Our ears have built-in secret "bodyguards" that protect them from extremely loud, potentially damaging noise, researchers say.
A newly detected connection between the inner ear and the brain can warn of strong incoming noise that can cause tissue damage leading to hearing loss, researchers at Northwestern University found.
A unique pain system found only in the ear tells us to jam our fingers in our ears when that dangerously loud siren sound of an ambulance or fire engine passes us by, they explain.
The ears need that special system because the kind of nerves that usually alert us to pain, when we've touched a hot burner on a stove for example, are not present in our inner ears, they've found.
Instead, there's a pathway researchers have dubbed the auditory nociception pathway, separate from the pathway for transferring information about sound from the ear to the brain.
This pain pathway contains just a single kind of neuron, one that is activated only by levels of noise that approach uncomfortable or dangerous intensities, the researchers report in Nature Reviews Neuroscience.
"It's very important for your system to have protection from damaging sound," says study author Jaime García-Añoveros, an anesthesiology professor at the university's Feinberg School of Medicine. "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."
Because we are exposed to noise throughout our lives and are living longer, hearing loss is the most common degenerative condition in humans, he says.
The discovery of the special "bodyguard" pathway offers a new way of considering painful and intractable hearing conditions including hyperacusis, an oversensitivity to everyday sounds often seen in soldiers exposed to explosives in the military, and tinnitus, a persistent and uncomfortable ringing in the ears, the researchers say.
"We do not know how to treat these debilitating conditions, and understanding what neuronal pathway might be involved is essential, " says García-Añoveros. "If we find they are actually pain syndromes rather than hearing syndromes, perhaps they could be treated effectively with analgesic pain medication that acts on the brain."
The researchers say the ear's special pain system may be triggering a defensive autonomic reflex of stiffening the inner ear muscles to reduce the level of sound entering the ear, or at the very least sending pain messages to the brain that prompt us to take protective measures such as the good old "fingers in the ear" response to loud noises.