The long standing mystery of what triggers the brain to reduce sound amplification in response to a loud environment may have been solved this week in a study in mice published in Nature Communications. This protective reflex, which the research reports is controlled by Type II fibres in the ear, has previously been shown to be necessary for speech discrimination in loud environments, sound localization and protection from noise-induced hearing loss.
Sound is detected and amplified by thousands of tiny hair cells that reside in the fluid-filled cavities of the inner ear. In a loud environment, the brain sends a signal to the ears to turn down the amplifyer system. However, prior to this study, it was unknown how sound actually controlled this protective, sound-evoked reflex. Gary Housley and colleagues now report that transgenic mice lacking a specific type of nerve fibre in the ear exhibited a complete loss of the sound-evoked reflex. These Type II nerve fibres carry information from the ear to brain, yet their function has long been an enigma as they are small in number, representing 5% of auditory nerve fibres, and extremely difficult to study. The authors demonstrated that mice lacking Type II fibres had normal hearing thresholds but exhibited no suppression of the amplifyer system in response to loud sounds. While the long-term effects of missing Type II fibres remains to be determined, this study identifies their role in controlling the sound-evoked reflex in mice.