Secondary vestibulocerebellar projections to flocculonodular lobe in a prosimian primate, Galago senegalensis.

The organization of vestibulocerebellar projections to the flocculonodular and adjacent cortices were studied in Galago using horseradish peroxidase (HRP). Implants of HRP pellets or injections (0.14-0.95 microliter) were placed in floccular, parafloccular, nodular, and uvular cortices. Following survival times of 18-25 hours animals were killed via transcardiac perfusion of heparinized saline followed by a buffered solution of paraformaldehyde and glutaraldehyde. Tissues were processed using DAB as the chromogen. Consequent to floccular implants HRP-positive cells are found bilaterally in medial (MVN), spinal (SpVN), and superior (SVN) vestibular nuclei. Labeled neurons are present in the ipsilateral subgroup y and interstitial nucleus of the eighth nerve. The prepositus hypoglossal nuclei also contained HRP-positive somata. A column of labeled cells is present exclusively in dorsomedial MVN subsequent to injection of the paraflocculus. Injections of nodular cortex reveal a distinct bilateral projection to this cortical area. Many labeled cells are located in SpVN, MVN, SVN, subgroups x and y, the interstitial nucleus, and the ganglion of the eighth nerve. Labeled somata are concentrated in dorsal and dorsolateral SpVN and in a bandlike configuration in subgroup x. HRP-reactive cells appear to have a differential rostrocaudal distribution in MVN, while the majority of positive cells in SVN are found in central portions of the nucleus. After HRP injection into the transition area between nodular and uvular cortices, labeled neurons are present in MVN, SpVN, and the prepositus hypoglossal nucleus. A similar distribution of HRP-positive cells is seen following injections of ventral uvula; however, cells are markedly fewer in number. In no case, subsequent to injection of the flocculonodular lobe and adjacent cortices, are HRP-labeled neurons found in the lateral vestibular nucleus.