Martin P, Hudspeth A J
Howard Hughes Medical Institute, Laboratory of Sensory Neuroscience, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Proc Natl Acad Sci U S A. 1999 Dec 7;96(25):14306-11. doi: 10.1073/pnas.96.25.14306.
To enhance their mechanical sensitivity and frequency selectivity, hair cells amplify the mechanical stimuli to which they respond. Although cell-body contractions of outer hair cells are thought to mediate the active process in the mammalian cochlea, vertebrates without outer hair cells display highly sensitive, sharply tuned hearing and spontaneous otoacoustic emissions. In these animals the amplifier must reside elsewhere. We report physiological evidence that amplification can stem from active movement of the hair bundle, the hair cell's mechanosensitive organelle. We performed experiments on hair cells from the sacculus of the bullfrog. Using a two-compartment recording chamber that permits exposure of the hair cell's apical and basolateral surfaces to different solutions, we examined active hair-bundle motion in circumstances similar to those in vivo. When the apical surface was bathed in artificial endolymph, many hair bundles exhibited spontaneous oscillations of amplitudes as great as 50 nm and frequencies in the range 5 to 40 Hz. We stimulated hair bundles with a flexible glass probe and recorded their mechanical responses with a photometric system. When the stimulus frequency lay within a band enclosing a hair cell's frequency of spontaneous oscillation, mechanical stimuli as small as +/-5 nm entrained the hair-bundle oscillations. For small stimuli, the bundle movement was larger than the stimulus. Because the energy dissipated by viscous drag exceeded the work provided by the stimulus probe, the hair bundles powered their motion and therefore amplified it.
为了增强其机械敏感性和频率选择性，毛细胞会放大它们所响应的机械刺激。尽管外毛细胞的胞体收缩被认为介导了哺乳动物耳蜗中的主动过程，但没有外毛细胞的脊椎动物却表现出高度敏感、调谐尖锐的听力和自发性耳声发射。在这些动物中，放大器必定位于其他地方。我们报告了生理学证据，表明放大作用可能源于毛束（毛细胞的机械感受细胞器）的主动运动。我们对牛蛙球囊中的毛细胞进行了实验。使用一个双隔室记录室，使毛细胞的顶端和基底外侧表面暴露于不同溶液中，我们在类似于体内的情况下检查了毛束的主动运动。当顶端表面浸泡在人工内淋巴中时，许多毛束表现出自发振荡，振幅高达50纳米，频率在5至40赫兹范围内。我们用一个柔性玻璃探针刺激毛束，并用一个光度系统记录它们的机械响应。当刺激频率处于包含毛细胞自发振荡频率的频段内时，小至±5纳米的机械刺激就能带动毛束振荡。对于小刺激，毛束的运动大于刺激。由于粘性阻力耗散的能量超过了刺激探针提供的功，毛束为其运动提供动力，因此放大了该运动。
