Mammal ears
From WikiCover
Announced July 27, 2007, a thirty-year scientific debate over 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 investigators at St. Jude Children's Research Hospital.
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."
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 a hundred 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.