Researchers discover primary role of the olivocochlear efferent system

March 27, 2013

New research from the Massachusetts Eye and Ear, Harvard Medical School and Harvard Program in Speech and Hearing Bioscience and Technology may have discovered a key piece in the puzzle of how hearing works by identifying the role of the olivocochlear efferent system in protecting ears from hearing loss. The findings could eventually lead to screening tests to determine who is most susceptible to hearing loss. Their paper is published today in the Journal of Neuroscience.

Until recently, it was common knowledge that exposure to a (concert, iPod, mechanical tools, firearm, etc.), could lead to permanent or temporary hearing loss. Most would assess the damage caused by this type of exposure by measuring hearing thresholds, the lowest level at which one starts to detect/sense a sound at a particular frequency (pitch). Drs. Sharon Kujawa and Charles Liberman, both researchers at Mass. Eye and Ear, showed in 2009 that noise exposures leading to a temporary in mice (when hearing thresholds return to what they were before exposure) in fact can be associated with neuropathy, a situation in which, despite having a normal threshold, a portion of auditory is missing).

The , the organ that converts sounds into messages that will be conveyed to and decoded by the brain, receives in turn fibers from the . Those fibers are known as the olivocochlear efferent system. Up to now, the involvement of this efferent system in the protection from acoustic injury – although clearly demonstrated – has been a matter of debate because all the previous experiments were probing its protective effects following noise exposures very unlikely to be found in nature.

Stephane Maison, Ph.D., investigator at the Eaton-Peabody Laboratory at Mass. Eye and Ear and lead author, explains. "Humans are currently exposed to the type of noise used in those experiments but it's hard to conceive that some , thousands of years ago, were submitted to stimuli similar to those delivered by speakers. So many researchers believed that the protective effects of the efferent system were an epiphenomenon – not its true function."

Instead of using loud noise exposures evoking a change in hearing threshold, we used a moderate noise exposure at a level similar to those found in restaurants, conferences, malls, and also in nature (some frogs emit vocalizations at similar or higher levels) and instead of looking at thresholds, we looked for signs of cochlear neuropathy, Dr. Maison continued.

The researchers demonstrated that such moderate exposure lead to cochlear neuropathy (loss of auditory nerve fibers), which causes difficulty to hear in noisy environments.

"This is tremendously important because all of us are submitted to such acoustic environments and it takes a lot of auditory nerve fiber loss before it gets to be detected by simply measuring thresholds as it's done when preforming an audiogram," Dr. Maison said. "The second important discovery is that, in mice where the efferent system has been surgically removed, cochlear neuropathy is tremendously exacerbated. That second piece proves that the efferent system does play a very important role in protecting the ear from cochlear neuropathy and we may have found its main function."

The researchers say they are excited about this discovery because the strength of the efferent system can be recorded non-invasively in humans and a non-invasive assay to record the efferent system strength has already been developed and shows that one is able to predict vulnerability to acoustic injury (Maison and Liberman, Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength, Journal of Neuroscience, 20:4701-4707, 2000).

"One could envision applying this assay or a modified version of it to human populations to screen for individuals most at risk in noise environments," Dr. Maison concluded.

Explore further: Study: Hearing impaired ears hear differently in noisy environments

More information: "Efferent Feedback Minimizes Cochlear Neuropathy from Moderate Noise Exposure," Journal of Neuroscience, 2013.

Related Stories

Study: Hearing impaired ears hear differently in noisy environments

September 11, 2012
(Medical Xpress)—The world continues to be a noisy place, and Purdue University researchers have found that all that background chatter causes the ears of those with hearing impairments to work differently.

Can you hear me now? New strategy discovered to prevent hearing loss

July 6, 2012
If you're concerned about losing your hearing because of noise exposure (earbud deafness syndrome), a new discovery published online in the FASEB Journal offers some hope. That's because scientists from Germany and Canada ...

Earphones 'potentially as dangerous as noise from jet engines,' according to new study

August 29, 2012
Turning the volume up too high on your headphones can damage the coating of nerve cells, leading to temporary deafness; scientists from the University of Leicester have shown for the first time.

Recommended for you

The neural codes for body movements

July 21, 2017
A small patch of neurons in the brain can encode the movements of many body parts, according to researchers in the laboratory of Caltech's Richard Andersen, James G. Boswell Professor of Neuroscience, Tianqiao and Chrissy ...

Faulty support cells disrupt communication in brains of people with schizophrenia

July 20, 2017
New research has identified the culprit behind the wiring problems in the brains of people with schizophrenia. When researchers transplanted human brain cells generated from individuals diagnosed with childhood-onset schizophrenia ...

Scientists reveal how patterns of brain activity direct specific body movements

July 20, 2017
New research by Columbia scientists offers fresh insight into how the brain tells the body to move, from simple behaviors like walking, to trained movements that may take years to master. The discovery in mice advances knowledge ...

Scientists discover combined sensory map for heat, humidity in fly brain

July 20, 2017
Northwestern University neuroscientists now can visualize how fruit flies sense and process humidity and temperature together through a "sensory map" within their brains, according to new research.

Team traces masculinization in mice to estrogen receptor in inhibitory neurons

July 20, 2017
Researchers at Cold Spring Harbor Laboratory (CSHL) have opened a black box in the brain whose contents explain one of the remarkable yet mysterious facts of life.

Speech language therapy delivered through the Internet leads to similar improvements as in-person treatment

July 20, 2017
Telerehabilitation helps healthcare professionals reach more patients in need, but some worry it doesn't offer the same quality of care as in-person treatment. This isn't the case, according to recent research by Baycrest.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.