Rep Prog Phys. 2014 Jul;77(7):076601. doi: 10.1088/0034-4885/77/7/076601. Epub 2014 Jul 9.
Most sounds of interest consist of complex, time-dependent admixtures of tones of diverse frequencies and variable amplitudes. To detect and process these signals, the ear employs a highly nonlinear, adaptive, real-time spectral analyzer: the cochlea. Sound excites vibration of the eardrum and the three miniscule bones of the middle ear, the last of which acts as a piston to initiate oscillatory pressure changes within the liquid-filled chambers of the cochlea. The basilar membrane, an elastic band spiraling along the cochlea between two of these chambers, responds to these pressures by conducting a largely independent traveling wave for each frequency component of the input. Because the basilar membrane is graded in mass and stiffness along its length, however, each traveling wave grows in magnitude and decreases in wavelength until it peaks at a specific, frequency-dependent position: low frequencies propagate to the cochlear apex, whereas high frequencies culminate at the base. The oscillations of the basilar membrane deflect hair bundles, the mechanically sensitive organelles of the ear's sensory receptors, the hair cells. As mechanically sensitive ion channels open and close, each hair cell responds with an electrical signal that is chemically transmitted to an afferent nerve fiber and thence into the brain. In addition to transducing mechanical inputs, hair cells amplify them by two means. Channel gating endows a hair bundle with negative stiffness, an instability that interacts with the motor protein myosin-1c to produce a mechanical amplifier and oscillator. Acting through the piezoelectric membrane protein prestin, electrical responses also cause outer hair cells to elongate and shorten, thus pumping energy into the basilar membrane's movements. The two forms of motility constitute an active process that amplifies mechanical inputs, sharpens frequency discrimination, and confers a compressive nonlinearity on responsiveness. These features arise because the active process operates near a Hopf bifurcation, the generic properties of which explain several key features of hearing. Moreover, when the gain of the active process rises sufficiently in ultraquiet circumstances, the system traverses the bifurcation and even a normal ear actually emits sound. The remarkable properties of hearing thus stem from the propagation of traveling waves on a nonlinear and excitable medium.
大多数有意义的声音都包含复杂的、随时间变化的、不同频率和可变幅度的混合音。为了检测和处理这些信号，耳朵使用了一种高度非线性、自适应、实时的频谱分析仪：耳蜗。声音激发鼓膜和中耳三个微小骨头的振动，后者作为活塞，在充满液体的耳蜗腔室内引发振荡压力变化。基底膜是一种弹性带，沿着耳蜗的两个腔室螺旋状延伸，它对这些压力的反应是为输入的每个频率分量产生一个基本独立的行波。然而，由于基底膜沿其长度在质量和刚度上是分级的，因此每个行波的幅度增大，波长减小，直到在特定的、频率相关的位置达到峰值：低频传播到耳蜗顶端，而高频则在基底结束。基底膜的振动使毛细胞的机械敏感细胞器毛束发生偏转。当机械敏感的离子通道打开和关闭时，每个毛细胞都会产生一个电信号，该信号通过化学方式传输到传入神经纤维，然后传入大脑。除了转换机械输入外，毛细胞还通过两种方式放大它们。通道门控赋予毛束负刚度，这种不稳定性与肌球蛋白-1c 相互作用，产生机械放大器和振荡器。通过压电膜蛋白 prestin 的作用，电反应也使外毛细胞伸长和缩短，从而将能量泵入基底膜的运动中。这两种运动形式构成了一个主动过程，该过程放大了机械输入，提高了频率分辨力，并赋予了响应的压缩非线性。这些特性的产生是因为主动过程在 Hopf 分岔附近运行，Hopf 分岔的一般特性解释了听觉的几个关键特征。此外，当主动过程的增益在超静音环境中足够高时，系统会穿越分岔，甚至正常的耳朵实际上也会发出声音。因此，听觉的惊人特性源于非线性和兴奋介质上行波的传播。
