Loud noise can do permanent damage to our hearing without any other warning signs other than occasional ringing in the ears. Previous hearing loss research has focused on the link between hearing loss and the loss of hair cells in the inner ear. For decades, the hair cells have been considered the most vulnerable elements in the ear.
However, a study has analysed the mechanism behind noise-induced hearing loss and found that the nerve fibres in the inner ear are even more vulnerable to damage. The researchers hope to use this knowledge to explore potential therapies that can re-establish the damaged connections between the nerves and hair cells.
Hair cells and synapses
A research group from Harvard Medical School in the U.S. has investigated noise-induced hearing loss in three mammalian ears: mice, guinea pigs and chinchillas.
Taking a closer look at a hair cell, the cell is connected to the cochlear nerve terminal by its synapses. A normal hair cell has roughly 20 synapses, in other words 20 connections to the cochlear nerve.
By using confocal microscopes, the researchers discovered no loss of hair cells in ears exposed to noise, but a striking loss of synapses, a part of the neurons.
“Each missing synapse represents a cochlear nerve fibre that has been disconnected due to retraction. It no longer responds to sound” explained Charles Liberman, lead researcher and Professor of Otology and Laryngology at Harvard Medical School.
The findings suggest that every time we go to a loud concert or use tools without ear protection, we might be losing synapses or connections to the cochlear nerve, increasing the degree of hearing loss.
According to the researchers, the current guidelines toward noise exposure are not adequate, since they are based on the assumption that noise-exposure causes only transient threshold elevation.
The researchers hope that the damage done to the synapses may possibly be reversible, leading to potential cure of noise-induced hearing loss in the future.
The study was conducted by researcher from Massachusetts Eye and Ear Infirmary's (MEEI) Eaton Peabody Laboratory at Harvard Medical School in the U.S.