Sound-induced mechanical
vibrations of the middle ear are transmitted into the fluid-filled
cochlea, and generate the movement of the basilar membrane (BM). Then,
sensory cells on the BM transduce mechanical vibrations into electrical
signals (nerve pulses). Thus, the cochlea is functioning as a
mechanoelectrical transducer in the auditory system.
Recently, as shown in Fig.3 of
`Measurement of the BM vibrations', the BM vibration measured directly
in the live cochlea is clarified to be larger than that in the postmortem
cochlea. Therefore, it is certain that the cochlea plays not only as a
transducer but also as an amplifier, and the sensory cells, i.e. outer hair cells (OHCs) in the cochlea are estimated
to be the origin of this amplification.
Although the
above observation has been made, the cochlea mechanics are still unclear,
because the position that provides data about the vibration of the BM is
limited and it is very difficult to measure without damage in the cochlea.
Therefore, in order to understand this mechanism, a three-dimensional
finite element method (FEM) model of a guinea pig cochlea (Fig.1) has been
established.
Figure 2 shows the dynamic animation of
the BM at the frequencies of 2kHz and 6kHz. The peak of the BM vibration
shifts toward the base with increasing frequency. This result suggests
that the BM has the frequency selectivity.
As shown
in Fig.3 of `Analysis of isolated OHC motility using
a high speed video system', OHCs have a function of motility. Hence,
the effect of this function on the BM vibration has been examined (Fig.3).
In the upper figure, the effect of OHC motility is included, and in the
lower figure, the effect is excluded. When the OHC motility is not taken
account of, the amplitude of the BM vibration is small. In contrast, when
the OHC motility is taken into account, the amplitude is much larger than
that of the passive case. This result indicates that the OHCs amplify the
BM vibrations.
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