New discovery helps explain the mechanics of hearing

Aug 28 2007

A 30-year scientific debate about how specialized cells in the inner ear amplify sound in mammals appears to have been settled more in favor of bouncing cell bodies rather than vibrating, hair-like cilia, according to St. Jude investigators.

The finding could explain why dogs, cats, humans and other mammals have such sensitive hearing and the ability to discriminate among frequencies. The work also highlights the importance of basic hearing research in studies into the causes of deafness. A report on this work appears in the advanced online issue of Proceedings of the National Academy of Science .

“Our discovery helps explain the mechanics of hearing and what might be going wrong in some forms of deafness,” said the paper's senior author, Jian Zuo, PhD, Developmental Neurobiology. “There are a variety of causes of hearing loss, including side effects of chemotherapy for cancer.”

The long-standing argument centers around outer hair cells, which are rod-shaped cells that respond to sound waves. Located in the fluid-filled part of the inner ear called the cochlea, these outer hair cells sport tufts of hair-like cilia that project into the fluid. The presence of outer hair cells makes mammalian hearing more than 100 times better than it would be if the cells were absent.

In mammals, the rod-shaped body of the outer hair cell contracts and then vibrates in response to the sound waves, amplifying the sound. While both mammals and non-mammals have cilia on their outer hair cells, only mammalian outer hair cells have prestin, a protein “motor” that drives this cellular contraction. This contraction pulls the tufts of cilia downward, maximizing the force of their vibration. In mammals, both the cilia and the cell itself vibrate. Thus far the question has been whether the cilia are the main engine of sound amplification in both mammals and non-mammals.

In the study, Zuo and his team conducted a complex series of studies that showed in mammals that the role of somatic mobility driven by prestin is not simply to modify the response of the outer hair cells' cilia to incoming sound waves in the cochlea fluid. Instead, somatic motility itself appears to dominate the amplification process in the mammalian cochlea, while the cilia dominate amplification in non-mammals.

Other St. Jude authors of this study include Jiangang Gao, PhD, Developmental Neurobiology; former graduate student Xudong Wu; and former POE student Manish Patel.