Position of hair cell important to sound amplification

A pleasant conversation is a result of more than just good company. It is due in part to our capacity to hear, but as we age this seemingly effortless ability begins to have limitations.

Looking to further explain the molecular mechanisms behind hearing loss, researchers at Baylor College of Medicine were able to show evidence that the ability of the outer ear hair cell to amplify sound depends on its position within the cochlea.

The findings, appearing in the online version of Biophysical Journal, also reveal a mathematical process that can be used to identify this position and the area with stronger amplification properties.

"We know that hearing loss is related to age in a lot of cases and that a common reason for age-related hearing loss is a reduction in the number of healthy hair cells found in the cochlea – a specialized structure within the inner ear," said Dr. Brenda Farrell, assistant professor of otolaryngology at BCM.

The loss of healthy cells can be a result of repeated exposure to loud noises or the side effect of therapeutics, such as certain types of antibiotics and chemotherapy drugs.

Sound travels through the ear as a mechanical signal. The outer hair cell amplifies this signal. This amplification is necessary because high-frequency signals weaken as they travel. Since these cells cannot be repaired and regenerated, the ability to amplify sound decreases when damaged.

In the current study, researchers examined outer hair cells isolated from mature male guinea pigs. They were able to see that when outer hair cells were positioned in high-frequency regions of the cochlea, the signal remained strong. When they were positioned in other areas, the signal diminished.

"This concept was proposed years ago, but our group's work gives it more credence," Farrell said. "We found that shorter hair cells in the high-frequency region provided greater boost of amplification when compared to longer cells in the low frequency areas."

The second finding in this research reveals that the size of the different regions of the outer ear hair cell can be determined through mathematical equations with variables that include the length of the outer ear hair cell and the membrane capacitance (the ability of a body to store an electrical charge) across the cochlea.

"There is a well-known relationship between the frequency of sound and the best place along the cochlea where it is sensed. This relationship is common across all mammalian species examined," said Farrell. "Being able to find further support for this idea and identify new relationships that predict the degree of amplification at different frequency of sounds will lend itself to the development of improved devices to mitigate hearing loss."

Other researchers who contributed to this work include first authors Dr. Christian Corbitt, medical resident, and Federica Farinelli, postdoctoral fellow, both in the department of otolaryngology at BCM; and Dr. William Brownell, professor of otolaryngology at BCM.

This work was supported by The National Institute on Deafness and Other Communication Disorders DC000354 and by the Bobby R. Alford Department of Otolaryngology - Head and Neck Surgery, Baylor College of Medicine.