How’s That?: The Aural Mechanics
Ears are incredible. They sense high whistles and deep thunder, the faintest rustle of a leaf and the trillion-fold more powerful shriek of a jet plane. Ears sense the world faster than eyes see or fingers feel--so fast, in fact, that scientists know that some very special biochemistry must lie behind their greased-lightning responsiveness.
Each part of the ear plays its part in hearing, though Charles Darwin dismissed the outer ear as a useless evolutionary leftover. After all, we can’t swivel our ears like dogs or cats, he wrote, and cited the case of a sailor who’d lost an ear in a brawl but could still hear fine out of that ear.
Since then, experiments (involving ears taped back against the head, shapes of ears altered with putty, and rubber ears) have revealed what Darwin missed. Outer ears, with their cunning swivels and swirls, act as cups to gather sounds. They also alter the quality of sounds funneling into the ear, helping us precisely pinpoint the source.
Having two ears is important for locating sounds as well--since a sound will reach one ear before the other, and be louder in the closer ear. (Even a baby knows instinctively what this means, and turns toward the source of a noise.)
But the most important parts of the ear lie deeper within our heads, in the middle and inner ear.
As sound waves enter the ear, they push up against the eardrum, causing it to vibrate. As the drum vibrates, it jiggles the first of three little bones--the hammer, which starts to move.
The hammer jiggles a second bone, the anvil, which jiggles a third bone, the stirrup. This jiggling chain of bones amplifies the sound signal.
The stirrup, meanwhile, plunges up against the entrance to the fluid-filled inner ear, swooshing the fluid with every movement it makes. It’s here, within the snail-like organ called the cochlea, that the real business of hearing takes place.
The cochlea is snail-like for a reason: Uncurled, it would take up way too much room, poking into our brain. But if you unroll that cochlea in your mind, you’ll see a long piece of rubbery skin with 17,000 cells studded along it. These “hair cells”--so named for the bundles of little hairy structures at their tips--allow our brains to sense and distinguish different sounds.
Hearing science isn’t knee-deep in Nobels, but the Hungarian-born physicist Georg von Bekesy got one in 1961 for figuring out the nuts and bolts of how this works. He did it with cadaver ears--of humans, rodents and even of elephants, which were nice and big and easy to study.
Here’s what Von Bekesy found: As the stirrup pushes the fluid at the entrance to the cochlea, the skin studded with hair cells starts flopping about like a piece of kelp swaying in an ocean swell.
Say we hear a high note--high C. Just one place in that skin will flop, stimulating only some hair cells, which send a “high C” signal to the brain. And what if that note is an octave lower? Now a region farther along will flop, and a “middle C” signal will go to the brain.
And so it goes down the length of the cochlea: Each region responds to a different frequency of sound. As if a whole piano keyboard is rolled up in our heads.
Hair Cells Are Central to Hearing
Since Von Bekesy’s time, researchers have learned much more about the ear, uncovering properties that give hearing its crispness and sensitivity. Certain hair cells, called “outer hair cells,” fine-tune and amplify sound signals. Then “inner hair cells” send the signal to our brains.
Our brains send signals to our ears too, telling us which sounds to amplify and pay attention to, and which ones to ignore.
With a structure this complicated, it’s no wonder that there are many different ways that hearing can be damaged.
Eardrums can perforate when children (and some adults) imprudently stick pointy objects in their ears, or when a buildup of fluid in the middle ear puts too much pressure on the drum, damping the ear’s sensitivity.
And fluid buildup in the middle ear--from bacterial and viral infections or from a blocked Eustachian tube, the airway that regulates pressure in the ear--temporarily dampens hearing. If infections happen too often, they may interfere with a child’s ability to learn. Occasionally, middle ear infections can irreversibly damage the hammer, anvil and stirrup, causing permanent hearing loss.
Sometimes, in a condition called otosclerosis, which tends to run in families, the three bones grow abnormally, and slowly lose the ability to transmit sound to the inner ear. (Otosclerosis can be treated by an operation in which surgeons implant an artificial stirrup.)
But the most common site for hearing problems is inside the inner ear, where the delicate hair cells reside. As people age, damage to these cells accumulates--from such things as exposure to loud noises and reduced blood circulation.
Some drugs--the aminoglycoside antibiotics, for instance, the anti-cancer drug cisplatin and even too much aspirin--can also damage hair cells. So can inner ear infections, small tumors that press against the hearing nerves, and a condition in which the body’s immune system starts attacking the inner ear.
And some people, even though every part of the outer, middle and inner ear is working perfectly, still can’t hear--because there’s some damage to the nerves transmitting hearing signals to the brain. Or there’s damage in the brain itself.
Many Genetic Reasons for Loss of Hearing
Myriad genes can cause hearing loss. Scientists estimate that more than half of all childhood cases of deafness are inherited (as well as many cases in which hearing loss sets in later). Some of the genes are well-known, like the ones behind Waardenberg syndrome, which causes not only deafness but traits like a white patch of hair on the head, and some of the genes that cause Usher syndrome, in which people lose their sight as well as their hearing. But most genes are still unidentified.
To find them, some scientists are turning to mice, whose petite, fuzzy ears are actually very similar to our own.
There are scores of mutant mouse strains with messed-up hearing. Some strains originated centuries ago in China, where they were prized for the strange way they moved. Balance as well as hearing is defective in such “waltzing” mice. Clap your hands together and a regular mouse will stiffen and start at the noise, but a waltzing mouse pays no heed: It just circles round and round.
More than mere curiosity drives such work: Scientists hope, by understanding how ears are formed and identifying the genes that cause hearing loss, that they will one day be able to do the miraculous: coax sensory cells to regrow inside the ear, restoring hearing to those who have lost it or bestowing it upon those who never had it.
Until very recently, there was never much hope that this could happen. The hair cells we’re born with, scientists knew, are all we ever get in this life.
But then came a discovery--that sharks and chickens can keep growing and regrowing their hair cells, and even regain some hearing after it’s damaged.
Of course, there’s no guarantee that this can also happen in people. After all, when surgeons spot a salamander or lizard regrowing a leg or a tail, they don’t immediately wax exuberant about regrowing legs in human amputees.
Still, scientists and biotech companies are trying hard to figure out just what chemical, or gene, might one day be dripped into the ear to trick those hair cells into sprouting up anew--a hair-restorer well worth working for.
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Hearing by the Numbers
28 million U.S. residents are hard of hearing or deaf, while many millions more have other hearing disorders, such as a ringing in the ears, or tinnitus. Here’s how the numbers break down.
Degrees of Hearing Loss
There are degrees of hearing loss, classified according to the softest sound someone can hear. (Sound levels are measured in decibles, or dBs.)
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Sources: American Speech-Language-Hearing Assn.; American Tinnitus Assn.; National Institute on Deafness and Other Communication Disorders; Boys Town National Research Hospital
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How We Hear
Our ears hear an incredibly wide range of sounds, from a soft whisper to a jet engine’s scream. Here’s how they do it:
