Martin P, Hudspeth A J, Jülicher F
Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Proc Natl Acad Sci U S A. 2001 Dec 4;98(25):14380-5. doi: 10.1073/pnas.251530598. Epub 2001 Nov 27.
Hearing relies on active filtering to achieve exquisite sensitivity and sharp frequency selectivity. In a quiet environment, the ears of many vertebrates become unstable and emit one to several tones. These spontaneous otoacoustic emissions, the most striking manifestation of the inner ear's active process, must result from self-sustained mechanical oscillations of aural constituents. The mechanoreceptive hair bundles of hair cells in the bullfrog's sacculus have the ability to amplify mechanical stimuli and oscillate spontaneously. By comparing a hair bundle's spontaneous oscillations with its response to small mechanical stimuli, we demonstrate a breakdown in a general principle of equilibrium thermodynamics, the fluctuation-dissipation theorem. We thus confirm that a hair bundle's spontaneous movements are produced by energy-consuming elements within the hair cell. To characterize the dynamical behavior of the active process, we introduce an effective temperature that, for each frequency component, quantifies a hair bundle's deviation from thermal equilibrium. The effective temperature diverges near the bundle's frequency of spontaneous oscillation. This behavior, which is not generic for active oscillators, can be accommodated by a simple model that characterizes quantitatively the fluctuations of the spontaneous movements as well as the hair bundle's linear response function.
听力依赖于主动滤波来实现极高的灵敏度和敏锐的频率选择性。在安静环境中，许多脊椎动物的耳朵会变得不稳定并发出一到几个音调。这些自发耳声发射是内耳主动过程最显著的表现，必定源于听觉成分的自持机械振荡。牛蛙球囊内毛细胞的机械感受性毛束具有放大机械刺激并自发振荡的能力。通过比较毛束的自发振荡与其对小机械刺激的反应，我们证明了平衡态热力学的一个普遍原理——涨落耗散定理的失效。因此，我们证实毛束的自发运动是由毛细胞内的耗能元件产生的。为了表征主动过程的动力学行为，我们引入了一个有效温度，对于每个频率成分，它量化了毛束偏离热平衡的程度。有效温度在毛束的自发振荡频率附近发散。这种行为并非主动振荡器所共有，一个简单的模型可以解释这种行为，该模型定量地描述了自发运动的涨落以及毛束的线性响应函数。
