The sensory cells of the inner ear have tiny hairs called stereocilia that play a critical part in hearing. It has long been known that these stereocilia move sideways back and forth when stimulated by a sound wave. After having designed a microscope to observe these movements, a research team at Karolinska Institutet in Sweden, in collaboration with a researcher at Baylor College of Medicine, have discovered that the hairs not only move sideways but also change in length.

The findings, published online in the journal Nature Communications, provide new fundamental knowledge about the mechanisms of hearing.

Observing ciliary motion

Before we can perceive speech, music and other sounds, the sound waves must be converted into electric impulses in the auditory nerve, a process mediated by the sensory cells of the inner ear. Previous studies revealed that sound causes a lateral movement of the tiny hairs that project from these cells that open and close mechanically sensitive ion channels to create the sensation of hearing.

It is impossible to study the movement of the human cilia because the sensory cells are deeply embedded in thick bone, but in guinea pigs and gerbils the inner ear is surrounded by thin more easily removed bone. Using a special in-house designed microscope, researchers were able to observe the sound-induced ciliary motion.

"This revealed something surprising—that the hairs not only bend sideways but also change in length," said Dr. Anders Fridberger, docent and physician at the Centre for Hearing and Communication Research at Karolinska Institutet’s Department of Clinical Science, Intervention and Technology. "These longitudinal changes have an important effect on the process of converting sound waves into electrical signals, which is necessary for hearing."

Ability to change in length

The scientists show that the stereocilia’s ability to change length was greater when the electric potential around the sensory cells was low, which is known to happen in connection with noise damage and age-related hearing loss. The voltage drop causes the hairs to become overly soft, thus impairing ear function.

"Our findings might possibly help us understand why the ear doesn’t work as well in such cases," says Fridberger. "And maybe one day they can be put to use in the development of a new treatment for impaired hearing. If we can use a drug to restore the cilia's normal stiffness we could make the ear work better, but this is something for the distant future, if it is even possible. What we must do now is to discover the exact mechanism that control ciliary stiffness."

Importance of inner ear environment

"The electric potential that stiffens the stereocilia also powers hair cell electromotility which we discovered 30 years ago," says Dr. William Brownell, professor of otolaryngology at BCM. "This new form of electromechanical signalling in the stereocilia is exciting because it further demonstrates the importance of the inner ear electrical environment for hearing health."

Funding for the study came from the Swedish Research Council, the Swedish Council for Working Life and social research, the Wallenberg Foundations, the Tysta Skolan (Silent School) Foundation, the Swedish Association of Hard of Hearing People and the National Institutes of Health, USA.

Others who took part in the study include: Drs. Pierre Hakizimana and Stefan Jacob, both with the department of clinical science, intervention and technology and the department of otolaryngology, 
Center for Hearing and Communication Research, Karolinska Institutet, Sweden.