A bodyguard for your ears

Our hearing has a secret bodyguard, a newly discovered connection from the cochlea to the brain that warns of intense incoming noise that causes tissue damage and hearing loss, according to new research by Northwestern Medicine scientists.

Scientists believe they have discovered the ear's own novel pain system that protects it from very loud or damaging noise. It may be the reason you jam your fingers in your ears when a fire engine or ambulance wails close by. The nerves that normally alert you to pain - like touching a hot burner on a stove - are not present in your inner ear. So, it needs its own private alert system.

The discovery may provide insight into the cause and treatment for such painful hearing conditions as hyperacusis, an oversensitivity and earache in response to everyday sounds, common in soldiers exposed to explosives in the military, and tinnitus, a persistent and uncomfortable ringing in the ears.

The pathway, which scientists named auditory nociception (pain), is different from the one that transfers information about sound to the brain and enables you to hear a bird singing or a friend gossiping. This pathway is populated by a single set of neurons activated only by noxious or dangerous levels of noise. Scientists aren't sure if the neurons are triggered by the death of hair cells (which detect normal level sound as part of hearing) or simply dangerous sound levels.

"It's very important for your system to have protection from damaging sound," said study senior author Jaime Garcia-Anoveros, associate professor of anesthesiology at Northwestern University Feinberg School of Medicine. "When sensory hair cells in the ear die, they are not repopulated. That's why hearing loss is irreversible. You need to be able to detect dangerous sound the way your nerve cells alert you to the danger of putting your hand on a hot iron."

Garcia-Anoveros also is an investigator at Northwestern's Knowles Hearing Center and a member of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.

The study will be highlighted Feb. 20 in Nature Reviews Neuroscience and was recently published in Current Biology. The research was conducted with mice, and Garcia-Anoveros believes the pathway has an equivalent in humans. Next, he wants to research the human pathway.

Hearing loss is the most common degenerative condition in humans, because we are exposed to noise throughout our lives and are living longer, Garcia-Anoveros said.

The discovery offers an entirely new way of looking at the painful and intractable hearing conditions hyperacusis and tinnitus.

Ear has its own pain system designed to protect it from noise that causes hearing loss

Ears don't have the nerve cells that normally detect pain