The Journal of Neuroscience, September 1, 2000, 20(17):6684-6693
Centrifugal Pathways Protect Hearing Sensitivity at the
Cochlea in Noisy Environments That Exacerbate the Damage
Induced by Loud Sound
Ramesh
Rajan
Department of Physiology, Monash University, Monash, Victoria 3800, Australia
Loud sounds damage the cochlea, the auditory receptor organ,
reducing hearing sensitivity. Previous studies demonstrate that the
centrifugal olivocochlear pathways can moderately reduce these temporary threshold shifts (TTSs), protecting the cochlea. This effect involves only the olivocochlear pathway component known as the
crossed medial olivocochlear system pathway, originating from
the contralateral brainstem and terminating on outer hair cells in the
cochlea. Here I demonstrate that even moderate noise backgrounds can
significantly exacerbate the cochlear TTSs induced by loud tones, but
this is prevented because in such conditions there is additional
activation of uncrossed olivocochlear pathways, enhancing protection of
cochlear hearing sensitivity. Activation of the uncrossed pathways
differs from that of the crossed pathway in that it is achieved only in
noise backgrounds but can then be obtained under monaural conditions of
loud tone and background noise. In contrast, activation of the crossed
pathway is achieved only by binaural loud tones and is not further
enhanced by background noise. Thus, conjoint activation of both crossed
and uncrossed efferent pathways can occur in noise backgrounds to
powerfully protect the cochlea under conditions similar to those
encountered naturally by humans.
Key words:
cochlea; olivocochlear efferents; loud sounds; hearing
damage; noise backgrounds; protection; TTS
Copyright © 2000 Society for Neuroscience 0270-6474/00/20176684-10$05.00/0
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