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Mechanism of hearing
The three auditory ossicles are localised in the middle ear. Of these, the malleus is attached to the TM, and the stapes to the membranous oval window on the medial wall. Incus articulates with these two bones. Thus vibrations of the TM are transmitted to the oval window. As the TM has an area of 90mm2 and the foot plate of the stapes 3.2mm2 and the lever system formed by the ossicles multiplies the force 1.3 times.
The vibrations of the oval window generate pressure waves in the fluid filling the vestibular canal. The pressure waves pass to the median canal and vibrate the basilar membrane. The tympanic canal is connected to a circular membrane called the round window just beneath the oval window. This arragement allows the pressure waves to transmit through the cochlear fluid.
Located on the basilar membrane is the Organ of Corti which contains the auditory receptors. Four rows of hair cells arise from the basilar
The site of maximum distortion in the organ of Corti is determined by the frequency of sound, for high pitched sounds the maximum height of the waves is near the base of the cochlea and for low pitched near the apex. The extent of distortion is determined by the loudness of the sound. Interpretation of these impulses is made in the auditory cortex.
Sound waves � vibrate tymphanic membrane � movements in ear ossicle chain � vibration of oval window � waves in perilymph (scala vestibuli) � waves in endolymph � deflection in the Reissner's membrane � basilar membrane disturbed � bending of hair cells � action potential � transmission by auditory nerve.
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