Scientists have revealed that humans get the ability to hear as a result of a Rube Goldberg-style process, wherein sound vibrations entering the ear shake and jostle a successive chain of structures until they are converted into electrical signals that can be interpreted by the brain.
Researchers at the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health (NIH), and the Scripps Research Institute in La Jolla, CA say that they have identified two key proteins that join together at the precise location where energy of motion is turned into electrical impulses.
The discovery is significant because scientists had long been trying to understand the pivotal point at which a person is able to discern sound.
"This team has helped solve one of the lingering mysteries of the field. The better we understand the pivotal point at which a person is able to discern sound, the closer we are to developing more precise therapies for treating people with hearing loss, a condition that affects roughly 32.5 million people in the United States alone," Nature magazine quoted Dr. James F. Battey, Jr., director of the NIDCD, as saying.
He said that sound vibrations entering the ear first bounce against the eardrum to cause it to vibrate, which in turn causes three bones in the middle ear to vibrate, amplifying the sound.
He further said that vibrations from the middle ear set fluid in the inner ear, known as cochlea, into motion and a travelling wave to form along a membrane running down its length.
Sensory cells (called hair cells) sitting atop the membrane "ride the wave", and bump up against an overlying membrane, causing bristly structures protruding from their tops (called stereocilia) deflect, or tilt to one side, said the researcher.
Dr. Battey said that the tilting of the stereocilia cause pore-sized channels to open up, ions to rush in, and an electrical signal to be generated that travels to the brain, a process called mechanoelectrical transduction.
Many scientists believe that the channel gates are opened and closed by microscopic bridges called 'tip links', which connect shorter stereocilia to taller ones positioned behind them. Determining what the tip links are made of may take them one step closer to understanding what causes the channel gates to open.
NIDCD and Scripps researchers have demonstrated that two proteins associated with hearing loss—cadherin 23 (CDH23) and protocadherin 15 (PCDH15)—unite and adhere to one another to form the tip link. Mutations in CDH23 are known to cause one form of Usher syndrome as well as a nonsyndromic recessive form of deafness, and mutations in PCDH15 are responsible for another form of Usher syndrome. Usher syndrome is the most common cause of deaf-blindness in humans.
"Cadherin 23 and protocadherin 15 have been implicated in a variety of forms of late- and early-onset deafness, and a whole range of mutations can produce different outcomes," says NIDCD's Kachar, a co-senior investigator on the study.
"Now that we know how these two proteins interact at the tip link, we can perhaps predict how different types of hearing loss can take place depending on where a mutation is located," Kachar added.