We've probably all had cause to use the phrase "painful to listen to", but now scientists in the US have discovered why. Writing in Nature, Johns Hopkins researcher Paul Fuchs and his colleagues show that a small group of nerve cells in the ear respond to sounds that would best be described as "traumatic". The ear converts sound waves into brain waves using a specialised structure called the cochlea. Here, vibrations of the eardrum are transferred to a group of cells called hair cells. The information from these hair cells is then transmitted to the brain by a family of nerve cells. 95% of these nerves are known as type I fibres and respond very strongly to even the quietest sounds. But the other 5%, which are known as type II neurones, have remained something of an enigma, because, in previous attempts to study them, they didn't seem to respond to sounds at all.
Now, the Johns Hopkins team have shed some light on the mystery by managing to record, in young rats, the electrical activity from inside some of these unusual nerve cells. They also labelled some of the cells with a dye so that they could study their structure and how they were wired up to the cochlea. What this has revealed is that the cells are activated by an excitatory nerve transmitter chemical, called glutamate, which is released from the sound-sensing hair cells. But the recordings suggest that the sounds have to be very loud indeed to trigger any response, meaning that the cells might be there to help to process and discriminate different types of very loud sound. The team also found that the nerve fibres respond to another nerve chemical called ATP, which is often associated with tissue damage, so they could also be providing the brain with a way to monitor the health and function of the of auditory system. This means that they might help us to better understand and even treat hearing problems like tinnitus, which causes sufferers to experience distracting high-frequency buzzing noises.
According to Fuchs, "no one thought recording them was even possible," he said. "We knew the type II neurons were there are now at lest we know something about what they do and how they do it."