The ear is the sensory organ dedicated to 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 relayed to the brain via the ear’s nerve cells in the cochlea. Previously, most of these cells were described as 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 states 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 have shown 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 the type 1 cells actually comprise three very different cell types, with neuronal type already established at birth, indicating 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 distinguished via genetic profiling.  Results provide a catalog of the genes expressed in the nerve cells, proffering the global medical community a solid foundation for a better understanding of the auditory system.

The team hypothesizes these three neuronal types may play a part in the decoding of sonic intensity or volume, a crucial function during conversations in a loud environment with the added attribute to filter out background noise. They go on to add this property is also important in different forms of hearing disorders, such as tinnitus or hyperacusis.

The team surmises 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.  For the future, the researchers state the next step is to show what effect these individual nerve cells have on the auditory system to provide a stepping stone to next-generation auditory aids such as cochlear implants.

Source: Karolinska Institutet

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