Motor performance depends on somatosensory feedback, and consistent with this finding, primary somatosensory (SI) cortex projects to several regions involved in motor control. Although the pathways mediating sensorimotor integration are known, few studies have compared their projection patterns. Therefore, in each animal, we injected two anterograde tracers into SI barrel cortex and compared the relative density and spatial extent of the labeled projections to the primary motor (MI) cortex, neostriatum, superior colliculus, and basal pons. Quantitative analysis revealed that these projections terminated most extensively in the neostriatum, to a lesser extent in MI cortex, and innervated the least amount of neuropil in the superior colliculus and pontine nuclei. Tracer overlap in the pontine nuclei was significantly higher than in the other three brains regions, and was strongly correlated with overlap in the superior colliculus, presumably because some projections to these two brain regions represent collaterals of the same neurons. The density of labeled varicosities was highest in the pons and lowest in MI. As a proportion of total labeling, densely packed clusters of labeled terminals were most prevalent in the pons, less prevalent in neostriatum and superior colliculus, and least prevalent in MI cortex. These results are consistent with physiological evidence indicating strong coherence between SI barrel cortex and the cerebellum during whisking behavior.
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