장소: 제1과학관 31214  e+첨단강의실 

초록:

Hair cells of the vertebrate inner ear are the mechanotransducers which have been proposed to amplify weak signals by generating active forces. While amplification in the mammalian cochlea is widely believed to originate from the membrane dynamics involving outer hair cell motility, non-mammalian vertebrates lack outer hair cells. Nevertheless the ear of lower vertebrates achieves acute hearing. The exact mechanism is not yet clearly known. Hair bundle motility probably underlies the amplification process. Unlike mammalian hair cells, spontaneous oscillations have been observed in individual hair cells of turtles and frogs. However, in recent experiments under more natural conditions than previously studied, frog hair bundles with an overlying membrane are found not to spontaneously oscillate, but are in fact quiescent.

 Here we introduce our recent theory[1] on the auditory transduction of the bullfrog sacculus, where  the coupling of hair bundles can induce the cessation of the spontaneous oscillation and enhanced signal-to-noise ratio. Using numerical simulations of models of bullfrog sacculi, we investigate the mechanotransduction properties of inhomogeneous hair bundles with elastic coupling and mechanical loading. We show that when inhomogeneous hair bundles are coupled with sufficient strength, all the spontaneous oscillations are suddenly quenched. This phenomenon is called amplitude death – the cessation of oscillation due to the coupling of oscillators, which was first noted in the 19th century by Rayleigh, who found that adjacent organ pipes can suppress each other's sound. We find that there exists an optimal value of the mass of the overlying membrane which gives the maximum signal-to-noise ratio. The hair bundles in this optimal condition turn out to be in the region where amplitude death is seen, which indicates the hair bundles are likely to exploit amplitude death for signal transmission.

[1] Ahn K-H. 2013 Enhanced signal-to-noise ratios in frog hearing can be achieved through amplitude death.  J R Soc Interface 10: 20130525.

Figure 1: Schematic figure for elastically coupled hair bundles with mechanical loading. (a)~(f) The power spectra of mechanical displacement for various inter-bundle coupling strength k and the stimulus amplitude F when 6 Hz signal is applied.