Researchers identify three previously unknown neuronal types in the inner ear.

The ear is the sensory organ of hearing and balance in mammals, and is usually described as having three parts, namely, the outer ear, middle ear and the inner ear.  When sound reaches the inner ear, it is converted into electrical signals which are relayed to the brain via the ear’s nerve cells in the cochlea. Previously, most of these cells were considered to be only type 1 and type 2 neurons, with the type 1 neurons transmitting most of the auditory information. Now, a study from researchers at Karolinska Institutet identifies four types of neurons in the peripheral auditory system, three of which are new to science.   The team state that the analysis of these cells can lead to new therapies for various kinds of hearing disorders, such as tinnitus and age-related hearing loss. The study is published in Nature Communications.

Previous studies show that spiral ganglion neurons of the cochlea convey all auditory waves to the brain, yet the cellular and molecular complexity necessary to decode the various acoustic features in the spiral ganglion is still unclear.  The current study shows that the type 1 cells actually comprise three very different cell types, with neuron type specification already established at birth, indicating is that neuronal programming is established by molecular processes independent of neuronal activity or sensory input.

The current study uses single-cell RNA sequencing in mice to identify four distinct types of adult spiral ganglion neurons, including three previously unknown subclasses of type I and type II neurons, which can be distinguished via genetic profiling.  Results provide a catalogue of the genes expressed in the nerve cells, which can give the global medical community a solid foundation for better understanding the auditory system.

The team hypothesize that these three neuron types probably play a part in the decoding of sonic intensity or volume, a function that is crucial during conversations in a loud environment, which rely on the ability to filter out background noise. They go on to add that this property is also important in different forms of hearing disorders, such as tinnitus or hyperacusis.

The team surmise that their data identifies four types of neurons, including three novel subclasses of type I and type II neurons in the spiral ganglion neurons of the cochlea, along with numerous new marker genes and provided a comprehensive genetic framework that may shape their synaptic communication patterns.  For the future, the researchers state that 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.

Source: Karolinska Institutet

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