Keywords: cochlea, round window membrane, basilar membrane, auditory activity, finite element method, coupled vibration.

The mammalian cochlea consists of a fluid-filled duct that is coiled like a snail shell. The vibration of the tympanic membrane which is induced by sound is transmitted to the cochlear fluid through the ossicular chain. The vibration of the cochlear fluid generates the traveling wave on the basilar membrane in the cochlea. Sensory cells which are lined up on the basilar membrane also vibrate in response to the vibration of the basilar membrane. The sensory cells transform the vibration the basilar membrane into action potentials in auditory nerve fibers.

The oval window and the round window are two openings of the cochlea. The oval window is occupied by the footplate of the stapes (one of the ossicles), and vibrations arriving at the tympanic membrane are transmitted through the oval window to the cochlear fluid. The round window is closed by the round window membrane, which moves out when the stapes moves in, and vice versa. This movement of the round window membrane allows the movement of the cochlear fluid and leads the movement of sensory cells on the basilar membrane. Therefore, the mobility of the round window membrane affects auditory activity.

Some studies have investigated the vibration of the cochlea using the finite element method (FEM) [1,2]. However, only intact cochlea has been modeled, and the effects of cochlear diseases on the vibration of the basilar membrane have not been clarified. A round window fistula is one of the major diseases of the cochlea caused by an external wound, sneezing, or diving, etc., and is usually closed by surgery using the fascias or perichondrium. In this case, the stiffness of the round window membrane changes and this change probably affects auditory activity.

In this study, the effect of the stiffness of the round window membrane on auditory activity was analyzed using a three-dimensional finite element model of the human cochlea. Dynamic behavior of the basilar membrane and the cochlear fluid caused by the vibration of the stapes footplate was analyzed, and the effect of Young's modulus of the round window membrane on the vibration of the cochlea was examined. The results suggest that when the round window membrane is more mobile than in a normal situation, its stiffness does not affect the vibration of the basilar membrane. In contrast, if the stiffness of the round window membrane abnormally increases or ossifies, the amplitude of the basilar membrane is suppressed. In other words, the auditory threshold increases if the round window membrane stiffens.

1

F. Bönke, W. Arnold, "3D-finite element model of the human cochlea including fluid-structure couplings", Journal for Oto-Rhino-Laryngology and Its Related Specialties, 61(5), 305-310, 1999.

2

M. Andoh, C. Nakajima, H. Wada, "Phase of neural excitation relative to basilar membrane motion in the organ of Corti: theoretical considerations", J. Acoust. Soc. Am., 118(3), 1554-1565, 2005. doi:10.1121/1.2000770

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