Researchers identify potential contributor to age-related hearing loss

Researchers at Johns Hopkins University School of Medicine have found that an increased number of connections between certain sensory cells and nerve cells in the inner ear may contribute to age-related hearing loss.

In a study published in The Journal of Neuroscience, researchers noted their findings on mice subjects challenge the conventional wisdom that dying sensory hair cells in the inner ear are almost entirely to blame for age-related hearing loss. Instead, they found an increased number of connections between certain sensory cells and nerve cells— which normally tamp down hearing when an animal is exposed to a loud sound— in the inner ear of aging mice. These new connections may contribute to age-related hearing loss in mice, and possibly humans.

“If confirmed, our findings give us new ideas for how physicians may someday treat or prevent age-related hearing loss,” Paul Fuchs, Ph.D., the John E. Bordley professor of otolaryngology—head and neck surgery at the Johns Hopkins University School of Medicine, said in a news release.

Sensory cells that lie inside the ear convert sound waves into electrical signals, which the nerve cells pick up and send to the brain, which tells animals what they “hear.” There is an inner and outer set of these sensory cells, called “hair cells” because of the filaments that act like antennae picking up sound waves. The inner tier is closer to the brain. The outer tier has a secondary function of amplifying the sound waves within the inner ear. Aging brings a loss of these outer cells, and is closely correlated with a loss of hearing.

The nerve cells act as a one-way street, either bringing signals from the brain to the ear, or releasing signals from the ear to the brain, Fuchs said. The incoming nerve cells that bring signals to the ear specifically lower the amplification when an animal is exposed to a noisy environment. Previous research suggested an age-related decrease of brain-to-ear, or outgoing, connections and an increase in ear-to-brain connections.

In their study, researchers recorded the electrical signals from within the inner hair cells of young and old mice to test the efficiency of the outgoing and incoming connections. They found that the incoming nerve cells were more active in animals that were harder of hearing. The more active those nerve cells, the more they turn down the amplification of the outer hair cells, and the harder it becomes to hear.

If the same phenomena is occurring in humans, researchers believe there may be ways to prevent those incoming nerve cells from forming connections with inner hair cells, which could prevent hearing loss during old age.

The paper was published July 1.