The major cortical-subcortical re-entrant pathways through the basal ganglia and cerebellum are considered to represent anatomically segregated channels for information originating in different cortical areas. A capacity for integrating unique combinations of cortical inputs has been well documented in the basal ganglia circuits but is largely undefined in the precerebellar circuits. To compare and quantify the amount of overlap that occurs in the first link of the cortico-ponto-cerebellar pathway, a dual tracing approach was used to map the spatial relationship between projections originating from the primary somatosensory cortex (S1) and the primary motor cortex (M1). Because overlap is most likely to occur among functionally related projections, we injected small amounts of the anterograde tracers biotinylated dextran amine and Fluoro-Ruby into electrophysiologically identified, homologous whisker representations in the above mentioned areas.
The ensuing pontine labeling patterns were analyzed using a computerized three-dimensional reconstruction approach. The results demonstrate that whisker-related projections from S1 and M1 are largely segregated but occupy partly adjacent territories of the pontine nuclei. At some locations, the two projections are adjoining and partly overlapping. Furthermore, S1 contributes significantly more corticopontine projections in strength than corresponding M1 representations.
Because the experimental animals previously were used to study corticostriatal projections, we compared the relative patterns of corticostriatal and corticopontine projections from the same pairs of cortical sites. Corticopontine and corticostriatal projections revealed a lower degree of S1-M1 overlapin the pontine nuclei than in the neostriatum. Compared with the neostriatum, the pontine nuclei provide more opportunities for integrating information from related sensory areas.