Central projections of auditory-nerve fibers of differing spontaneous rate. I. Anteroventral cochlear nucleus.

Auditory nerve fibers have been subdivided into three functional groups (Liberman, M.C. [1978] J. Acoust. Soc. Am. 63:442-455) differing in acoustic sensitivity and spontaneous discharge rate (SR). Using intracellular injection of horseradish peroxidase, the present study analyzes the projections of these three neuronal subclasses to the various subdivisions of the anteroventral cochlear nucleus (AVCN) and to the different cell types found therein. The average number of swellings and number of cells contacted decreased from low- to medium- to high-SR groups. However, these differences in terminal elaboration were not evenly distributed throughout the AVCN. The small cell cap was almost exclusively innervated by low- and medium-SR fibers, i.e., those with the highest acoustic thresholds. Within anterior AVCN, spherical-cell innervation was seen from all SR groups, whereas almost all multipolar cell innervation was from low- and medium-SR fibers. In the posterior AVCN, multipolar-cell innervation was equally likely from all SR groups, whereas globular cells were preferentially contacted by high-SR fibers. These SR-based trends in cochlear nucleus innervation help explain some of the known physiological properties of cell-types in each subdivision. They also suggest that additional physiological study of the small cell cap may be key in elucidating the functional significance of the low-SR population.