Hearing is the result of a joint effort between the ear and the brain. The ear’s job is to capture, amplify and guide the sound into the cochlea, which is found in the inner ear. Once in the cochlea, the sound vibrations are transformed into an electrical “language” that the brain understands, by the sound-sensing cells, the hair cells. Bridging the connection between the hair cells and the brain are a group of nerve cells called spiral ganglion neurons. Loss of either hair cells or spiral ganglion neurons leads to hearing loss, which is caused by several factors, including aging and inherited deafness.
Researchers in Professor Lisa Goodrich’s laboratory at Harvard Medical School in the USA published their new findings in the prestigious scientific journal Cell, showing that 425 genes are activated at different levels in different spiral ganglion neurons. By looking at the different levels of gene activity, they were able to group the nerve cells into three distinct subtypes: Ia, Ib and Ic. Researchers already knew that spiral ganglion neurons can have different structures and functions from each other, but they did not know these differences could also be seen inside the cells at the level of gene activity. They also found that the spiral ganglion neuron subtypes developed and became established during the first week after birth in the mice they were studying.
Moreover, the researchers discovered that the proportions of the different subtypes of spiral ganglion neurons changed along the length of the snail-shaped cochlea; there are more nerve cells of subtype Ia and fewer nerve cells of subtype Ib at the bottom of the cochlea than at the top. This differing distribution of the subtypes along the cochlea may contribute to how the cochlea is able to separate sounds based on their pitch. Cells at the bottom of the cochlea detect sounds at higher pitches, while cells at the top detect sounds at lower pitches. This organisation is important for the brain to be able to perceive sounds correctly.
But what happens to the subtypes of spiral ganglion neurons during aging or deafness at birth?
The researchers looked at the proportion of the three subtypes of spiral ganglion neuron in the cochlea of mice over a longer time. They found that, although there was a loss of cells of all subtypes with aging, subtype Ic in particular seems to be more vulnerable to aging and was lost at a higher rate than the other subtypes as the mice got older.
The researchers also concluded that communication between hair cells and spiral ganglion neurons is essential for the three subtypes to form correctly. If this communication is compromised, the proportions of the different subtypes of spiral ganglion neuron change, and become unbalanced. When studying mice that are deaf from birth, the researchers saw that the differences in gene activity were lost and more of the cells were of subtype Ia; the other subtypes did not form correctly.
Inner ear of a mouse (image provided by Professor Ed Rubel, University of Washington)
Why is this work important?
Knowing that there are three different subtypes of spiral ganglion neuron and that hair cells are essential for their formation is important to help develop future treatments. This study raises important questions such as: will restoring hair cells in the inner ear also lead to recovery of the correct proportions of spiral ganglion neuron subtypes, and therefore good communication between the ear and the brain? The identification of these subtypes will also make researchers aware that future treatments need to re-establish these distinct subtypes in the correct proportions for proper hearing recovery.
The different vulnerability of the subtypes to aging is another essential piece of the puzzle of how aging contributes to hearing loss. The higher loss of subtype Ic with aging that the researchers saw in mice may be an important cause of the increasing difficulty that we have in listening in noisy environments while we age.
Increasing our knowledge of how the ear communicates with the brain to allow us to hear is essential to develop appropriate treatments to protect and restore hearing fully.
Find out more
The image in this blog post, a view of the inner ear of a mouse, was kindly provided by Professor Ed Rubel at the University of Washington. The image shows a single row of inner hair cells (green) and three rows of smaller outer hair cells (also green) spiral upwards from the base of the cochlea. Nerve fibres (red) may be seen radiating outwards from the centre and are brightly labelled at the base of the inner hair cells and extending outwards towards the outer hair cells.
This research was published earlier this year in the journal Cell – you can read the abstract here: https://www.ncbi.nlm.nih.gov/pubmed/30078709.
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