Evidence of an x zone in lobule V of the squirrel monkey (Saimiri sciureus) cerebellum: the distribution of corticonuclear fibers.

The distribution of corticonuclear fibers to medial-most parts of the posterior interposed nucleus (NIP) from lateral areas of the vermis was studied in the squirrel monkey (Saimiri sciureus), using a silver impregnation method. The origin and course of degenerated fibers were studied in serial sections. The distribution pattern of corticonuclear fibers from a series of small well localized lesions placed in the vermis and paravermal cortex of lobule V is compatible with the interpretation that an x zone is present in Saimiri. A comparison of the positions of lesions and the trajectory of fibers arising therein suggests that corticonuclear input to medial-most parts of the NIP originated from a narrow cortical area (about 0.5-0.7 mm wide) located between a cortical area projecting into the medial cerebellar nucleus (the A zone) and a laterally adjacent area (the B zone) which related to the lateral vestibular nucleus. This NIP-projecting cortical area, located about 1.7 mm to 2.5 mm off the midline in lobule V, is interpreted as the x zone in this primate; it extends from lobule IV into lobule VI in squirrel monkey. Corticonuclear fibers of zone x in this primate form a comparatively small terminal field in the medial-most portions of NIP. This contrasts with the distribution of corticonuclear fibers of the C2 zone which consistently distribute to terminal fields that are shifted into more central areas of NIP. There appears to be no overlap of the corticonuclear terminal fields in the NIP for zone x versus the C2 zone. These results were correlated with data from the literature on the distribution of olivocerebellar fibers to the x zone and the C2 zone and the arrangement of cerebellar nucleoolivary projections into the inferior olive from the NIP. The x zone and the C2 zone both receive input from the contralateral medial accessory olive (MAO), both zones project into the NIP, and the NIP projects into those regions of the MAO which, in turn, project to these respective cortical zones and into the NIP. This suggest that the x zone is a component of the NIP-MAO circuit. Furthermore the proposed function of the x zone would support the view that this sagittal strip may have a more extensive rostrocaudal distribution in primates as compared to the cat.