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Doing the 'Stereocilia Wave'

Are the hair cells in your inner ear doing a line dance, or is it more like the wave?

Whenever you hear a sound, clusters of tiny strands sprouting from the tops of key sensory cells in your inner ear begin to move, which triggers an electrical signal that rapidly scuttles off to your brain. But how exactly do they move? Most researchers picture them bowing in unison, as if they were in a three-row line dance doing the Cupid shuffle or the electric slide. Sonya Smith, Ph.D., a professor of mechanical engineering at Howard University, and Richard Chadwick, Ph.D., chief of the NIDCD Section on Auditory Mechanics, see things differently. To them, it’s more like they’re doing the wave.

The difference, they say, is that the inner ear is filled with fluid, so forces that are acting on the individual strands (called stereocilia) are different than if they were surrounded by air. Dr. Smith, who specializes in numerical modeling and acoustics on a large scale, such as with aircraft, and Dr. Chadwick, who has expertise in the mechanics of the inner ear, have been employing a model that is commonly used to predict the movement of submarines, airplanes, and parachutes to determine the movement of those tiny stereocilia on the sensory cells (called inner hair cells) deep inside your ear. For example, when the stereocilia begin to move, nearby fluid will move at the same velocity, which in turn affects further movement of the stereocilia.

Drs. Smith and Chadwick captured the fluid-stereocilia interactions in their model, then plugged in experimental data from the inner ears of guinea pigs, using the only two measurements that can be made of stereocilia—the motion of the thin layer that anchors the stereocilia to the top of the hair cell, and the motion of the tips of the stereocilia occupying the third and tallest row. Not only did the measurements fit well with the model, but they were able to use the model to figure out what can’t be measured on stereocilia. Namely, they were able to determine how tip links—the thin filaments connecting one row of stereocilia to the row behind it—stretch to open key channel gates, and how the channel gates, the entry points for potassium and calcium ions, let in enough ions to kick-start the electrical signal that travels to the brain.

A video representation of how stereocilia (strands on top of sensory cells in the inner ear) move in response to sound.
Click on this image to view/download
a video representation of how stereocilia
(strands on top of sensory cells in the
inner ear) move in response to sound.
Source: PLoS ONE
Credit: Daniella Chadwick

The researchers found that rather than bowing in unison, the first and third rows of stereocilia rotate to one side, while the middle row doesn’t rotate at all, but stretches up and down, as if it were in a stadium doing the wave. They also found that the layer anchoring the stereocilia moves in an orbital fashion and the membrane lying overhead (called the tectorial membrane) remains stationary at a key height above the stereocilia tips. If the membrane were any higher, the channel gates would not open properly. Finally, they discovered that the sudden movement of the channel gates opening creates a vortex that helps usher the ions inside, which helps start the electrical signal. (Watch the stereocilia perform their dance here.)

Dr. Smith conducted this work, which is published in the March 31 issue of PLoS ONE, under an Oak Ridge Institute for Science and Education (ORISE) fellowship sponsored by the NIDCD. As part of her fellowship, she and Dr. Chadwick plan to develop an ongoing program in which Dr. Smith’s students at Howard University will be able to continue using the technique she’s developed to further delve into the mysteries of the inner ear.

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