogeneous subdivision of that same digit area. In contrast, the
connections of the heterogeneous region of each digit zone were much more
extensive and usually included projections from nonadjacent, as well as
neighboring digit zones. The density of HRP- positive neurons declined more
gradually with distance from the injection site, and considerable labeling
was present in the heterogeneous sectors of adjacent digit zones. The
intracortical projections of both functional subdivisions were often, but
not always, reciprocal, and the cells of origin tended to be distributed in
clusters. The laminar distributions of labeled neurons were similar for
both sectors; HRP-filled cells were concentrated more in the supragranular
layers, especially in layer III; fewer were found in the infragranular
layers, mainly in layer VI and rarely in layer V. These results show that
the intrinsic connections of the glabrous cortical subdivisions are fairly
localized, whereas those of the heterogeneous cortical subdivisions are
more diffuse and highly convergent. The differing intracortical
connectional patterns of the 2 sectors are consistent with their
contrasting thalamocortical projection patterns and may contribute to the
unique functional properties of neurons located within each
sector.(ABSTRACT TRUNCATED AT 400 WORDS)
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