The Inner Ear
The mammalian ear has three major components: the outer, middle, and inner ears (Fig. 20.1A). The outer ear consists of a cartilaginous flange, the pinna, incorporating a resonant cavity that connects to the ear canal and finally to the tympanic membrane. It performs an initial filtering of the sound waves, increasing the sound pressure gain at the tympanic membrane in the 2 to 7 kHz region. It also aids sound localization. Bats, for example, have highly developed pinnae, with a high degree of directional selectivity. Although less efficient in humans, the outer ear accounts for our ability to distinguish whether sounds come from above or below, in front or behind.
The function of the middle ear is to transmit the sound vibrations from the tympanic membrane to the cochlea. Because of the much higher impedance of the cochlear fluid, the middle ear also functions as an impedance-matching device, focusing the energy of the tympanic membraneon the oval window of the cochlea. Ifnot for impedance matching, much of the energy of the sound waves in air would be reflected by the cochlear fluid. This impedance matching is carried out by the ossicles, three small bones, the malleus, incus, and stapes, that connect the tympanic membrane to the oval window. The tympanic membrane has a much higher surface area than the oval window, and the ossicles act as levers that increase the force at the expense of velocity, resulting in the required concentration of energy at the oval window.
KeywordsHair Cell Hopf Bifurcation Tympanic Membrane Outer Hair Cell Basilar Membrane
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