Researchers at Linköping University in Sweden have made several discoveries about the working mechanisms of inner hair cells in the ear, which convert sounds into nerve signals processed in the brain. The results, presented in the scientific journal Nature communications, call into question the current image of the anatomical organization and functioning of the auditory organ, which has prevailed for decades. A better understanding of how hair cells are stimulated by sound is important for issues such as the optimization of hearing aids and cochlear implants for people with hearing loss.
To hear sounds, we need to convert sound waves, which are compressions and decompressions of air, into electrical nerve signals that are transmitted to the brain. This conversion takes place in the part of the inner ear called the cochlea, due to its shape reminiscent of a snail shell. The cochlear canal houses the auditory organ, with numerous hair cells that are divided into outer and inner hair cells. The outer hair cells amplify sound vibrations, which allows us to hear faint sounds and better perceive the different frequencies of human speech. Inner hair cells convert sound vibrations into nerve signals. In the current study, the researchers investigated how the conversion takes place. It is still unknown how inner hair cells are stimulated by sound vibrations to produce nerve signals.
It has long been known that outer hair cells are connected to a membrane that rests on them. The outer hair cells have hair-like protrusions called stereocilia that are bent and activated when sound vibrates the membrane and the auditory organ. However, the current opinion is that the stereocilia of the inner hair cells are not in contact with this membrane, which is known as the tectorial membrane, and that they are stimulated by sounds by a completely different mechanism. This is the model that the new study challenges.
The relationship between hair cells and the tectorial membrane has been studied in detail by electron microscopy since the 1950s. But it is extremely difficult to study how this gelatinous membrane works, as it shrinks as soon as it is removed from the ear. . This makes it extremely difficult to preserve the relationship between the inner hair cells and the tector membrane. In addition, this membrane is transparent, and has therefore been essentially invisible. So far. LiU researchers noticed that the tectorial membrane reflected green light. This discovery made it possible to visualize the tectorial membrane under a microscope.
“We don’t see any space between the tectoral membrane and the hair cells. In contrast, the stereocilia of the outer and inner hair cells are completely integrated into the tectorial membrane. Our results are incompatible with the generally accepted idea that only the outer hair cells are in contact with the tectoral membrane, ”explains Pierre Hakizimana, senior research engineer in the Department of Biomedical and Clinical Sciences at Linköping University and lead author of the article.
Pierre Hakizimana and his colleagues studied the inner ear of guinea pigs, very similar to that of humans. When the researchers further investigated the relationship between the membrane and hair cells, they made another discovery.
“We found calcium channels with an appearance that we had never seen before. These calcium channels span the tectoral membrane and connect to the stereocilia of internal and external hair cells, ”explains Pierre Hakizimana.
The research group, led by Professor Anders Fridberger, has already discovered that the tectorial membrane functions as a reservoir of calcium ions, necessary for hair cells to convert vibrations evoked by sound into nerve signals. The researchers followed the movement of calcium ions through the ducts, and their results suggest that calcium ions flow through the ducts to the hair cells. This may explain how hair cells get the large amounts of calcium ions necessary for their function. The study also showed that the stereocilia on the inner and outer hair cells are folded by the tector membrane in a similar fashion. The next step in the research will be to understand in more detail how calcium ions are transported and to identify the protein (s) that make up the newly discovered calcium channels.
“Our results allow us to describe a mechanism of the functioning of hearing, incompatible with the model accepted for more than fifty years. Classic illustrations in textbooks showing the hearing organ and how it works need updating. The models used in research to study hearing should also be updated to incorporate these new discoveries, ”explains Pierre Hakizimana.
New information about how our hearing works may be important in the long term for the development of cochlear implants. These are hearing aids that are inserted into the cochlea and use electrical stimulation to enable children and adults with hearing loss to perceive sounds.
“Cochlear implants are an amazing solution to treating hearing loss, but they can be improved. It is important to better understand how internal hair cells are stimulated by sound to optimize the way in which cochlear implants stimulate the auditory nerve, ”explains Pierre Hakizimana.
The study received financial support from the Tysta Skolan Foundation, the Swedish Research Council and the National Institutes of Health in the United States.
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Material provided by Linköping University. Original written by Karin Söderlund Leifler. Note: Content can be changed for style and length.