Hearing Loss: Sensory Cells In Mice May Explain Age-Related Hearing Loss

New findings published in The Journal of Neuroscience reveal how studies in mice have verified an increased number of connections between certain sensory and nerve cells in the inner ear of aging creatures. As these connections normally tamp down hearing when an animal is exposed to loud sound, the researchers said they believe that the connections may also be contributing to age-related hearing loss in mice and even humans.

"The nerve cells that connect to the sensory cells of the inner ear are known to inhibit hearing, and although it's not yet clear whether that's their function in older mice, it's quite likely," said Paul Fuchs, Ph.D., the John E. Bordley Professor of Otolaryngology-Head and Neck Surgery at the Johns Hopkins University School of Medicine, in a news release. "If confirmed, our findings give us new ideas for how physicians may someday treat or prevent age-related hearing loss."

Though conventional wisdom once blamed age-related hearing loss to the death of certain sensory hair cells found in the inner ear, new findings suggest that nerve cells could indeed be responsible. Yet this new research builds on knowledge that inside the ear lies a coiled row of sensory cells to the brain that process and tell animals what they actually "hear."

Two of these sets are so-called hair waves and exist based on inner tier closest to the brain to the outer tier. The outer ones have a secondary function that amplifies the sound waves within the inner, not surprisingly Fuchs notes, to a loss of outer hair cells that closely correlate with a loss of hearing.

Yet previous studies have also shown that within the last decade, changes relating to certain connections between hair cells and nerve cells to which they are attached may form.

According to Fuchs, each of the nerve cells functions as a one-way street, taking signals either from the ear to the brain or vice versa. The nerve cells take signals to the ear to turn down amplification that's provided by outer hair cells when an animal is exposed to a noisy environment over an extended period of time.

Furthermore, previous researchers has shown that age, inner hair cells in mice and even humans experience a decrease in outgoing nerve cell connections, with incoming nerve cells connections increase.

To discoverer if the new connections worked out normally, researchers recorded electrical signals from within the inner hair cells of young and old mice.

Findings revealed that incoming nerve cells were active and that their activity levels correlated with the animals' hearing abilities. Furthermore, the harder of hearing an animal was, the higher the activity of its incoming nerve cells. 

"These nerve cell connections seem to be reverting back to the way they worked during early development before the animals' sense of hearing was operating," said Fuchs. "We don't know why the new connections form, but it might be as simple as a lack of competition for space once the outgoing nerve cells have retracted."

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