Noise-induced hearing loss link to 'touch-sensing' nerve cells

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Over-exposure to noise can cause lasting changes to hearing and lead to tinnitus, according to new research from the University of Michigan in the US.

Millions of people worldwide are afflicted by incurable tinnitus, ranging from intermittent and slightly annoying to chronic, severe and debilitating. While the condition is most usually related to noise-induced hearing loss, tinnitus can also be caused by head and neck injuries or dental work.

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The UoM study found that somatosensory (or 'touch-sensing') neurons in the neck and face are highly active after exposure to loud noise, even after actual hearing returns to normal.

Although the study was conducted on guinea pigs, the findings have important implications for people with tinnitus, said the senior researcher Dr Susan E. Shore.

"The animals that developed tinnitus after a temporary loss in their hearing after loud noise exposure were the ones who had sustained increases in activity in these neural pathways," said Dr Shore. "In the future it may be possible to treat tinnitus patients by dampening the hyperactivity by reprogramming these auditory-touch circuits in the brain."

In normal hearing, an area of the brain called the dorsal cochlear nucleus (DCN) is where signals first arrive via the auditory nerve from the ear. However, this region is also where neurons process touch and other sensory signals, along with hearing information.

According to earlier research by Dr Shore, other sensory signals entering the DCN are amplified during loss of hearing. This overcompensation by the somatosensory neurons, which carry information relating to vibration, touch, pain and skin temperature, is believed in many cases to cause tinnitus.

Dr Shore noted that touch-sensing nerves in the head and neck explain why some tinnitus sufferers can alter the pitch and volume of the sound by clenching their jaw or moving their head and neck.

"This is the first research to show that, in the animals that developed tinnitus after hearing returned to normal, increased excitation from the somatosensory nerves in the head and neck continued," said Dr Shore. "This dovetails with our previous research, which suggests this somatosensory excitation is a major component of tinnitus.

"The better we understand the underlying causes of tinnitus, the better we'll be able to develop new treatments."


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