(Inside Science) -- Researchers have long debated why so many people lose their hearing as they age. Some have argued that the problem lies mainly with the stria vascularis, a sort of battery in the inner ear that powers the transformation of sound energy into nerve impulses. But a new study places the blame largely with hair cells -- the same sensory organs that are damaged by loud noises.
Most details of the inner ear are impossible to examine while people are alive. To really see what’s going on, researchers must take thin slices from cadavers and examine them with microscopes.
When researchers did this in the past, they found that older people often had damage in a structure called the stria vascularis, or stria for short. The stria acts like a battery, maintaining a high concentration of positively charged ions in the fluid that bathes the sensory tips of hair cells. When sound waves reach those sensory tips, the movement opens channels, allowing positive ions to flow into the hair cells and produce an electrical current. In this way, the stria allows sound to be transformed into electrical signals the brain can decode.
In past studies, researchers didn’t notice much damage to the hair cells themselves. But according to Charles Liberman, an auditory neuroscientist at the Massachusetts Eye and Ear Infirmary at Harvard Medical School, that’s because they weren’t looking hard enough. In each region past researchers examined, they simply noted whether they could see any surviving hair cells, rather than determining what fraction had been destroyed.
It is possible to count how many are destroyed, because hair cells are arranged in a regular pattern like a checkerboard, said Liberman.
"If you looked at a checkerboard and you shot a bullet through some of the squares, you could look at that whole checkerboard and you could see how many were blown out," he said. "Everybody previously just sort of looked and said, 'Yeah, there's a checkerboard there.'"
Counting the checkerboard
In the new study, Liberman and his colleagues examined the inner ears of 120 cadavers -- far more than any prior study. In 77 cases, they also had access to hearing tests conducted within a few years prior to the person's death. The subjects ranged in age from birth to 104, although the vast majority were over 50 when they died.
A section of the inner ear of a 25-year-old woman. The row of structures at the top are inner hair cells, which transform sound waves into nerve impulses. The array at the bottom is made up of outer hair cells, which are responsible for amplifying sound. Several of the outer hair cells are missing.
Image Credits: Courtesy of Charles Liberman
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Liberman's team observed extensive damage to both the hair cells and the stria. However, damage to the stria bore no detectable relationship to people's actual hearing sensitivity. In contrast, missing hair cells were associated with significantly worse hearing, especially when people were missing a type of hair cell responsible for amplifying sound. By the time the average person in the study turned 60, they had already lost half of these sound-amplifying hair cells. Hair cell loss was also generally worse in people who were exposed to loud noises in the military or in their jobs.
Hair cells in different parts of the cochlea detect and amplify different frequencies of sound, and people tended to hear less clearly in the frequencies for which they were missing the most hair cells. This was easy to see in graphs known as audiograms that display the results of people's hearing tests.
"The hair cell loss is mirroring the shape of the audiogram," said Peizhe Wu, an auditory neuroscientist who worked with Liberman on the study, which appeared last week in the Journal of Neuroscience.
The findings suggest that hair cells are far more important in age-related hearing loss than previously appreciated, said Dwayne Simmons, an auditory neuroscientist at Baylor University in Waco, Texas, who was not involved in the research. Simmons found it especially compelling that when the researchers reanalyzed specimens from previous studies using their new methods, they found hair cell loss that the earlier research had missed.
"To me, that's extremely powerful," said Simmons. "Those previous studies may have missed some really, really interesting data."
What it means for your ears
If damaged hair cells are the main culprit in age-related hearing loss, then much of the hearing loss people experience could potentially be avoided by wearing ear protection. Indeed, past research has shown that in traditional societies in Sudan and Easter Island, where there is very little noise pollution, elderly people have better hearing than one would expect for people of the same age in industrialized societies.
The new study also raises hopes for treatments currently in development to regrow hair cells, said Liberman. Such treatments have been shown to work in lab animals, and the new findings suggest they could potentially benefit a large swathe of the population.
Robert Frisina, an auditory neuroscientist at the University of South Florida in Tampa who was not involved in the research, agreed that the study makes a compelling case for the importance of hair cell loss. But he said that doesn't mean the stria isn't important too.
The researchers analyzed the stria by measuring its cross-sectional area -- a relatively crude metric, said Frisina. Strial tissue may start to break down and stop working before cell death causes visible changes in size. Moreover, cadaver tissues will shrink and degrade to varying degrees before being fixed on slides, which could throw off size-based measurements. That could be why the researchers didn't find a correlation between strial damage and hearing loss, said Frisina.
But Liberman doesn't think their measurement techniques were a big factor. He suspects that by the time the stria is bad enough to impact hearing, most people in our society have already lost so many hair cells that it no longer matters.
Hair cell loss seems to be a less important part of the aging process in other species, a fact that may have led researchers to underestimate the importance of hair cell loss in humans, said Liberman. Scientists often get clues about human biology by studying other animals, and there is extensive research showing that old gerbils lose their hearing because of damage to the stria.
Richard Schmiedt, one of the scientists who conducted the gerbil research, noted that his team protected their gerbils from noise exposure in order to isolate the effects of aging. In the absence of noise damage, humans might follow the same pattern as gerbils, with the stria being the first thing to go.
"[The new study] demonstrates that many humans in our western civilization are noise exposed over their entire lifetimes, and that pure aging effects may be masked by this noise exposure," wrote Schmiedt in an email.
If that's true, then protecting or regrowing hair cells will only help up to a point. Even people with intact hair cells may eventually lose their hearing as their stria degrades. Nevertheless, the new study suggests there's a lot you can do to keep your hearing sharp -- a message Wu and Liberman have taken to heart.
"I never used to wear ear plugs when I mowed the grass," said Liberman. "Now I do."