Now hear this: Scientists show how tiny cells deliver big sound

Oct 22, 2009

Deep in the ear, 95 percent of the cells that shuttle sound to the brain are big, boisterous neurons that, to date, have explained most of what scientists know about how hearing works. Whether a rare, whisper-small second set of cells also carry signals from the inner ear to the brain and have a real role in processing sound has been a matter of debate.

Now, reporting on rat experiments in the October 22 issue of Nature, a Johns Hopkins team says it has for what is believed to be the first time managed to measure and record the elusive of the type II in the snail-shell-like structure called the . And it turns out the cells do indeed carry signals from the ear to the brain, and the sounds they likely respond to would need to be loud, such as sirens or alarms that might be even be described as painful or traumatic.

The researchers say they've also discovered that these sensory cells get the job done by responding to glutamate released from sensory of the inner ear. Glutamate is a workhorse throughout the nervous system and it excites the cochlear neurons to carry acoustic information to the brain.

"No one thought recording them was even possible," says Paul A. Fuchs, Ph.D., the John E. Bordley Professor of Otolaryngology-Head and Neck Surgery and co-director of the Center for Sensory Biology in the Johns Hopkins University School of Medicine, and a co-author of the report. "We knew the type II neurons were there and now at last we know something about what they do and how they do it."

Working with week-old rats, neuroscience graduate student Catherine Weisz removed live, soft tissue from the fragile cochlea and, guided by a powerful microscope, touched electrodes to the tiny type II beneath the . Different types of stimuli were used to activate sensory hair cells, allowing Weisz to record and analyze the resulting signals in type II fibers.

Results showed that, unlike type I neurons which are electrically activated by the quietest sounds we hear, and which saturate as sounds get louder, each type II neuron would need to be hit hard by a very loud sound to produce excitation, Fuchs says.

The cell bodies of both type I and type II neurons sprout long filaments, or axons that head to the brain, and some others that connect to sensory hair cells. Unlike the big type I neurons, each of which make one little sprout that touches one sensory hair cell in one spot, the type II cells have projections that contact dozens of hair cells over a relatively great distance.

"Somewhat counter-intuitively, the type II cell that contacts many hair cells receives surprisingly little synaptic input," Fuchs says. "In fact, all of its many contacts put together yield less input than that provided by the one single hair cell touching a type I neuron."

Fuchs and his team postulate that the two systems may serve different functional roles. "There's a distinct difference between analyzing sound to extract meaning — Is that a cat meowing, a baby crying or a man singing? — versus the startle reflex triggered by a thunderclap or other sudden loud sound." Type II afferents may play a role in such reflexive withdrawals from potential trauma."

Source: Johns Hopkins Medical Institutions

Explore further: Hearing quality restored with bionic ear technology used for gene therapy

add to favorites email to friend print save as pdf

Related Stories

Surviving dance club music (noise) with hearing intact

Jan 21, 2009

By tweaking a system in the ear that limits how much sound is heard, a global team of researchers has discovered one alteration that shows that the ability of the ear to turn itself down contributes to protecting against ...

Ears ringing? Scientists ID the brain's own clarion

Oct 31, 2007

Brain scientists at Johns Hopkins have discovered how cells in the developing ear make their own noise, long before the ear is able to detect sound around them. The finding, reported in this week’s Nature, helps to exp ...

Streamlining brain signals for speed and efficacy

Oct 22, 2008

Life exists at the edge of chaos, where small changes can have striking and unanticipated effects, and major stimuli may go unheard. But there is no space for ambiguity when the brain needs to transform head motion into precise ...

Recommended for you

LED exposure is not harmful to human dermal fibroblasts

Apr 23, 2014

There was a time when no one thought about light bulbs—one blew, you screwed another one in. Nowadays, it's more complicated, as energy efficiency concerns have given rise to a slew of options, including ...

User comments : 0

More news stories

New breast cancer imaging method promising

The new PAMmography method for imaging breast cancer developed by the University of Twente's MIRA research institute and the Medisch Spectrum Twente hospital appears to be a promising new method that could ...

Breast cancer replicates brain development process

New research led by a scientist at the University of York reveals that a process that forms a key element in the development of the nervous system may also play a pivotal role in the spread of breast cancer.

Research proves nanobubbles are superstable

The intense research interest in surface nanobubbles arises from their potential applications in microfluidics and the scientific challenge for controlling their fundamental physical properties. One of the ...

Using antineutrinos to monitor nuclear reactors

When monitoring nuclear reactors, the International Atomic Energy Agency has to rely on input given by the operators. In the future, antineutrino detectors may provide an additional option for monitoring. ...