A new understanding of how we can hear such a wide range of sounds is reported in the online issue of Nature [06 February 2005]. The study, by researchers at the universities of Bristol, Wisconsin and Cambridge, describes a new mechanism for amplifying sounds within the inner ear.

To understand speech, sounds must be processed accurately and at high speed. When a sound is absorbed by the ear, vibrations of fluids within the inner ear are detected by special sensory cells. At the top of each cell is the ‘hair bundle’, so called because it looks like a bundle of hairs.

These hairs cells are sensitive enough to detect very tiny movements of the fluid in the inner ear. However, in order to achieve the high levels of sensitivity required for hearing sounds such as speech, the sound vibrations reaching the hair cells must be amplified in some way.

Dr Helen Kennedy from the Physiology Department at Bristol University said: “Our work shows how the hair bundles respond when stimulated by sounds. We have discovered that they are able to produce substantial mechanical forces, and that this force is linked to activity within tiny channels at the tips of the hairs. These forces amplify the sound, and may explain how we are able to achieve high sensitivity at all frequencies.”

Understanding how sounds are processed by the ear in this way provides insight into how damage to this sensitive amplifier leads to hearing loss, and may lead to improved therapies in the future.

Paper: Force generation by mammalian hair bundles supports a role in cochlear amplification by H. J. Kennedy, A. C. Crawford & R. Fettiplace.

Image: Courtesy Dr Dawn Davies, Department of Physiology, University of Bristol.