Researchers reverse hypothesis underlying the sensitivity of the mammalian auditory system

A new study from the University of Colorado Anschutz Medical Campus challenges a decades-old hypothesis about adaptation, an important function in how sensory cells of the inner ear (hair cells) detect sound.

The paper, out today Science Advances, investigates how hair cells transform mechanical forces arising from sound waves into a neural electrical signal, a process called mechanoelectric transduction (MET). Hair cells have an intrinsic ability to fine-tune the sensitivity of the MET process (designated adjustment), which underlies our ability to recognize a wide range of sound intensities and frequencies with very high precision. So far, 30+ years of research have convinced auditory scientists that the molecules and proteins responsible for adaptation have been invented. First published in 1987, the predominant model of how adjustment works is that the sound-sensitive “antenna” of the hair cell (called the hair bundle) undergoes a mechanical change during adjustment, so that a decrease in the stiffness of the hair bundle decreases. in MET sensitivity.

Additional experiments performed in the following decades have suggested that a motor protein, myosin 1c, is required for MET adaptation. Through multiple experiments and a variety of controls, Anschutz researchers have determined that this existing hypothesis needs to be re-examined; that although adaptation requires myosin motors, it is not a mechanical change in the hair bundle.

Anschutz researchers have conducted a series of refined experiments to investigate the relationship between the mechanical properties of the hair bundle and the electrical response of the hair follicle. Giusy Caprara, Ph.D., postdoctoral fellow at the University of Colorado School of Medicine and lead author of the study, used a custom-built high-speed imaging technique, while simultaneously making electrical recording and imaging of hair cells in a variety of mammalian species at 10,000 frames per second to investigate the mechanical changes to the hair bundle during adaptation, an extreme departure from the 1987 experiments using photodiodes. “The reason this was not discovered earlier is because there are very few experiments that test the mechanical properties of the hair bundle,” says Anthony Peng, Ph.D., supervising author and assistant professor in physiology and biophysics from the University of Colorado School of Medicine. “Technology drove and made this discovery possible.”

Understanding the mechanism of adaptation is important in determining how the sensory cells of the inner ear work. Although the research is not directly translational, it is an important first step in fixing and replacing cochlear function, potentially leading to technological improvements for better sound processing and treatment of below-the-line hearing function.

“The discovery that the original model of adaptation was wrong was important in a few ways,” Peng says. In basic science, this has opened up opportunities for more research, including proposing a new model of how adaptation works. More importantly, listener sensitivity and the range of listening we can achieve in this process, so understanding this will help us better understand the different types of hearing loss that people experience. ”