Corticocortical connections within the primary somatosensory cortex of the rat.

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, PHAL, 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. The cellular basis for these connections was assessed by following single axons (filled by extracellular HRP or PHA-L injections) for long distances through thick sections. Axons of layer III pyramids travelled in a U-shaped trajectory, first ramifying locally, then descending to the deep cortical layers and ramifying further before coursing through deep cortical layers or white matter to their termination zones. It was concluded that neurons in the perigranular zones as well as the granular zones may receive VP thalamocortical information projecting to layer IV in the granular zones, and that this may be transmitted (I) to deep layers of adjacent granular zones and (3) to topographically more distant dysgranular zones. This is consistent with neurophysiological findinge that receptive fields are smallest in granular zone layer IV, and largest in layerV, and in all layers in perigranular zones and in dysgranular zones.