Auditory nerve terminals in the cochlear nucleus magnocellularis: differences between low and high frequencies.

Primary auditory nerve fibers were labelled in the barn owl by localized horseradish peroxidase (HRP) injections into the cochlear nucleus angularis. They were followed to their terminal sites in the hearing organ (basilar papilla), confirming that they were auditory, and to the cochlear nucleus magnocellularis. The terminal sites of low-frequency fibers within nucleus magnocellularis always included an area previously described as the lagenar part, i.e., an area receiving primary input which is probably only vestibular. Furthermore, a number of differences were recognized between these low-frequency (up to 0.64 kHz) and the high-frequency (1.8 kHz and above) auditory nerve projections to nucleus magnocellularis. Most importantly, the collaterals given off by low-frequency fibers into the nucleus typically showed multiple terminal branching, with both en passant and terminal bouton-like swellings. High-frequency fiber collaterals, in contrast, terminated unbranched in a single endbulb of Held. Nucleus magnocellularis is the first station in a brainstem auditory pathway processing stimulus timing information, coded through neuronal phase locking. The prominent difference in terminal shape found between its low- and high-frequency input fibers is interpreted as reflecting different requirements of the absolute temporal precision for significant phase locking. Terminals in the shape of endbulbs of Held are probably a specialization to improve the temporal precision of synaptic transmission, allowing phase locking to higher frequencies.