On the organization of cerebellar efferent pathways in the nurse shark (Ginglymostoma cirratum).

Abstract

AbstractThe organization of the efferent fiber systems of the nurse shark cerebellum was studied with the Nauta and Fink‐Heimer techniques. Lesions of the cerebellar cortex produce a pattern of terminal degeneration restricted to select regions of the cortex, as well as ipsi‐ and contralateral cerebellar nuclei. We found no evidence of cortical axons projecting beyond the nuclei, to the medulla and spinal cord as has often been reported.Lesions involving the cerebellar nucleus reveal three efferent pathways. The ipsilateral descending cerebello‐bulbar (IDC) pathway extends as far caudal as the first spinal segment and issues fibers to the lateral parts of the reticular formation. The brachium conjunctivum splits into a larger ascending limb (BCA) and a smaller descending one (BCD). The latter descends to the caudal medulla terminating along its course in the medial reticular formation. The ascending limb provides an input to (1) a nucleus we have tentatively labelled the “red nucleus,” (2) the trochlear nucleus, (3) the oculomotor nuclei, (4) the midbrain central gray, and (5) a poorly differentiated nucleus in the dorsal thalamus.