An unusual dance recital was videotaped in David Corey's lab at Massachusetts General Hospital recently. The star of the performance, magnified many times under a high-powered microscope, was a sound-receptor cell from the ear of a bullfrog, called a hair cell because of the distinctive tuft of fine bristles sprouting from its top.
The music ranged from the opening bars of Beethoven's Fifth Symphony and Richard Strauss' "Thus Spake Zarathustra" to David Byrne and the Beatles.
As the music rose and fell, an electronic amplifier translated it into vibrations of a tiny glass probe that stimulated the hair cell, mimicking its normal stimulation in the ear. The bristly bundle of "stereocilia" at the top of the cell quivered to the high-pitched tones of violins, swayed to the rumblings of kettle drums, and bowed and recoiled, like tiny trees in a hurricane, to the blasts of rock-and-roll.
The dance of the hair cell's cilia plays a vital role in hearing, Corey explains. Now an HHMI investigator at MGH and Harvard Medical School, Corey was a graduate student at the California Institute of Technology when he began working with James Hudspeth, a leading authority on hair cells. Together, the two researchers have helped discover how movements of the cilia, which quiver with the mechanical vibrations of sound waves, cause the cell to produce a series of brief electrical signals that are conveyed to the brain as a burst of acoustic information.
In humans and other mammals, hair cell bundles are arranged in four long, parallel columns on a gauzy strip of tissue called the basilar membrane. This membrane, just over an inch long, coils within the cochlea, a bony, snail-shaped structure about the size of a pea that is located deep inside the inner ear.
Sound waves generated by mechanical forces, such as a bow being drawn across a string, water splashing on a hard surface, or air being expelled across the larynx, cause the eardrumand, in turn, the three tiny bones of the middle earto vibrate. The last of these three bones (the stapes, or "stirrup") jiggles a flexible layer of tissue at the base of the cochlea. This pressure sends waves rippling along the basilar membrane, stimulating some of its hair cells.
These cells then send out a rapid-fire code of electrical signals about the frequency, intensity, and duration of a sound. The messages travel through auditory nerve fibers that run from the base of the hair cells to the center of the cochlea, and from there to the brain. After several relays within the brain, the messages finally reach the auditory areas of the cerebral cortex, which process and interpret these signals as a musical phrase, a dripping faucet, a human voice, or any of the myriad sounds in the world around us at any particular moment.
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David Corey and James Hudspeth, who played major roles in discovering how the ear's hair cells respond to sound, discuss the cells' conversion of vibrations into nerve signal. In the background, a slide shows hair cells in the inner ear.
Photo: Paul Fetters