It's estimated that there are 100 billion neurons in the human body, so it's no surprise that some of them have evaded detection by scientists. Now, a team of Swedish researchers have discovered three new types of neurons, which help carry auditory signals from the ear to the brain and may hold the secrets to treating hearing disorders like tinnitus.

Technically, these neurons have been known to science for a long time, but the finer nuances of their function have been mostly overlooked. What researchers from Karolinska Institutet have done in the new study is break down groups of these nerve cells into more accurate sub-classes, after realizing they had different roles. In that way, it's kind of like scientists dividing one species of animal into two on closer examination of differences between individuals.

Nerve cells in the cochlea are responsible for translating sound waves into electrical signals, which are then carried to the brain. It's long been thought that these cells generally fall into two categories dubbed type 1 and type 2 neurons, with the former handling most of the auditory information.

But the new study has identified no less than four types of neurons in the peripheral auditory system. Type 2 remains unchanged, but type 1, according to the team, actually comprises three different kinds of cells.

"We now know that there are three different routes into the central auditory system, instead of just one," says François Lallemend, lead researcher on the study. "This makes us better placed to understand the part played by the different neurons in hearing. We've also mapped out which genes are active in the individual cell types."

These new neurons appear to be involved in decoding the sonic intensity – in other words, the volume – of what we're hearing. That might make them responsible for the "cocktail party effect," where the brain is able to filter out loud background noise to focus on the voice of a friend. On the flipside, they might also play a part in developing hearing disorders like tinnitus and hyperacusis, which is an oversensitivity to sound. But, as the researchers say, identifying this might open up new ways to treat these conditions.

"Our study can open the way for the development of genetic tools that can be used for new treatments for different kinds of hearing disorders, such as tinnitus," says Lallemend. "Once we know which neurons cause hyperacusis we'll be able to start investigating new therapies to protect or repair them. The next step is to show what effect these individual nerve cells have on the auditory system, which can lead to the development of better auditory aids such as cochlear implants."

The team discovered the neurons in mice, using single-cell RNA sequencing. This technique allows researchers to understand the functions of individual cells by looking at which genes they express. The study also showed that these different neurons are all present at birth – at least in mice.

The research was published in the journal Nature Communications.

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