Age-related Hearing Loss: Probable New Causal Factor Found
Age-related hearing loss may not be due entirely to the death of sensory hair cells in the inner ear, recent research on the workings of nerve cells at Johns Hopkins suggests.
It’s studies in mice, the Johns Hopkins team says, verified a higher number of connections between some sensory cells and nerve cells in the inner ear of aging mice.
Since these connections usually dampen hearing when an animal is exposed to loud sound, the scientists believe these newer connections may also be contributing to age-related hearing loss in the mice, and potentially in 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. If confirmed, our findings give us new ideas for how physicians may someday treat or prevent age-related hearing loss.”
This research builds on the knowledge that inside the ear there is a coiled row of sensory cells responsible for converting sound waves into electrical signals sent through nerve cells to the brain, which processes and tells animals what they hear.
Two sets of these so-called hair cells, named for the filaments that act like antennae picking up sound waves, exist.
There is an inner tier closest to the brain and an outer tier. The outer hair cells have a secondary function to amplify the sound waves within the inner ear.
Not surprisingly, Fuchs notes, a loss of outer hair cells closely correlates with a loss of hearing.
Studies over the last decade, however, have suggested that alterations over time also occur in the connections between hair cells and the nerve cells to which they are attached.
Each of those nerve cells is like a one-way street, Fuchs explains, receiving signals either from the ear to the brain or vice versa. The nerve cells taking signals to the ear are known to turn down the amplification provided by outer hair cells when an animal is, for example, exposed to a noisy environment for an extended period of time.
Earlier research suggested that inner hair cells in mice and humans experience, with age, a decrease in outgoing nerve cell connections, while incoming nerve cell connections increase.
To determine if the new connections worked normally, Stephen Zachary, grad student in Fuchs’ lab, recorded electrical signals from within the inner hair cells of young and old mice.
He found that the incoming nerve cells were active. Activity levels also correlated with the animals’ hearing abilities. 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,” says 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.”