Morphology, Development, and Neurotrophic Regulation of Cochlear Afferent Innervation

Abstract

Spiral ganglion neurons (SGNs) are primary sensory neurons of the audi- tory system that send auditory information encoded by the inner ear to the central nervous system. The success of cochlear implant therapy is totally dependent on the status of SGN function. Therefore, information regarding the neurogenesis, sur- vival, and neurite growth of SGNs is important not only to understand the patho- physiology of sensorineural hearing loss but also to improve cochlear implant therapy. SGNs are anatomically and functionally divided into two subtypes, type I and type II. Type I SGNs connecting inner hair cells contribute to the transmission of sound information into the central auditory pathway, while type II SGNs con- necting outer hair cells are involved in active tuning of frequency in the cochlea. In the developing cochlea, the survival and neurite formation of SGNs are strongly regulated by neurotrophic factors, especially neurotrophin 3 (NT-3) and brain- derived neurotrophic factor (BDNF). Also, in the adult cochlea, the loss of hair cells induces secondary loss of SGNs presumably because of a loss of neurotrophic sup- port. When the deafened ear is treated with exogenous BDNF or NT3, there is a significant enhancement of SGN survival and resprouting of neurites. Therefore, chronic application of neurotrophic factors in the cochlea may improve the efficacy of cochlear implants.