The Effects of Complex Stapes Motion on the Response of the Cochlea
Alexander M. Huber, Damien Sequeira, Christian Breuninger, and Albrecht Eiber
Otol Neurotol. 2008 Dec;29(8):1187-92
Department of Otorhinolaryngology, Head and Neck Surgery, University
Hospital of Zurich, Zurich, Switzerland (1-2); and Institute of Engineering
and Computational Mechanics, University of Stuttgart, Stuttgart, Germany
HYPOTHESIS: The piston-like motion of the stapes footplate is the only
effective stimulus to the cochlea, and rocking-like stapes motions have
no effect on hearing.
It has been established for several years that the stapes moves both piston-like and rocking-like, the first type being dominant at low frequencies and the second type being more important at middle and high frequencies.
Interestingly, only the piston mode can produce a pressure difference between scala vestibuli and scala tympany and thus generate a traveling wave along the basilar membrane. The rocking mode only shifts fluid locally from one side of the oval window to the other, without influencing the pressure in the scala vestibuli. This mode thus cannot generate a traveling wave. However, it can generate a compressional sound wave in the scala vestibuli that can reach the hair cells directly.
Until now it was unknown if the rocking mode has any influence on the excitation of hair cells. Huber et al. (2008) have now answered this question by a series of well-designed experiments. They succeeded in sufficiently separating the two modes, and they observed that the rocking mode indeed strongly excites the hair cells.
In conclusion, this study adds an important further piece to the huge bulk of evidence (link A, link B) that a cochlear traveling wave is no necessary element in hair cell excitation and that this excitation is most probably caused directly by compressional sound waves in the cochlear fluids. (Comment Martin Braun)