Shhh! Did you hear that? The ghostly whispers that grab
your attention could be the result of chit-chatting nerve
cells in your ears that were there in the womb.
The finding, reported in the Nov. 1 issue of the journal
Nature, has implications for treating a phenomenon
called tinnitus in which people hear annoying high-pitched
sounds with no apparent source.
While the study involved rats, the scientists say the
results likely apply to humans who are equipped with similar
Once a person's (or other mammal's) auditory
machinery is fully developed, it works something like
this: Sound waves travel through the ear canal and get
detected by hair cells in the cochlea. This snail-shaped
organ translates the sound signals into electrical impulses
that travel along the auditory nerve into the brain's
Dwight Bergles, a neuroscientist at Johns Hopkins University
in Baltimore, and his colleagues examined how hearing
works in young rats whose cochleae were not yet fully
developed. Even without mature auditory machinery, the
rats showed nerve activity in the brain regions associated
The activity showed up even without sound input.
Further lab tests revealed non-sensory hair cells called
support cells could explain how the rats could hear without
the needed equipment and even without sound input. These
support cells reside in the cochlea and had been thought
of as silent spectators, uninvolved in nerve communication.
However, they showed strong electrical activity in the
rats' ears during the study.
The team found evidence that the "bystanders"
spontaneously release ATP, the energy molecule of cells,
which triggers a cascade of events that lead to electrical
impulses being sent to the brain—no sound input
“It is as if ATP substitutes for sound when the
ear is still immature and physically incapable of detecting
sound,” Bergles said. "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.”
Other mammals, including humans, also have support cells
in their ears, so the rat findings could have implications
for people as well, the scientists say.
For instance, the finding suggests these support cells
in the developing human ear make their own noise long
before babies are born and before they can even detect
sound. Bergles speculates the premature hearing ability
could prep the auditory system for when it goes "live."
The ability to hear subtle sound differences, such as
the inflection in a person's voice, “requires
a lot of fine-tuning based on where in the brain the nerves
connect," Bergles explained. "It could be that
brief bursts of electrical activity in just a few nerve
cells at a time help do that fine-tuning so the system
By the time rats developed mature ears and could hear
sound, the spontaneous ATP-release ceased. That way the
sideshow wouldn't interfere with detection of actual
sounds, Bergles suggests.
The ear's hair cells held onto their ability to
respond to ATP, however, even though none was around.
Bergles suspects if a malfunction leads to anomalous release
of ATP in people, the support cells might whip into action,
making one think they are hearing something—as is
the case with tinnitus.
"If ATP were released by the remaining support
cells, it may cause the sensation of sound when there
is none,” Bergles said.