Why do mammals have a flexible three-bone ossicular chain?

描述（由申请人提供）：哺乳动物的中耳在四足动物（哺乳动物、两栖动物、爬行动物和鸟类）中是独一无二的，因为它包含三个不同的听骨（锤骨、母耳和镫骨），它们在鼓膜和耳蜗之间形成一个间接的、通常是灵活的耦合路径，因此大部分听骨肿块集中在远离耳蜗入口轴的地方。这与非哺乳动物四足动物的中耳明显不同，后者的鼓膜运动或多或少是通过杆状小柱结构以直线传递到耳蜗的。关于这种特殊的中耳结构的功能后果和可能的优势，已经提出了许多理论，特别是考虑到哺乳动物能够听到比非哺乳动物高得多的频率（bbb100 kHz vs. <12 kHz），这些理论包括允许小骨在较高频率下采用低惯性振动模式；提供额外的灵活性，以保护耳蜗免受耳道内的高静压或脉冲刺激；利用离轴质量分布和关节灵活性来减少听骨惯性传递到耳蜗，以响应由发声、呼吸、泵血等自生声音引起的头骨振动，从而使更多的注意力集中在对生存至关重要的外部声音上。在这项提议中，哺乳动物听骨链的功能将被测试，使用测量由空气和骨传导刺激引起的3D运动和耳道内正静压和负静压引起的3D位移；并建模，产生解剖学上准确的基于ct的完整中耳3D模型，准确表示听骨关节的灵活性，并可选择结合一个卷曲的耳蜗模型来测试骨传导反应。这些测量和模型将以人类、猫和老鼠的颞骨为基础进行，其中包括：1)天然的听骨链，以及2)通过融合一条来改变其自然运动自由的修改版本

DESCRIPTION (provided by applicant): The mammalian middle ear is unique among tetrapods (mammals, amphibians, reptiles, and birds), in that it contains three distinct ossicles (the malleus, incus, and stapes) that form an indirect and usually flexible coupling path between the eardrum and cochlea, for which the majority of the ossicular mass is concentrated away from the cochlear entry axis. This differs markedly from the middle ears of non-mammalian tetrapods, in which eardrum motions are transmitted to the cochlea more or less in a straight line via a rod-like columella structure. Many theories have been presented as to the functional consequences and possible advantages of this peculiar middle-ear arrangement, especially considering that mammals are capable of hearing to much higher frequencies than non-mammals (>100 kHz vs. <12 kHz), and these theories range from allowing the ossicles to adopt lower-inertia vibrational modes at higher frequencies; to providing added flexibility to protect th cochlea against high static pressures in the ear canal or impulsive stimuli; to leveraging the off-axis mass distribution and joint flexibility to reduce the amount of ossicular inertia transmitted o the cochlea in response to skull vibrations from self-generated sounds due to vocalizations, breathing, pumping blood, etc., and thus allowing more attention to be focused on the external sounds critical for survival. In this proposal, the functioning of the mammalian ossicular chain will be tested, using measurements of 3D motion due to air and bone-conducted stimuli and 3D displacements due to positive and negative static pressure in the ear canal; and modeled, producing anatomically accurate �CT-based 3D models of the full middle ear with accurate representations of ossicular-joint flexibility and the optional incorporation of a coiled cochlear model for testing bone-conduction responses. The measurements will be performed, and models based, on human, cat, and mouse temporal bones, with their: 1) natural ossicular chains, as well as with modified versions that 2) alter its natural freedom of motion by fusing one or both ossicular joints, and that 3) remove its indirect coupling path and off-axis mass distribution by replacing the incus with a columella-like prosthesis directly connecting the long process of the malleus to the footplate or head of the stapes. The measurements and resulting models will be used to comparatively examine the role of middle-ear structure on: 1) sound transmission from the ear canal to the cochlea, especially at higher frequencies; 2) protection of the cochlea; and 3) the transmission of sound to the cochlea via bone conduction. Comparisons across the three proposed scientifically and clinically important mammalian species will help to clarify any functional changes caused by differences in mammalian middle-ear anatomy, while comparisons across the proposed ossicular chain modifications will allow clarification of the functional implications of the structural features unique to mammalian middle ears. The measurements and models with a prosthesis replacing the incus may additionally lead to new ideas for improving the clinical outcomes of middle-ear reconstruction surgeries.