Cerebellar corticonuclear projections in the red-eared turtle Pseudemys scripta elegans.

In the present study the organization of the corticonuclear projections of the cerebellum in the red-eared turtle Pseudemys scripta elegans was investigated. To find out whether a zonal pattern exists in the cerebellar cortex, a topological analysis was made of the Purkinje cell layer of a Nissl-stained series of the cerebellum. This analysis showed a distinct, longitudinally oriented zonal pattern of Purkinje cells. In each cerebellar half a medial, an intermediate, and a lateral zone could be distinguished. This pattern appeared to correspond in part to an earlier subdivision of the reptilian cerebellum suggested by Larsell ('26, '32, '67). In the experimental part of this study, the corticonuclear projections to the cerebellar nuclei and the vestibular nuclear complex were demonstrated with the horseradish peroxidase (HRP) technique. All projections of the Purkinje cells appeared to be strictly ipsilateral. After HRP slow-release gel implantations at various levels of the vestibular nuclear complex, longitudinally oriented areas of labeled Purkinje cells were found in the lateral part of the medial zone, the intermediate zone, and the lateral zone. Two strips of Purkinje cells remained unlabeled: One in the rostrolateral part of the intermediate zone, and another in the medial part of the medial zone. After HRP gel implantations in the cerebellar peduncle aimed at the medial and lateral cerebellar nuclei, respectively. Purkinje cells in the latter two strips were labeled. The present results indicate that the corticonuclear projections to the medial cerebellar nucleus arise in the medial part of the medial zone and those to the lateral cerebellar nucleus in the rostrolateral part of the intermediate zone. It can be concluded that cerebellar zones as demonstrated in the turtle Pseudemys scripta elegans, are phylogenetically very old and may represent the basic functional circuit of the cerebellum.