delanceyplace.com 5/20/13 - hearing loss is from ear hair damage
In today's selection -- in most cases, when our hearing is impaired, it is damage to ear hair. Although that damage becomes evident when we are older, it often relates to damage incurred when we are young:
"By far the most common [condition that destroys hearing is] exposure to either long-term moderately loud noise or sudden very loud noise. ... What actually happens in the inner ear when it is exposed to ... loud noise?
"The inner ear is home to the cochlea, a bony spiral cavity about the size of a pea, which turns on itself two and a half times and looks like a snail shell ('cochlea' comes from the Latin term for 'snail'). Sound waves, or vibrations, enter the cochlea (having been given a boost by the middle ear's three interconnected bones, including the stapes, the smallest bone in the body). As this happens, fluid in the cochlea sets in motion the thousands of hair cells located in the organ of Corti, deep in the inner ear.
"The hair cells in the organ of Corti are organized into four rows. The three outer rows of cells pick up the movement and change it into a mechanical impulse, which amplifies the signal -- now traveling through the cochlear bath and thus dulled, as sound would be if you were underwater. The inner hair cells, in a single row, each respond to a particular frequency. They are activated to release a neurotransmitter to the auditory nerve fibers, which also number in the thousands and also each respond to a different frequency. The neurons transmit the sound via the auditory nerve to the brain, ultimately reaching the auditory cortex, which translates the sound into something that we recognize as speech or birdsong or a car passing on the road. The translation that occurs in the auditory cortex allows us to distinguish between similar speech sounds like 'ah' and 'eh,' 'b' and 'p,' 'ch' and 'sh.' How the cortex does this is beyond the scope of this book. Suffice it to say that you hear with your brain. The auditory system merely transmits the signals. But if the signals can't get to the brain, then the brain can't do its job.
"In a lot of deafness, the first things you lose are the outer hair cells. The inner hair cells may be undamaged, but because you've lost the mechanical response of the outer cells, the cochlea is not as sensitive, not as fine-tuned in its response. The result is that some neurons respond to more frequencies than they should, sending a muddled signal to the brain. The primary damage is to speech recognition. 'Bet' sounds like 'pet,' 'church' sounds like 'shirts.' Brad May, of Johns Hopkins, calls this 'brain deafness.' ...
"[This] type of hearing loss [is] often referred to as nerve damage but [it is] not, technically, since [it doesn't] affect the acoustic nerve, only the hair cells that communicate with it. ...
"A person with mild to moderate hearing loss can still hear in a quiet room or other favorable environment. But when too many frequencies are destroyed, he or she may not understand speech, even under the best of conditions. The muddled transmissions also make it difficult for the auditory system to filter unwanted noise: the din and clatter of a restaurant, the engine of a bus, the hum of a fan or air conditioner. Intrusive noise may be simply two or three people talking at once, creating a background sound of indistinguishable voices, or it may be a large, resonant room echoing sound off the walls. ... Since hearing aids aren't as good as the human ear at screening out unwanted noise, using them can be frustrating, especially in noisy environments.
"Assuming my hair cells are damaged, they probably look flattened, like a field of wheat after a hailstorm. ... Each cell in those four rows of cells (the single inner row, which communicates with the brain, and three outer rows) is topped by a tiny standing hair, or stereocilium. The hair cells, she said, are 'connected to each other with fine little filaments, so that when sound comes in and they bend, it allows currents to flow through.' This movement triggers the release of the neurotransmitter substances. After intense noise exposure, the hair cells lie flat. If the noise is not too loud, they eventually right themselves. The threshold shift is temporary.
"But Kujawa and Liberman have found that even though the threshold reverts to normal, permanent damage may have occurred. ... [They] found that the damage occurs not in the hair cells themselves, which may recover, but in the spiral ganglion cells (SGCs -- the cells in the cochlear neurons). The hair cells communicate with SGCs in the process of passing information to the brain.
"Although hearing is restored, the damage is done almost instantaneously. ... Even though we think of this kind of hearing loss as related to aging, the truth is that ears are most vulnerable to noise damage when they're young. ... Teenagers -- with their ubiquitous iPods and MP3 players, not to mention noise exposure from video games, loud stadiums, and rock concerts -- are experiencing these loud noises at an especially vulnerable age. Another vulnerable population, newborn infants, might suffer damage from continuous noise in a neonatal ICU or from a white noise machines parents sometimes use to help fussy infants sleep."