Touch-sensing nerve cells step up their activity in the brain after hearing cells are damaged. Hyperactivity of these touch-sensing neurons likely plays an important role in tinnitus, often called "ringing in the ears."
People with tinnitus sense ringing or other sounds in their ears or head when there is no outside source. Whether it's mild and intermittent or chronic and severe. The condition commonly occurs with hearing loss, but also after head or neck trauma such as whiplash or dental work.
The animal-based study, led by Susan E. Shore, Ph.D., research professor in the Department of Otolaryngology, they suggest that available treatments such as acupuncture, if used to target nerves in the head and neck, may provide relief for some people plagued by tinnitus.
"In this study, we showed that when there is a hearing loss, other parts of the brain that normally convey signals to the cochlear nucleus have an enhanced effect," Shore said.
"When you take one source of excitation away, another source comes in to make up for it. The somatosensory system is coming in, but may overcompensate and help cause tinnitis," she said.
The somatosensory system is a nerve network in the body that provides information to the brain about touch, vibration, skin temperature and pain. The part of the system that provides sensations from the face and head, called the trigeminal system, brings signals to the cochlear nucleus that help us hear and speak.
But when people experience hearing loss or some other event, such as having a cavity filled or a tooth implanted, these neurons from the face and head can respond like overly helpful relatives in a family crisis. The resulting neuron firings in the cochlear nucleus, like too many phone calls, create the din of tinnitus, a "phantom sound" produced in the brain.
In the study, the researchers measured the patterns of activity of neurons in the brains of normal and deafened guinea pigs. They used a 16-electrode array to measure signals from the trigeminal nerve, which is responsible for sensation in the face, and multisensory neurons in the dorsal cochlear nucleus. When they compared results in the two groups, they found clear differences in trigeminal nerve activity.
"The study shows that in deafened animals, the somatosensory response is much stronger than in animals with normal hearing. This study shows that it is only those neurons that receive somatosensory input that become hyperactive," she said.
The study is published in the European Journal of Neuroscience.