Central trajectories of type II (thin) fibers of the auditory nerve in cats.

This paper describes the central projections of thin fibers of the auditory nerve in cats. Both thin (type II) and thick (type I) fibers are labeled by extracellular injections of horseradish peroxidase (HRP) into the auditory nerve. Type I and almost all type II fibers bifurcate upon reaching the auditory nerve root of the cochlear nucleus. For a given bundle of auditory nerve fibers labeled by a discrete injection of HRP, bifurcations of type II and type I fibers are restricted to a narrow region of the nerve root. After the bifurcation, the pathways of type II branches within the anteroventral cochlear nucleus (AVCN) and posteroventral cochlear nucleus (PVCN) are similar to those of type I branches. This similarity in bifurcation and course of type I and type II fibers was observed in the ventral as well as dorsal parts of the ventral cochlear nucleus. The complete axonal course of most type II fibers could not be reconstructed, however, due to fading of the reaction product. Type II fibers produce very few collaterals in the cochlear nucleus (CN), but possess many 'en passant' swellings along their main processes and collaterals. Compared with type II fibers previously studied in mice (Berglund and Brown, 1989; 1994; Brown and Ledwith, 1990), cat type II fibers are similar in their general projections within the main body of the nucleus and in the frequency of 'en passant' swellings per length of fiber, but cat fibers have a higher percentage of 'complex' or pedunculated 'en passant' swellings.