Researchers from Texas and the U discovered the mechanism of sound amplification in the ear8212;an answer that could lead to further breakthroughs in fighting hearing loss.
Although previous research suggested that hair cells in the inner ear flex to intensify sound, a recently published study shows sound is also amplified when tiny nanotubes sitting atop hair cells in the ear vibrate back and forth.
“Your ear basically has a big battery in it that is charged by other cells. The hair cells use the power from the battery to amplify the sound,” said Richard Rabbitt, the study’s main author and chairman of the bioengineering department at the U. “It’s just like when you turn the wheel in your car and power steering system takes over and adds power.”
The discovery could help researchers studying gene therapy for deteriorated hair cells, but Rabbitt warns that people with hearing loss have years to wait for a solution.
Study co-author William Brownell, an ear, throat and nose specialist and professor at Baylor College of Medicine, said the nanotubes are related to microvilli, parts of cell membranes useful in absorbing and secreting cells, and scientists might be able to derive applications based on what was discovered in this study.
“It’s been postulated for a number of years that microvilli would be able to change their shape,” Brownell said. “They are also implicated in memory. We still don’t understand memory completely.”
The study, which was published in the journal PLoS ONE on Wednesday, shows that the nanotubes’ flexing converts the movement into an electrical signal.
To hear most conversations, humans require the amplifying tubes to work, or fluid in the ear would dilute sound too much.
“Imagine trying to suck a chocolate malt through a tiny straw,” Rabbitt said. “The viscosity of the fluid in the ear tends to drag.”
Brownell has been studying flexoelectric motors, the mechanical motor that works when nanotubes “dance” back and forth on hair cells, since 1983 when he and other researchers noticed hair cells moving in a petri dish.
“It provided evidence that hair cells had the ability to generate force needed for the cochlear amplifier,” Brownell said. “People became excited.”
Now, research at the U shows the exact technique by which humans can hear the quietest 35 decibels of incoming sound.
Katie Breneman, a study author and bioengineering doctoral student, said in a press release that the study suggests that sensory cells are “compelled to move” when sound comes through the ear.
Although sound and following ear vibrations trigger the nanotubes, Rabbitt said the brain can also control amplification.
“If you’re at a cocktail party trying to listen to a woman nearby but there’s lots of other sounds, the brain has neurons going to the ear that can turn off the motors amplifying those frequencies,” Rabbitt said. The brain can change the recognized frequency, be it the higher pitch from a woman talking or lower pitch from a man.
Hairs cells eventually deteriorate with age, causing hearing loss and problems with balance.
“In humans, hair cells do not regenerate8212;you just lose them,” Rabbitt said. “It’s a good reason not to listen to really loud music or take certain drugs.”
The research leaves many unanswered questions in the minds of scientists but offers new paths of study.
Rabbitt is currently studying how this amplifier affects balance in humans. Brownell is also studying the amplifying motor, and how cholesterol plays a role in amplifying sound generated in the ear.
University of UtahU bioengineering doctoral student Katie Breneman stands next to a large laboratory model of the cochlea, the part of the inner ear where incoming sound vibrations are converted into nerve impulses and sent to the brain. Breneman and colleagues conducted a study explaining how faint sounds are amplified mechanically by the movements of bundles of hair-like tubes atop “hair cells” in the cochlea.