Regeneration of the eight cranial nerve. I. Anatomic verification in the bullfrog.

The regenerative capacity of the eight cranial nerve of the bullfrog, Rana catesbeiana, was investigated. The nerve was sectioned between the ganglion and the brain stem in 16 experimental animals. Horseradish peroxidase was extracellularly injected distal to the site of the nerve section five or more weeks postoperatively. Labeled fibers were identified histologically and their central nervous system projections studied. The findings were as follows: 1. The eight cranial nerve of the bullfrog regenerated after preganglionic axotomy. 2. Fibers of all sizes regenerated. The regenerated fibers in the nerve were disorderly at the site of nerve section. Most fibers entered the brain stem and formed a vestibuloauditory tract, although some "escaped" and coursed along the brain stem. 3. The regenerated vestibuloauditory tract was situated more laterally than normal, with thick and thin fibers intermixed. 4. Regenerated tertiary fibers were long and tortuous and had unusual trajectories. 5. All vestibuloauditory nuclei received thick and thin afferent fibers.