Projections of the Vestibular Nuclear Complex in the Bullfrog (Rana catesbeiana)

Abstract

Electrolytic lesions were made in the vestibular nuclear complex (VNC) in 25 adult bullfrogs. Following postoperative survival times of 4–35 days, the frogs were perfused with 10% formol-saline. The brains were embedded in egg yolk and frozen sections cut at 33 µm. Sections were processed according to various modifications of Nauta and Fink-Heimer techniques and the degenerating fibers traced to their termination.The VNC distributes to (1) some of the cranial nerve nuclei, (2) the cerebellum, (3) the contralateral VNC, and (4) the spinal cord. Fibers from the VNC pass ventromedially to enter the medial longitudinal fasciculus (MLF), and eventually reach the nuclei of the extraocular muscles. Fibers to the cerebellum pass rostrally from the VNC to terminate within the molecular and granular layers. The projection to the contralateral VNC passes toward the midline, and then turns sharply toward the ventral surface, where it continues its course to the contralateral VNC. The fibers destined for the spinal cord appear to terminate in the medial parts of the ventral horn.