Cerebellar corticonuclear fibers of the dorsal culminate lobule (anterior lobe?lobule V) in a prosimian primate,Galago senegalensis

Abstract

The organization of corticonuclear fibers from lobule V of the anterior lobe of the lesser bushbaby, Galago senegalensis, was studied utilizing the Fink and Heimer ('67) method. Corticonuclear fibers of lobule V are ipsilateral, topographically arranged in their trajectory into the deep cerebellar nuclei and organized into six identifiable zones. No evidence is seen to support the contention that these fibers may project diffusely into the cerebellar nuclei, either in a rostro-caudal or medio-lateral sequence, or that they may project to any of the contralateral nuclei. Lesions in lateral lobule V cortex produce degeneration in rostroventral magnocellular NL. No debris is seen in parvicellular NL and no degenerated axons enter the NIA, NIP or NM. The terminal field in the NL is roughly pyramid shaped with the base being located at the periphery of the nucleus and the apex directed toward the hilus. Based on the totality of its efferent projections the intermediate cortex (IC) of lobule V is a relatively wide region which is divisible into three separate areas. Lateral portions of the IC project heavily into the lateral NIA and send a sparse number of fibers into the NIP. The middle area of the IC sends fibers almost exclusively to a bifid terminal field located in the NIP. From this area of cortex no degenerated fibers enter either the NL or NM and only extremely sparse amounts of debris are seen in rostrolateral NIA. Following damage to medial regions of IC degeneration is seen primarily in medial portions of the NIA and in the juxtarestiform body, if the lesion involves the lateral portions of the vermal cortex. These three regions of the IC, identified here based on the arrangement of their corticonuclear fibers, represent the cortical correlates of compartments C₁, C₂ and C₃ (of Voogd, '69). Lesions involving the lateral aspect of the vermal cortex produce degeneration in the vestibular complex, primarily its lateral nucleus, and in the NIP. These observations may represent a subdivision of this area, a fact previously reported in the cat (Oscarsson and Sjölund, '77a). The medial portions of the vermal cortex project primarily to rostrodorsal NM. The results of this study provide data concerning the arrangement of zones in the anterior lobe cortex of a primate. Due to the fact that differential projections into magnocellular versus parvicellular NL were not seen, the lateral cortex is designated only as zone D. The IC contains, from lateral to medial, zones C₃, C₂ and C₁. The lateral edge of the vermal cortex is designated zone B, although it is recognized that subdivisions of this area may exist (Oscarsson and Sjölund, '77a) while zone A is the more medial area of vermal cortex. These zone designations represent those portions of cortex which overlie previously identified compartments (Voogd, '69) in the subcortical white matter.