New insight into sound amplification processes in inner ear

Sound waves decrease in amplitude as they pass through the fluid-filled cochlea of the inner ear. The inner ear’s outer hair cells (OHCs) are specially equipped to mitigate this loss in amplitude so that sounds remain perceptible, but how this happens isn’t fully understood.

New research from Baylor College of Medicine suggests that multiple voltage-sensing pathways exist in the lateral membrane of OHCs. These pathways help to facilitate sound wave amplification across the audible range of mammals. The findings are published in the Journal of General Physiology.

It is unclear how voltage sensing occurs at both low and high frequencies because the same sensing components reside in the lateral membrane of OHCs across the entire frequency range, even though amplification is understood to be most needed at high frequencies. Baylor College of Medicine researchers started investigating that question when they observed some unusual correlations in their data.

“One of our measured parameters varied along the gradient from the higher to lower frequency regions,” said Dr. Brenda Farrell, corresponding author and associate professor of otolaryngology – head and neck surgery at Baylor. “If you have one sensor or one pathway, you’d expect this parameter to be constant, and it wasn’t.

This led Baylor College of Medicine researchers to investigate whether a new voltage-sensing model, developed by the late Dr. I. Kim and Dr. A. Warshel of the University of Southern California, would provide a framework to interpret their data.  

When applying their data and those published by other researchers to this theoretical framework, Farrell and her colleagues identified two reaction pathways in OHCs and now suggest that multiple pathways allow these cells to respond to acoustic vibrations.

“These findings help us to understand how we amplify sound at high frequencies and dial down the response for low frequencies. One pathway may be used to amplify higher frequency sounds, while the other is used for lower frequencies,” Farrell said.