Recent advances in the fish auditory efferent system

The octavolateral efferent neurons in fish project axons to innervate the ear and lateral line organs. However, the function of the octavolateral efferent system is not well understood. This review focuses on recent progress in structure and function of octavolateral efferent system in a teleost, the sleeper goby (Dormitator latifrons). The sleeper goby has a single octavolateral efferent nucleus that is composed of about 19 large, densely packed somata in the medulla. Most efferent neurons have bilateral dendrites and single axons innervating ipsilateral peripheral octavolateral organs. The efferent neurons have distinctive characteristics of frequency, temporal, directional, and dynamic responses, which differ from those of otolithic organ afferents. How efferent stimulation affects afferent response is investigated by electrically stimulating efferent somata and recording compound action potentials from afferents innervating the saccule, the main auditory organ in this species. There are different efferent modulatory effects on afferent response, depending on the level of background noise. Efferent activation inhibits afferent response in quiet conditions but can enhance signal detection in noisy environments. Results of these studies indicate that the fundamental role in efferent modulation of afferent input is likely conserved in both nonmammalian vertebrates and mammals. [Work supported by NIH/NIDCD Grant R01DC003275.]