We know that as we age our hearing deteriorates. This decline in hearing can be picked up using standard hearing tests. But there are plenty of people out there who, although they perform typically on standard hearing tests, still struggle to hear in noisy environments. For example, have you ever struggled to hold a conversation in a busy restaurant? This is known as ‘hidden hearing loss’, and appears to be the first stage of age-related hearing loss. Fortunately, by studying changes within our inner ear, we are starting to understand what is happening in hidden hearing loss.
What’s happening in the cochlea to cause hidden hearing loss?
Our cochlea is part of the inner ear. When sound enters the cochlea it is detected by inner hair cells. These are highly specialised sensory cells that convert sounds into electrical signals. The inner hair cells are connected to sensory nerve cells, which receive the electrical signals and carry them to the brain, where they are then perceived. Within the last 10 years, scientists have found that the connection between the inner hair cells and the sensory nerve cells can become damaged following loud noise exposure. This faulty connection has been suggested to be one of the causes of hidden hearing loss. Many people are now working on ways to repair these connections, but as with all biology, it may not be this simple.
Another player in the game…motor nerve cells
In addition to the sensory nerve cells, which carry information from the cochlea to the brain, there are also motor nerve cells (known as efferent fibres). These feed information from the brain back to the cochlea. The efferent fibres communicate with a number of types of cell, including the same sensory nerve cells that receive signals from inner hair cells. The role of the efferent fibres is to protect the cochlea from noise-induced damage.
How do efferent fibres change during age-related hearing loss?
Recent work in mice (that are used as a model of early age-related hearing loss in people) has found that, over time, efferent fibres change which cells they communicate with. They stop communicating with sensory nerve cells and start communicating directly with inner hair cells. There are many questions about this phenomenon that need to be addressed, and my research will start to answer them.
Do efferent fibres really change the way they communicate?
The first and most important question is whether efferent fibres really change the way they communicate during age-related hearing loss. We need to ask this because this change may only be seen in the specific mouse model that was originally studied. This mouse model develops early onset age-related hearing loss because it has a mutation in a gene called cadherin-23. This gene produces a protein that is essential for inner hair cells to be able to detect sound. However, when the gene is faulty, the protein doesn’t work properly. It is possible that when this protein doesn’t work correctly, the efferent fibres could become rearranged in a way that is not normally seen in age-related hearing loss.
How can we study the rearrangement of efferent fibres?
Scientists have been able to repair the faulty cadherin-23 gene in the mice mentioned above. This has produced mice that develop hearing loss at older ages, which is more similar to what we normally see in people. I will be using these new, repaired mice to investigate whether their efferent fibres still become rearranged. As the mice will be otherwise identical to the original mice, any changes seen between them will be because of the faulty cadherin-23 gene. To study the rearrangement of efferent fibres, I’ll be using a technique called electrophysiology. This takes advantage of the fact that cells within our bodies generate very small electrical currents, just like the currents generated by electrical appliances, such as a TV. I will be measuring the electrical currents produced by inner hair cells in response to communication from the efferent fibres; if there is no electrical response from the inner hair cells, then I will know that the efferent fibres have not started communicating with them.
How will this improve our understanding of age-related hearing loss?
Looking at whether efferent fibres are rearranged in a better model of age-related hearing loss will lead us closer to understanding if this also happens in humans. If it does still happen in the repaired mice, then establishing whether it happens before or after inner hair cells and sensory nerve cells stop communicating will tell us whether it is the first sign of hidden hearing loss, or whether it is a result of other changes. This will help us to identify which types of cells and processes to target to develop the best treatments to prevent age-related hearing loss.