Organization of the cerebellum in the pigeon (Columba livia): I. Corticonuclear and corticovestibular connections.

The projections of the cerebellar cortex upon the cerebellar nuclei and the vestibular complex of the pigeon have been delineated using WGA-HRP as an anterograde and retrograde tracer. Injections into individual cortical lobules (II-IXa) produce a pattern of ipsilateral terminal labeling of both the cerebellar and vestibular nuclei. The pattern of corticonuclear projections indicates both a rostrocaudal and a mediolateral organization with respect to the lobules and is consistent with a division of the cerebellar nuclei into a medial (CbM) and a lateral (CbL) nucleus. The retrograde experiments indicate that these nuclei receive projections, respectively, from Purkinje cells within medial (A) and lateral (C) longitudinal zones, which alternate with longitudinal zones (B, E) projecting upon the vestibular complex. Purkinje cells in (vestibulocerebellar) lobules IXb-X show only limited projections upon the cerebellar nuclei, but do project extensively upon the cerebellovestibular process (PCV), as well as upon the medial, superior, and descending vestibular nuclei. As the injection site shifts from medial to lateral, there is a corresponding shift in focus of the projection within PCV from areas bordering CbM to those abutting CbL. The topographic organization of corticovestibular projections is less clear-cut than those of the corticonuclear projections. Lobules II-X project upon the lateral vestibular nucleus (anterior lobe) or the dorsolateral vestibular nucleus (posterior lobe). These projections originate from either side of the lateral (C) zone. Projections originating from the medialmost (B) zone are interrupted in lobules VI and VII. The anterior and posterior portions of the lateralmost (E) zone overlap along lobules VI and VII. In addition, the E zone of the anterior lobe is the source of projections upon the medial, the descending, and the superior vestibular nuclei. Projections from the auricle and adjacent lateral unfoliated cortex (F zone) focus upon the infracerebellar nucleus, the medial tangential nucleus, and the medial division of the superior vestibular nucleus. The data suggest that the cerebellar cortex of the pigeon, like that of mammals, may be subdivided into a mediolaterally oriented series of longitudinal zones, with Purkinje cells in each zone projecting ipsilaterally to specific cerebellar nuclei or vestibular regions. For cortical regions exclusive of the auricle and lateral unfoliated cortex, three such zones (A, B, and C) are defined that are comparable in their efferent targets with the A, B, and C zones of mammals. There does not appear to be a D zone in the pigeon. The results are discussed in relation to comparative data on amphibians, reptiles, and mammals.