|Mature hair cells, left, lose connections to outgoing neurons (blue) and gain connections to incoming neurons (red) as they age, right.--Courtesy of Paul Fuchs, Johns Hopkins Medicine|
New research published in The Journal of Neuroscience by a John Hopkins team led by Paul Fuchs points to a new therapeutic approach to tackling a causal mechanism for age-related hearing loss.
Hair cells in the inner ear convert sound waves to electrical signals which are conveyed to the brain through networks of nerve cells. It's known that with aging, hair cells found in the outer tier of the inner ear that process sound waves die off, which contributes to hearing loss. Or at least that's the conventional dogma among scientists in the field.
Challenging this long-held view, Fuchs and colleagues instead focused on an increased number of connections within the inner ear of aging mice, between a certain type of sensory and nerve cell. These types of connections are important, as they protect against exposure to loud sounds by damping down the sensory input. Fuchs is confident these connections are responsible for hearing loss.
"The nerve cells that connect to the sensory cells are known to inhibit hearing," notes Fuchs, "and although it's not yet clear whether that's their function in older mice, it's quite likely."
A graduate student, Stephen Zachary, recorded electrical signals from within the inner hair cells of young and old mice. He showed that incoming nerve cells were active and their relative activity corresponded with the hearing ability of the animal.
Fuchs comments: "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 ... 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 cells have retracted."
If these findings translate in humans then there could be a therapeutic way to inhibit these new connections--improving hearing in aging people afflicted with hearing loss, or indeed trauma.
- here's the release