Cells in the developing ear can make their own noise, long before the ear can detect sounds around them. This may explain why people sometimes hear sounds that appear to come from nowhere. Reported in the current issue of Nature, the study results also helps explain how the developing auditory system generates brain activity in the absence of sound.
Non-nerve cells in the ears of young rats were studied, and to the researchers’ surprise these support cells showed strong, spontaneous electrical activity, similar to nerve cells.
"It’s long been thought that nerve cells that connect auditory organs to the brain need to experience sound or other nerve activity to find their way to the part of the brain responsible for processing sound," says the study’s lead author, Dwight Bergles, Ph.D., an associate professor of neuroscience at Hopkins. "So when we saw that these supporting cells could generate their own electrical activity, we suspected they might somehow be involved in triggering the activity required for proper nerve wiring."
Suspecting that a chemical might be involved in the cells’ ability to generate electrical pulses, Bergles’ team applied a number of drugs to the cochlea—the small, hollow and liquid-filled inner ear chamber that converts sound waves to electrical signals—hoping to block the mystery trigger.
The breakthough came when it was discovered that ATP (adenosine triphosphate) also caused the supporting cells to change their shape. The team videotaped the developing cochlea to monitor where and where ATP was released. It was found to be released near hair cells, which transfer sound information to auditory nerves. ATP then signals the hair cells to release another chemical, glutamate, which then activate the nerve cells that project into the brain.
"It’s as if ATP substitutes for sound when the ear is still immature and physically incapable of detecting sound," says Bergles, adding that "the cells we have been studying seem to be warming up the machinery that will later be used to transmit sound signals to the brain."
The research was funded by the National Institutes of Health. Authors on the paper are Nicholas Tritsch, Eunyoung Yi, Elizabeth Glowatzki, and Bergles, all of Hopkins, and Jonathan Gale of University College, London.
Source: Johns Hopkins Medicine