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New insight into aged-related hearing loss

Scientists at the University of Brighton have moved a step closer to understanding the underlying cause of age-related hearing loss which affects 11 million people in the UK.
Hearing loss is believed to originate in non-sensory cells in the cochlea, the auditory portion of the inner ear containing the organ which produces nerve impulses in response to sound.
The cells are coupled together by ‘gap junctions’ which are formed of two proteins called connexin 26 and connexin 30. It is mutations or failures in these proteins that cause most cases of hearing loss.
However, experiments by our Sensory Neuroscience Research Group have shown that one particular mutation in the connexion 30 protein actually prevents deafness to high-frequency sound.
Professor Ian Russell, Professor of Neurobiology here at university and a member of the group, said: “This was a great surprise: The mutation should have impaired the function of the cochlea, not aided it.”
He said: “Other members of the research team are now making direct measurements from these supporting cells to understand how the mutation changes the properties of the gap junctions. They should obtain measurements that will enable us to understand how the mutation alters the electrical and mechanical properties of the cochlea and eventually lead to our understanding how sensitivity is preserved in a cochlea that would otherwise be decimated by age-related-hearing-loss.”
The Sensory Neuroscience Research Group’s findings were published on 21 February in Nature Communication.
Professor Russell explained how the research has led to new insight into how we hear: “The convention is that sound energy is converted into electrical energy in the cochlea through modulation by sound of a flow of current through the sensory cells, which is provided by a battery in the cochlea.
“Modulation of this current flow causes voltage and mechanical changes in the sensory hair cells that are the basis for voltage-dependent amplification in the cochlea that provides it with exquisite sensitivity and frequency tuning.
“We discovered that the voltage of the cochlear battery is almost halved in the mutants, but voltages across the entire sensory cells of the cochlea remain unchanged.
“The significance of our findings is that cochlea amplification is driven not by voltages that are due to current flowing through the membranes of the sensory hair cells but by voltage changes that occur in the fluid spaces that surround them; an idea first put forward 20 years ago.
“This way of driving cochlear amplification provides a reason for why it is that amplification can work at incredibly high frequencies. Up to 20 kHz in humans and to more than 200 kHz in some bats and dolphins.”

Stephanie Thomson • February 22, 2017

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