Research Explains Mystery of Inner Ear Function

The mystery of how the inner ear works appears to have been solved. For over 30 years, the scientific community has debated how cells in the inner ear of mammals amplify sound. The conclusion by researchers at St. Jude Children's Research Hospital is that the cell bodies actually bounce to amplify sound rather than vibrate the long hair-like cilia of the inner ear. The results will be published in the Proceedings of the National Academy of Science.

The results of this study could help scientists better understand why mammals have sensitive hearing and can differentiate between frequencies. It also validates the need for even basic hearing research in studies that examine the cause of deafness.

"Our discovery helps explain the mechanics of hearing and what might be going wrong in some forms of deafness," said Jian Zuo, Ph.D. in a press release. Zuo was the senior author of the paper and an associate member of the St. Jude Department of Developmental Neurobiology. "There are a variety of causes for hearing loss... researchers have the ability to ask some very basic questions about how the body works, and then use those answers to solve medical problems in the future."

The debate over how the inner ear works has focused on the outer hair cells. This column shaped cells respond to sound waves and are located in the cochlea. The outer hair cells are covered in cilia that project into the fluid filled cochlea, creating should. It is estimated that the outer hair cells allows mammals to hear 100 times better than if these hair cells did not exist.

Sound waves cause waves in the cochlear fluid, which causes the cilia to move in both mammal and non-mammal species. However, in mammals, the column shaped outer hair cell contracts and vibrates when it feels sound waves, which then amplifies the sound.

In previous research, Zuo proved that a protein causes the contraction in the outer hair cells. Then Zue used a mutated form of this protein, called prestin, which caused the cell to extend itself during vibration. They used this type of protein in genetically modified mice, essentially altering how the inner ear functioned.

The scientists found that the genetically modified mice showed not evidence of altered hearing, allowing them to conclude it was the presence of prestin, not the contraction or extension of the cells that caused the amplification of hearing.

SOURCES:

"St. Jude study solves mystery of mammalian ears." St. Jude Children's Research Hospital. URL: (http://www.stjude.org/media/0,2561,453_2816_23471,00.html)