For optimal hearing, hair cells located in the inner ear and the spiral ganglion neurons must be correctly connected to each other, since they are the first auditory nerve relays to the central nervous system. If they're not correctly connected, the transduction of sound to the brain is affected, and worse, if no connection is made between these two types of cells, there is no hearing at all. But there are two types of hair cells and spiral ganglion neurons and they cannot come together at random:  the inner hair cells connect to type I neurons while the outer hair cells connect to type II neurons. A team of researchers at the University of Liège's GIGA-Neurosciences research unit, led by Brigitte Malgrange, has established that outer hair cells emit a message that repels type I neuron projections to ensure they reach their targets, that is the inner hair cells! This major discovery, which could contribute to hearing restoration, was published in Nature Communications.

For most us, hearing is as natural as breathing. Unconsciously, each second of our lives, sounds all around us are sent to the brain and give us information about our environment, allowing us to react, to communicate, or even to relax. As simple and natural as hearing may seem, it can be faulty. According to statistics from the FFSB (French-Speaking Federation of the Deaf in Belgium), around 400,000 people in Belgium are deaf or hearing-impaired, and 40,000 of them are profoundly deaf. In Europe there are 15 million deaf and hearing-impaired people. ONE (Office of Birth and Childhood) estimates that profound hearing loss is detected in one in 1,000 newborns during their first year, and that an additional one in 1,000 newborns are diagnosed during their second year.

At the University of Liège's GIGA-neurosciences research unit, led by Brigitte Malgrange, researchers are trying to understand the development of the auditory portion of the internal ear. "We specifically want to understand why the inner ear cells don't regenerate after having been destroyed," says the FNRS Research Director. (see article "The mysteries of the broken ear").   

How sounds travel through the central nervous system

After entering through the auricle and passing the ossicles in the middle ear, sound is transmitted to the auditory portion of the inner ear. "This part is called the Organ of Corti and contains the hair cells among other things. The stereocilia on the surface of these cells vibrate when sound reaches them. This allows potassium and calcium to enter the hair cells and this modification creates a nerve signal," explains Brigitte Malgrange. This signal is then transmitted to the neurons of the spiral ganglion, also located in the inner ear. "These neurons are essential because they form the first auditory nerve relay to the central nervous system," states Brigitte Malgrange. For optimal hearing, the hair cells and the spiral ganglion neurons must be correctly connected to one another. If they're not, the transduction of sound to the brain is affected, and worse, if there is no connection between these two types of cells, there is no hearing at all.