HRP study of cerebellar corticonuclear‐nucleocortical topography of the dorsal culminate lobule—lobule v—in a prosimian primate ( Galago ): with comments on nucleocortical cell types

Abstract The distribution of retrogradely labeled cerebellar nucleocortical (NC) cells and anterogradely labeled corticonuclear (CN) fibers was investigated in a prosimian primate ( Galago ) by means of horseradish peroxidase as a tracer. Iontophoretic and pressure injections were made in the cortex of lobule V and the resultant patterns of label were determined in the cerebellar nuclei. Following iontophoretic injections in vermal (zone A), intermediate (zones C 1 , C 3 ), and lateral (zone D) cortices, retrogradely labeled cells were present in medial (NM), anterior interposed (NIA), and lateral (NL) cerebellar nuclei, respectively. Larger injections that involved A‐C 2 zones resulted in NC label in NM, medial NIA, and throughout the posterior interposed (NIP) nucleus. Retrogradely labeled NC cells were usually found in areas of their respective nuclei that also contained anterogradely filled CN axons. In addition, retrogradely labeled cells were seen contralateral to some injections. Contralateral NC cells were found mainly in the NM and NIP and seemed to be labeled in response to injections that involved zones A, C 2 , and possibly x on the opposite side. No contralateral CN labeling was seen. It appears that the NC projections of lobule V follows a basic zonal (sagittal) orientation and that most are reciprocal to CN fibers arising from the same cortical area. There is evidence of zonal heterogeneity in the ipsilateral NC projection. Iontophoretic injections placed in adjacent zones resulted in markedly different numbers of retrogradely labeled NC cells in their respective nuclei. Also, after pressure injections that involved two or more adjoining zones, the number of labeled NC cells was large in one nucleus but minimal in an adjacent nucleus. These data suggest that different cerebellar cortical zones have quantitatively different NC input; this may relate to specific functional demands placed on each nucleus and its corresponding cortical zone. On the basis of their known connections, it is hypothesized that there are at least three and possibly four categories of NC cells. Ipsilateral reciprocal NC cells are found in, or on the periphery of, CN terminal fields formed by axons originating from the same cortical area to which the NC cells project. Ipsilateral nonreciprocal NC cells are located outside the CN terminal field and may even be found in an adjacent nucleus; these are fewer in number than the reciprocal population. Contralateral NC cells are found in the opposite cerebellar nuclei and appear to be topographically related to the ipsilateral contingent as well as to the injection site. It is probable that different types of NC cells have individualized functional characteristics.