Neuroanatomical aspects of the gerbil inner ear: light microscope observations.

A specialization of the Mongolian gerbil inner ear allows easy access for electrophysiological recording from the auditory nerve and other structures of the internal auditory meatus. The round-window membrane is recessed, creating a cavity connecting the bulla and the external wall of the modiolus. This cavity has been named the round-window antrum. A small opening made in the dorsomedial wall of the round-window antrum gives access directly to the modiolus and internal auditory meatus, with no other openings in the cranium required. This surgery is less drastic than that needed for similar access in most other mammals. The anatomy of the specialized region is presented from studies with the light microscope. Emphasis is placed on the relationship between external features of the wall of the round-window antrum and the internal anatomy of the modiolus and internal auditory meatus. In addition, these studies reveal that cell bodies of the acoustic-nerve nucleus are located in the auditory nerve, central to the glial dome. About 22 +/- 8 cells are found in an auditory nerve. Their appearance with a Nissl stain is similar to cells found in rat, mouse, and man. The cell dendrites are preferentially oriented perpendicular to the longitudinal course of the auditory-nerve fibers. The anatomy of the gerbil auditory system should allow direct electrophysiological recording from these cells.