EVIDENCE THAT INNER HAIR-CELLS ARE THE MAJOR SOURCE OF COCHLEAR SUMMATING POTENTIALS

The role of the inner hair cells (IHCs) in generating the cochlear sum mating potentials (SP) was assessed by measuring SP, cochlear nerve ac tion potentials (CAP), cochlear microphonics (CM) and 2f(1)-f(2) disto rtion product otoacoustic emissions (DPOAEs) in 15 chinchillas with ei ther acute chemical de-afferentation, accomplished by applying kainic acid to the round window, or surgical de-afferentation and basal IHC l oss, which developed within two months after sectioning the auditory n erve. In the auditory nerve sectioned ears, type I ganglion cells disa ppeared whereas most, if not all, type II ganglion cells were still pr esent. Histological analysis of surface preparations and sections thro ugh the modiolus verified the de-afferentation in both models and show ed a large IHC loss at the base of the cochlea in the ears with the au ditory nerve sectioned while other structures of the cochlea remained intact. Unoperated (left) ears of 9 animals served as controls. CM and DPOAEs were normal in all ears whereas the CAP was substantially depr essed in de-afferented ears. Comparisons among the SP input-output fun ctions suggest that (1) the IHCs are the major generator of SP recorde d from the round window in chinchilla, in particular at low to moderat e stimulus levels, (2) the SP recorded from the round window largely r eflects the responses from hair cells al the base of the cochlea, and (3) kainic acid results in an increase of SP amplitude to high-level s timuli whereas the SP to low-to moderate-level stimuli remains in the normal range.