The human inner ear has a type of cell called the outer hair cell. These were thought to amplify sounds from the environment.
However, a new study based on hearing in gerbils shows these cells are probably involved in regulating the sensitivity of the ear to sound. This could help find ways to preserve good hearing by protecting these cells from destruction.
The ear has three zones, the outer, middle and inner ear. The inner ear contains hair cells which are tiny cells with protruding microscopic hair-like projections.
They work somewhat like microphones, changing sound-induced vibrations into electrical nerve impulses that the brain translates into the sensation of sound, and simultaneously amplifying the signals before sending them on.
In the current study, the researchers tried to resolve the question by using a technology called optical coherence tomography vibrometry (OCTV) to measure the corner frequency and the electromotility bandwidth in live gerbils.
They used OCTV to measure the small movements that occur in the outer hair cells when the gerbils hear a sound. They used sounds between 13-25 kHz to test outer hair cell motility.
The conclusion they reached was that the outer hair cells are quite similar to inner hair cells in their corner frequency, which doesn’t go beyond a few kHz. Thus they cannot amplify high-frequency ultrasonic sounds (which gerbils do hear quite well). Therefore this is not their function.
Instead, rather than picking up and responding to individual sound waves, they act as envelope detectors. They can correctly pick up changes in the loudness of sounds and modulate them before passing them on to the inner ear.
This protects the inner hair cells from extremely violent vibrations in response to very loud sounds. This is called automatic gain control and is used in numerous devices that receive and transmit sound waves to achieve dynamic range compression (DCR).
Researcher Anna Vavakou says, “Rather than amplifying sound, the cells seem to monitor sound level and regulate sensitivity accordingly.” This hypothesis is radically different from the current concept of outer hair cell function at present.
According to Marcel van der Heijden, “Figuring out their exact role could help guide efforts to prevent or even cure common forms of hearing loss.”