Adaptive mechanism protects the ear from loud noise
THE temporary hearing loss from an outing at a noisy nightclub may not indicate damage to our ears as traditionally thought, new research shows.
University of Auckland scientists and colleagues overseas have found that the temporary dulling of hearing after exposure to the blast of loud music or a racket of machinery is a normal adaptive process of the inner ear.
This is where the structure called the cochlea converts sound energy to electrical nerve impulses that go to the brain.
"We demonstrate that what we traditionally regard as a temporary hearing loss from noise exposure is in fact the cochlea of the inner ear adapting to the noisy environment, turning itself down in order to be able to detect new signals that appear in the noise," says the university's Professor Peter Thorne.
"... what we have always thought was temporary noise damage - that is, the temporary hearing loss experienced in nightclubs or a day's work in factories - may not be this, but instead is the ear regulating its sensitivity in background noise."
But this is not a licence to crank up the iPod or ditch the earmuffs with the jackhammer, he warns.
"If we exceed a safe dose of noise, our ears can still be damaged permanently despite this apparent protective mechanism," Professor Thorne says.
The researchers, whose findings from a 20-year collaboration are published in the Proceedings of the National Academy of Sciences, say the temporary noise-induced hearing loss is due to a molecular signalling pathway in the cochlea which is mediated by a chemical compound called ATP.
This is released by cochlear tissue during noise and activates specific ATP receptors.
If this signalling pathway is removed, such as by genetic manipulation, the adaptive mechanism does not occur and the ear becomes very vulnerable to longer-term noise exposure and age-related wear and tear, eventually leading to permanent deafness.
This shows the adaptive mechanism also protects the ear, Professor Thorne says.
In a second study, the scientists found a new genetic cause of deafness involving the same mechanism.
Two families in China who had a mutation in the ATP receptor experienced rapidly progressive hearing loss which accelerated if they worked in noisy environments.
"This work is important because it shows that our ears naturally adapt to their environment, a bit like pupils of the eye which dilate or constrict with light, but over a longer time course," Professor Thorne says.
If people do not have the genes that produce the protection, they are more susceptible to deafness.
"This may go some way to explaining why some people are very vulnerable to noise or develop hearing loss with age and others don't."