Corticothalamic projections from the rat primary somatosensory cortex

Abstract

To study the cells of origin of corticothalamic inputs to the ventral posterior and posterior medial nuclei of the somatosensory thalamus in rats, we injected small aliquots of tracer into each nucleus and analyzed the pattern of retrograde labeling in the posteromedial barrel subfield of primary somatosensory cortex, which can be divided into barrel and nonbarrel zones. The ventral posterior nucleus is innervated by neurons in layer VIa of both zones, whereas the posterior medial nucleus is innervated by neurons in layers Vb and VIb of both zones with additional innervation from layer VIa of nonbarrel cortex. Thus, only the posterior medial nucleus receives a layer Vb input. Because the layer Vb input is interpreted as the initiation of a feedforward cortico-thalamocortical pathway, this implies that the target of the posterior medial nucleus, which includes the nonbarrel cortex, is a higher-order cortical area. We thus suggest that this cortical zone, which is classically considered part of the primary somatosensory cortex, should be reclassified as higher-order cortex.

Figure 1.

Injection sites and resultant retrograde labeling. A, B, Examples of injection sites limited to the ventral posterior or posterior medial nucleus. The dashed line indicates the border between these two nuclei. C, Retrograde labeling of primary somatosensory cortex after injection into the ventral posterior nucleus. The layers are indicated and were determined from staining with bisbenzamide and viewed through the appropriate filter (image not shown). Furthermore, alternate sections were stained with cytochrome oxidase to demonstrate the location of barrel and nonbarrel cortex as in F and G. D, E, Retrograde labeling of primary somatosensory cortex after injection into the posterior medial nucleus. The layers are indicated and were determined as in C. F, G, Alternate sections showing cytochrome oxidase staining (F) and retrograde labeling (G). The arrows in F indicate barrels and are repeated in G.

Figure 2.

Schematic representation of relationships between thalamus and cortex involving the ventral posterior and posterior medial nuclei on the one hand and the primary somatosensory cortex on the other. See Results for details. SI, Primary somatosensory cortex; SII, secondary somatosensory cortex.