Descending projections from the dysgranular zone of rat primary somatosensory cortex processing deep somatic input.

In the mammalian somatic system, peripheral inputs from cutaneous and deep receptors ascend via different subcortical channels and terminate in largely separate regions of the primary somatosensory cortex (SI). How these inputs are processed in SI and then projected back to the subcortical relay centers is critical for understanding how SI may regulate somatic information processing in the subcortex. Although it is now relatively well understood how SI cutaneous areas project to the subcortical structures, little is known about the descending projections from SI areas processing deep somatic input. We examined this issue by using the rodent somatic system as a model. In rat SI, deep somatic input is processed mainly in the dysgranular zone (DSZ) enclosed by the cutaneous barrel subfields. By using biotinylated dextran amine (BDA) as anterograde tracer, we characterized the topography of corticostriatal and corticofugal projections arising in the DSZ. The DSZ projections terminate mainly in the lateral subregions of the striatum that are also known as the target of certain SI cutaneous areas. This suggests that SI processing of deep and cutaneous information may be integrated, to a certain degree, in this striatal region. By contrast, at both thalamic and prethalamic levels as far as the spinal cord, descending projections from DSZ terminate in areas largely distinguishable from those that receive input from SI cutaneous areas. These subcortical targets of DSZ include not only the sensory but also motor-related structures, suggesting that SI processing of deep input may engage in regulating somatic and motor information flow between the cortex and periphery.