Problems in Coclear Micromechanics

作为与NIDCD的理查德·查德威克博士及其团队长期合作的一部分，在过去的一年里，我们在许多项目中发挥了重要作用。我们主要利用原子力显微镜(AFM)完成了覆盖膜(TM)弹性模量的测量，该膜在听力中的作用是至关重要的，但其确切的作用机制仍存在争议。研究发现，TM在径向上是不均匀的，这种特性有利于毛细胞的剪切效率--对感觉细胞的刺激作用。 另一个被提及的问题与哺乳动物耳蜗特有的螺旋形状有关，这一形状多年来一直让听力研究人员感到困惑。各种研究都没有得出结论，有人认为这种形状的唯一目的是将一条长长的通道包装在小体积中。然而，最近的一些研究表明，低频灵敏度与螺旋线中的圈数之间存在很强的相关性。我们的NIDCD合作者感兴趣的是研究一个简化的螺旋耳蜗数学模型，以研究形状对沿螺旋流动的各种参数的影响。在这种情况下，计算表明，尽管沿螺旋通道的能量流保持不变，但不断变化的曲率将能量集中到通道外壁。能量聚焦效应向螺旋形尖端逐渐增强，在螺旋形尖端分析低频。这种现象可能会对低频阈值产生深远的影响，从而可能解释哺乳动物螺旋形状的进化效用。我们与伍兹霍尔海洋研究所的海洋哺乳动物专家合作，分析了大量哺乳动物耳蜗的组织切片、CT扫描和其他解剖图像，以建立低频限制与总曲率沿螺旋变化之间的假设相关性。研究发现，螺旋耳蜗几何形状的简单测量与海洋和陆地哺乳动物的低频率限制有着惊人的相关性。这种相关性对听力进化的研究有意义，但它也可能有助于指导未来的人工耳蜗术更有效地实施。

As part of a long-standing collaboration with NIDCD's Dr. Richard Chadwick and his group, we have been instrumental in a number of projects during the past year. Using primarily the atomic force microscope (AFM), we completed the measurements of the elastic modulus of the tectorial membrane (TM) whose role in hearing is critical but exact mechanism of action remains controversial. It was found that the TM is radially inhomogeneous and that such a property is favorable to the shearing efficiency of the hair cells -- the stimulatory action on the sensory cells. Another issue addressed has to do with the characteristic spiral shape of the mammalian cochlea that has mystified hearing researchers for many years. Various studies have been inconclusive and it has been argued that the only purpose of the shape is packaging of a long channel in a small volume. Some recent work, however, showed strong correlation between low frequency sensitivity and number of turns in the spiral. Our NIDCD collaborators were interested in studying a simplified mathematical model of a spiral cochlea to investigate the effect of the shape on various parameters of energy flow along the spiral. In this instance, computations showed that although energy flow along the spiral channel remains constant, the continuously changing curvature concentrates energy toward the outer channel wall. The energy focusing effect becomes progressively stronger toward the spiral apex where low frequencies are analyzed. Such a phenomenon could have profound effect on low frequency thresholds leading to a potential explanation of the evolutionary utility of the spiral shape in mammals. In collaboration with marine mammal experts at Woods Hole Oceanographic Institute we have analyzed histological sections, CT-scans and other anatomical images of a large number of mammalian cochleae to establish the hypothesized correlations between low frequency limits and total curvature change along the spiral. It was found that a simple measure of the spiral cochlea geometry correlates surprisingly well with the low frequency limit of both marine and land mammals. Such a correlation has implications for the study of the evolution of hearing but it may also help guide future cochlear implants toward more effective implementation.