Corticocortical connections within the primary somatosensory cortex of the rat

Abstract

Corticocortical connections within the primary somatosensory (SI) cortex of rat were investigated by using discrete injections of retro- and orthogradely transported neuroanatomical tracers (including HRP, WGA, PHA-L, and 3H-leucine). Tangential and vertical connections were defined with respect to the cytoarchitectonic divisions within the rat SI, specifically: (1) the "granular zones" (GZs), characterized by their dense layer IV granular aggregates, which receive the majority of direct ventroposterior (VP) thalamocortical terminations, (2) the "perigranular zones" (PGZs), the less-granular cortical matrix just surrounding the GZs, and (3) the "dysgranular zones" (DZs), the larger dysgranular regions lying centrally within and just lateral to the SI. Receptive fields recorded in the granular zones are small and discrete, whereas in the perigranular zones and especially in dysgranular zones they exhibit complex sensory convergence. A major aim of this study was to determine whether the pattern of intracortical connectivity within the SI is compatible with these observed physiological differences. In general, the perigranular and dysgranular zones contained more profuse systems of corticocortical connections than did the granular zones. For example, discrete tracer injections in the perigranular zones produced "walls" of labelling throughout the adjacent perigranular zones, while adjacent granular zones were relatively empty. Nevertheless, the granular zones were filled with dendritic branches of neurons in adjacent perigranular zones. Since these dendrites could presumably receive direct VP thalamocortical contacts, they represent one path through which this thalamic sensory information might be transmitted to the perigranular zones. Further transmission to the dysgranular zones might be subserved by a topographically organized system of reciprocal interconnections that was found between the perigranular zones and dysgranular zones. In coronal sections, labelling produced by relatively distant injections of either retro or orthograde tracers generally appeared in a columnar distribution, and was localized in perigranular zones and dysgranular zones. Within these zones, orthograde labelling consisted of vertically oriented axons emitting collateral sprays of terminals in all layers. Retrograde neuronal labelling (composed almost exclusively of pyramidal cells) was greatest in supragranular layers. Proximal to the injection site, labelling tended to spread out from these columns into supra- and infragranular layers in adjacent granular zones.(ABSTRACT TRUNCATED AT 400 WORDS)

Whisker-Related Axonal Patterns and Plasticity of Layer 2/3 Neurons in the Mouse Barrel Cortex

“…Previous works using neuronal tracers also suggested the existence of intracortical connections with whisker-related patterns in the rat barrel cortex (Chapin et al, 1987;Hoeflinger et al, 1995). When neuronal tracers were focally injected in the septal region of the rat barrel cortex, the labeled intracortical connections in layer 4 had a pattern complementary to whisker-related patches of TCAs (Chapin et al, 1987;Hoeflinger et al, 1995).…”

“…In contrast to the rather diffuse horizontal connections within layers 2/3 and 5, it has been suggested that layer 4 of the rat barrel cortex contains intracortical circuitry with distinct whisker-related patterns (Chapin et al, 1987;Hoeflinger et al, 1995). When neuronal tracers were focally injected into the septal region of the barrel cortex, the labeled fibers in layer 4 preferentially distributed in a manner complementary to whisker-related patterns of TCAs (Chapin et al, 1987;Hoeflinger et al, 1995). Such intracortical connections are proposed to be intercolumnar circuits connecting layer 4 septal neurons across the barrel cortex (Fox, 2008a).…”

Excitation and Inhibition Jointly Regulate Cortical Reorganization in Adult Rats

“…In the inhibition zone, the RF enlargement was smaller than at the stimulation site and in the EZ (Haupt et al, 2004). The ICMS-induced changes are, on the one hand, functionally limited by this inhibition zone, and, on the other hand, structurally restricted by the range of horizontal connections and anatomical rules such as the fact that, within the hindpaw representation, layer IV cells maintain relatively few axonal connections with adjacent more dysgranular cortical regions (Chapin et al, 1987). In addition, layer VI cell connectivity has to be considered because the changes of IR after ICMS extend over the whole column.…”

Cross-Sensory Modulation of Primary Sensory Cortex Is Developmentally Regulated by Early Sensory Experience

“…The DGZ receives auditory input from A1 cortex, from the ventral division of the MGN, and also from the multisensory neurons in the mMGN (Brett-Green et al, 2003). DGZ also projects directly to S1 barrel cortex (Chapin et al, 1987;Koralek et al, 1990). Moreover, since horizontal connections in cortex including somatosensory cortex have been shown to recruit inhibitory networks under some conditions (Tucker and Katz, 2003;Pluto et al, 2005;Keniston et al, 2010), it is possible that the projections from the DGZ to barrel cortex terminate on inhibitory FSUs that could cause subthreshold changes leading to an increased excitability in inhibitory neurons.…”