Supporting cells contribute to control of hearing sensitivity

The mammalian hearing organ, the organ of Corti, was studied in an in vitro preparation of the guinea pig temporal bone. As in vivo, the hearing organ responded with an electrical potential, the cochlear microphonic potential , when stimulated with a test tone. After exposure to intense sound, the re sponse to the lest tone was reduced. The electrical response either recover ed within 10-20 min or remained permanently reduced, thus corresponding to a temporary or sustained loss of sensitivity. Using laser scanning confocal microscopy, stimulus-induced changes of the c ellular structure of the hearing organ were simultaneously studied. The cel ls in the organ were labeled with two fluorescent probes, a membrane dye an d a cytoplasm dye, showing enzymatic activity in living cells. Confocal mic roscopy images were collected and compared before and after intense sound e xposure. The results were as follows. (1) The organ of Corti could be divid ed into two different structural entities in terms of their susceptibility to damage: an inner, structurally stable region comprised of the inner hair cell with its supporting cells and the inner and outer pillar cells; and a n outer region that exhibited dynamic structural changes and consisted of t he outer hair cells and the third Deiters' cell with its attached Hensen's cells. (2) Exposure to intense sound caused the Deiters' cells and Hensen's cells to move in toward the center of the cochlear turn. (3) This event co incided with a reduced sensitivity to the test tone (i.e., reduced cochlear microphonic potential). (4) The displacement and sensitivity loss could be reversible. It is concluded that these observations have relevance for und erstanding the mechanisms behind hearing loss after noise exposure and that the supporting cells take an active part in protection against trauma duri ng high-intensity sound exposure.