[Funicular structure and nerve fiber topography in the extratemporal facial nerve of the guinea pig].

The intratemporal portion of the facial nerve has a single bundle, while the extratemporal portion has multiple funiculi. In the guinea pig, the funiculi repeatedly divide and fuse between the stylomastoid foramen and the origin of the inferior labial branch. It remains unknown whether the nerve fibers in each funiculus are topographically organized or not. In this paper, the topographic organization of nerve fibers in a funiculus was clarified in the extratemporal facial nerve trunk of the guinea pig using an anterograde nerve degeneration technique. The author developed a technique in which one funiculus is cut selectively proximal to the origin of the cervical branch or peripheral to the origin of the inferior labial branch using microscissors under an operation microscope. Four to six days after sectioning, the main trunk just peripheral to the sectioned site was examined with toluidine blue staining to confirm that the resultant degeneration was limited to only one funiculus. Then all six main peripheral branches were sectioned and the distribution of degenerated nerve fibers in each branch was examined with toluidine blue staining. In six out of eleven nerves sectioned just proximal to the origin of the cervical branch, the degenerated nerve fibers were localized in one main peripheral branch. In the remaining five nerves, the degenerated nerve fibers spread into two or three main peripheral branches. In all nerves sectioned just peripheral to the origin of the inferior labial branch, the degenerated nerve fibers were localized in only one main peripheral branch, even though they distributed to several funiculi in the same branch.(ABSTRACT TRUNCATED AT 250 WORDS)