Cerebellar corticonuclear – nucleocortical topography: A study of the tree shrew (Tupaia) paraflocculus

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Abstract
Following injections of horseradish peroxidase (HRP) into parafloccular cortex, the location of nucleocortical cells in the cerebellar nuclei was determined and compared to the distribution of corticonuclear fibers arising from the same cortical region. Parafloccular corticonuclear fibers, visualized utilizing the Fink and Heimer (1967) method, project to caudal and lateral NIP and caudal and ventral NL. No degeneration is seen in either the NIA or NM. HRP‐positive nucleocortical cells (DAB‐technique) are found in caudal and lateral NIP and caudal NL with the former containing greater numbers of reactive somata. Rostral regions of the NL, medial NIP as well as the NIA and NM are divoid of cells containing HRP reaction product.At least two identifiable populations of HRP‐positive nucleocortical cells are seen. Numerous reactive cells are found in the NIP and NL ipsilateral to the injection site and sparse numbers of HRP‐containing somata are present in homologous areas of the contralateral NL and NIP. Contralateral nucleocortical cells are not only sparse in number but do not receive direct input from the opposite cortex to which they project. Consequently, these particular cells represent non‐reciprocal neurons in corticonuclear –; nucleocortical feedback loops. Ipsilateral nucleocortical cells are numerous and found almost exclusively within the distribution of parafloccular corticonuclear fibers. Also seen on the ipsilateral side interspersed among the HRP‐positive somata are numerous fibers which contain non‐granular homogeneous reaction product. These fibers represent diffusely filled axons of Purkinje cells which were damaged during the injection process. The location of ipsilateral nucleocortical cells within the borders of corticonuclear terminal fields coupled with their intimate opposition to some orthogradely filled terminal branchlets of Purkinje axons, indicates the probability of a precise interaction between these two fiber systems. Although a few HRP‐positive ipsilateral cells are found outside the boundaries of corticonuclear terminal fields they represent only a small fraction of the total number of reactive cells and, indeed, may function as part of the reciprocal cell population.