Why Is Our Hearing So Keen?

Humans can hear sounds between 20 Hz and 20 kHz. Bats and dolphins go much further: they can hear sounds over 20 kHz (ultrasounds), while dogs and elephants hear sounds under 20 Hz (infrasounds). For 30 years, researchers stated various hypotheses on how specialized cells in the mammals' inner ear amplify sounds. A recent research at St. Jude Children's Research Hospital found that the bouncing cell bodies rather than vibrating, hair-like cilia, are responsible for this.

"Our discovery helps explain the mechanics of hearing and what might be going wrong in some forms of deafness. There are a variety of causes for hearing loss, including side effects of chemotherapy for cancer," said senior author Dr. Jian Zuo, associate member of the St. Jude Department of Developmental Neurobiology.

Outer hair cells are rod-shaped cells sensitive to sound waves, located in the fluid-filled cochlea of the inner ear and got their name from their tufts of hair-like cilia projecting into the fluid. These cells boost mammalian hearing at a sensitivity 100 times higher than in their absence.

The energy of the sounds, even if dissipated by the cochlear fluid, triggers waves like a pebble thrown into a pond. The hair cell cilia in both mammal and non-mammal species swing back and forth rapidly in a steady rhythm as a response.

In mammals, the body of the outer hair cell reacts by contraction and vibration to the sound waves, amplifying the sound. The same team revealed that the protein named prestin makes the contraction in the mammalian hair cells, while in non-mammals, this protein is not present and the cells do not contract.

The tufts of cilia go downward by contraction, increasing the power of their vibration. Thus, in mammals, both the cilia and the cell itself vibrate. The debate is if the cilia are the main "culprit" of sound amplification in both mammal and non-mammal species.

Some think that mammalian somatic motility in outer hair cells is just a method to change the height of the cilia in the fluid to increase the force with which the cilia oscillate, amplifying the sound.

Others insist that even if the vibration of the outer hair cell body itself increases the cilia vibration, the cell body functions independently of its cilia, being the main factor of sound amplification in mammals. "If somatic motility is the dominant force for amplifying sound in mammals, this would mean that prestin is the reason mammals amplify sound so efficiently," said Zuo.

The team made tests that showed the role of the somatic mobility determined by prestin is not just to change the response of the outer hair cells' cilia to incoming sound waves. It appears that body cell movements are the main factor for sound amplification in the mammalian cochlea, while the cilia is more important for amplification in non-mammals.

Mutated prestin makes the outer hair cells contraction to result in an extension, not vibration. Changing the position of the cilia in the fluid should have impaired their ability to amplify sound, affecting the hearing in mutant individuals. But mice carrying the mutation did not display any hearing impairment, and if the outer hair cells contracted or extended themselves, this did not affect the amplification process.