Cerebral Cortex, Vol 7, 100-109, Copyright © 1997 by Oxford University Press
LL Porter
One feature of the cerebral cortex circuitry is the complex network of
fibers which links its different functional regions. Our knowledge of the
specific relationships between neurons which form these pathways is
limited. The cortico-cortical connections between primary somatosensory
cortex (SI) and primary motor cortext (MI) were the focus of the study. The
aims were twofold: first, to identify characteristics of inter- areal
cortico-cortical connections; and second, to determine if pathways exist
which support the notion that peripheral signals are integrated in the
somatosensory cortext before being relayed to the motor cortex. Neurons in
area 2 of SI, which projected to the motor cortex were identified. The
morphological characteristics of these neurons and the pattern of input
that they received from the area 3a were determined. The fluorescent
retrograde tracer, fast blue, was injected into the electrophysiologically
defined forepaw representation of motor cortex and the anterograde tracer,
dextran- tetramethylrhodamine (DR), was injected into the somatotopically
matched region of area 3a. Labeled neurons in area 2 which were located in
a field of labeled axons arising from area 3a were identified in fixed
tissue sections. Some of these labeled cells were impaled with a Lucifer
yellow (LY)-filled micropipette and were intracellulary labeled by
iontophoretic injection of LY Cells in area 2 that projected to the motor
cortex were located primarily in layers II-III. They were all classified as
pyramidal neurons and were morphologically similar. Their apical dendrites
for the most part did not extend beyond layer II. Their apical tufts
exhibited 2-4 branches within layers II-III, while basal dendrites
exhibited more numerous tertiary basal dendritic branches. Light
microscopic (LM) examination revealed the presence of appositions between
LY-filled profiles and DR-labeled axons. Appositions were observed between
swellings along DR-labeled axons and dendritic shafts or spines of 1
degrees, 2 degrees and 3 degrees branches of apical and 1 degrees and 2
degrees branches of basal dendrites. The appositions were primarily on
proximal segments of labeled dendritic shafts. Fewer appositions with
distal dendrites were observed and some of these were with dendritic
spines. No appositions with the somata were observed. Only one or two
appositions were observed for individual cells. The pattern of
cortico-cortical synaptic input arising from area 3a onto this population
of cells was predicted from these LM findings. An ultrastructural analysis
was performed to confirm the existence of contacts and the predicted
pattern of connectivity. Neurons in area 2 which projected to the motor
cortex, and area 3a axons which projected to area 2, were identified with
electron dense retrograde and anterograde tracers respectively. Labeled
neurons located in a field of labeled axons were examined throughout a
sequential series of ultrathin sections. Electron microscopic analysis
revealed a similar pattern, but with a slightly higher density of synaptic
input (1-8 contacts per target cell) than that predicted from the LM
studies. These results revealed a specific density and pattern of
coritco-cortical input onto an identified population of cortico- cortical
projection neurons. Individual target cells received only sparse input from
a functionally different but somatotopically related region of the cortex.
The pattern of input onto cells was unexpected in that most axons contacted
the shafts of proximal dendrites. This aspect of the connection may
exemplify a unique feature of the cortical circuit which helps to define
its functional role. The significance of these results in defining cortical
function is that the particular cortical circuit described may provide an
anatomical substrate for the modulation of motor cortex activity by
integrated signals from the sensory cortex. The synaptic relationships of
neurons in this pathway may be characteristic of i
ARTICLES
Morphological Characterization of a Cortico-cortical relay in the cat sensorimotor cortex
Uniformed Services University of the Health Sciences, Department of Anatomy and Cell Biology, Bethesda, MD 20814, USA.
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