Ear’s ‘power steering’ helps decode quiet sounds

April 22nd, 2009 - 1:42 pm ICT by ANI  

Washington, Apr 22 (ANI): Ever wondered how you could hear those whispers while gossiping with friends in class? Well, scientists have now found that a nano motor of sorts in the ear makes us comprehend quiet sounds.

Human ears have bundles of tiny, hair-like tubes atop “hair cells” in the ear, which move back and forth and act like miniature ‘flexoelectric’ motors to amplify sound, according to Utah and Texas researchers.

Working like a power steering system of a car, the nanoscale motors magnify quiet sounds.

“We are reporting discovery of a new nanoscale motor in the ear. The ear has a mechanical amplifier in it that uses electrical power to do mechanical amplification,” said Richard Rabbitt, the study’s principal author and a professor and chair of bioengineering at the University of Utah College of Engineering.

He added: “It’s like a car’s power steering system. You turn the wheel and mechanical power is added. Here, the incoming sound is like your hand turning the wheel, but to drive, you need to add power to it. These hair bundles add power to the sound. If you did not have this mechanism, you would need a powerful hearing aid.”

Also, the researchers speculated that flexoelectrical conversion of electricity into mechanical work might also be involved in processes such as memory formation and food digestion.

In an earlier research, other researchers indicated that hair cells within the cochlea of the inner ear could “dance” - elongate and contract - to help amplify sounds.

But, in the current study, scientists have shown that sounds also may be amplified by the back-and-forth flexing or “dancing” of “stereocilia”- the 50 to 300 hair-like nanotubes projecting from the top of each hair cell.

Such flexing converts an electric signal generated by incoming sound into mechanical work - namely, more flexing of the stereocilia - which in turn amplifies the sound by what is known as a flexoelectric effect.

“Dancing hairs help you hear,” said Katie Breneman, the first author of the study.

The study “suggests sensory cells in the ear are compelled to move when they hear sounds, just like a music aficionado might dance at a concert. In this case, however, they’ll dance in response to sounds as miniscule as the sound of your own blood flow pulsating in your ear,” she added.

And in an unpublished upcoming study, researchers have found evidence that the hair cells themselves - like the stereocilia bundles atop those cells - also amplify sound by getting longer and shorter due to flexoelectricity.According to estimates, the combined flexoelectric amplification - by both hair cells and the hair-like stereocilia atop hair cells - could make humans hear the quietest 35 to 40 decibels of their range of hearing.

The researchers claimed that the flexoelectric amplifiers are needed to hear sounds quieter than the level of comfortable conversation.

“The beauty of the amplifier is that it allows you to hear very quiet sounds,” said a co-author of the study.

Rabbit says that because hair cells die as people age, older people often “need a hearing aid because amplification by the hair cells is not working.”

The new study is published in PLoS ONE, a journal published by the Public Library of Science. (ANI)

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