Most of us are aware that exposure to very loud sounds (such as a chainsaw, a rock concert, or a jet engine) leads to permanent hearing loss (the noise damages the sensory hair cells in your inner ear).
Unfortunately, it's becoming clear that even moderately loud sounds (imagine a congested city street or a busy restaurant) can damage the inner ear. In these cases, it's not the hair cells themselves that are damaged, but rather the 'connections' they make with the nerves which carry information about sound to the brain. These connections, or synapses, are not easily repaired once broken, and their loss means that less information can be sent to the brain from the ear.
If hair cells are over-stimulated, for example, by exposure to moderate noise, they release large amounts of a chemical called glutamate. The hair cells normally use glutamate to signal to the nerve cells they connect to. But very high levels of glutamate can damage the nerve cells and destroy the synapses.
Each hair cell is connected to multiple nerve cells, all of which have a different role. Some transmit information about loud sounds to the brain, while others transmit information about quiet sounds. For some reason, it seems that the nerve cells which carry information about loud sounds are more sensitive to damage caused by too much glutamate.
So, following exposure to moderate noise, a person may still be able to hear a quiet sound (as the nerves carrying this information are unaffected), but their ability to listen to loud sounds could have been compromised. The nerves carrying information about loud sounds are thought to be important for picking out specific sounds, such as speech, when there’s a lot of background noise. This kind of hearing damage won't affect someone's 'thresholds' (the quietest sound someone can hear when there's no background noise) but it may affect how well they can listen in noisy backgrounds.
We want to find out more about how high levels of glutamate damage nerve cells in the inner ear. Dr Rutherford and his team are looking at the effect glutamate has on the 'protein machinery' in nerve cells that they use to receive and respond to signals from hair cells. The researchers are identifying differences between the different types of nerve cell, to try to understand why some are more sensitive to glutamate damage than others.
"Knowing more about how some nerve cells in the ear are more easily damaged than others could help us understand more about 'hidden hearing loss'."
Understanding how glutamate damages nerve cells in the inner ear and why it affects some more than others may lead to ways of preventing or reversing the damage. Understanding these differences will also provide basic information on how sound is processed in the inner ear, which could lead to improvements in the design and performance of medical devices to restore hearing, such as cochlear implants.