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Photo of Dr. Iwasa

Kuni H. Iwasa, Ph.D.

Section on Biophysics
Laboratory of Cellular Biology

50 South Drive MSC 8027
Room 4–4152
Bethesda, MD 20892
Phone: (301) 496–3987
Fax: (301) 480–0827
Experimental configuration

Experimental configuration
(for details, see Dong et al. 2002)

Research Statement

The Section on Biophysics studies biophysical properties of sensory cells in the auditory and vestibular systems.

Recent Accomplishments

Of the properties of cochlear hair cells, the sensory cells in the ear that convert sound-induced mechanical vibration into electrical signal, reverse transduction, in which hair cells act as a motor, appears the most paradoxical. This function is essential for the sharp frequency discrimination by the ear because the tuning mechanism is based on mechanical resonance, which pumps energy into mechanical vibration. Mammalian outer hair cells have a voltage-dependent motor in the cell body. Non-mammals, which do not have outer hair cells, reverse transduction must be carried out by hair bundles. It has been shown that hair bundles can have negative stiffness. We have found that negative stiffness requires cooperative interactions between mechanotransducer channels in a hair bundle.

Current elicited by stretching outer hair cell Current elicited by stretching outer hair cell
(for details, see Dong et al. 2002)
For clarifying the mechanism of the membrane motor in outer hair cells, we have been testing our hypothesis called the "area motor model." This model proposes that the hair cell motor has an electric charge that is transferable across the membrane and that charge transfer is coupled with changes in the membrane area of the motor. We found such a mechanism theoretically belongs to a class of piezoelectricity in that energy conversion is direct and reciprocal. Namely, the motor converts mechanical energy back into electrical energy in a symmetric manner. We experimentally showed that the piezoelectric reciprocity is satisfied, demonstrating the piezoelectric nature of the motor.

Force produced by outer hair cells depends on the voltage oscillation (receptor potential) in the cells due to transducer current in hair bundles. Studies on the electric properties of these cells indicate that the receptor potential is highly attenuated by the hair cell's intrinsic electric circuit (RC filter). As the result, receptor potential appeared too small to affect vibration in the cochlea. This is known as the RC time constant problem. We found that the receptor potential at the resonance frequency is not heavily attenuated up to 10 kHz because piezoelectric resonance can overcome the cells' RC filter. The frequency limit arises from the condition because that force produced by outer hair cells needs to cancel out viscous drag. For frequencies higher than 10 kHz, we expect that fast potassium channels enhance the receptor potential.


Lab Personnel

Ghanshyam Sinha (Send e-mail)
Jie Fang (Send e-mail)
Bora Sul (Send e-mail)

Selected Publications


National Institute on Deafness and Other Communication Disorders. Celebrating 20 years of research: 1988 to 2008