ENT CLERKSHIP NOTES HUGH ASHMAN
OTOLOGY The Physiology Of Hearing Sound is a waveform of mechanical energy, which exerts pressure throughout the medium in which it is transmitted. Hearing (the perception of sound) is essential for speech and language development, and is thus a prime prerequisite for communication.The pinna functions as a funnel that collects sound and transmits it to the external auditory canal. We can tell the direction of sound by the fact that sound willtend to be louder in the ipsilateral earand will also arrive earlier at that ear. The external auditory canal amplifies sound in the frequency range 3-4 kHz. This is due to the resonating properties of the external auditory canal.For hearing to occur, sound needs to pass from air in the external canal into the fluid medium of the cochlea, wherein lie the pressure sensors –the hair cells. If sound should pass directly into the cochlea, over 99% of sound energy would be reflected at the interface. To match the impedance (resistance to sound transmission) of air to that of the cochlear fluid, we need an intervening mechanism for sound transmission. The tympanic membrane and the ossicular chain (malleus, incus and stapes) are responsible for "impedance matching‖.A much greater pressure is required to transmit sound through liquid as compared to transmission by air. Pressure equals force divided by area. By the hydraulic principle, the same force that is exerted on the tympanic membrane, when exerted on the footplate of the stapes, will exert a pressure at the footplate that is 13 times greater than that at the tympanic membrane. This occurs because the ratio of the functional area of the tympanic membrane to that of the footplate of the stapes is 13 to 1. The pressure at the footplate of the stapes is further increased by the mechanical advantage gained by the lever systemof the malleus and the incus. The relative lengths of the handle of the malleus to that of the long process of the incus is 1.3 to 1.0. The tympanic membrane and ossicular chain therefore increase the pressure at the footplate of the stapes 17 fold (1.3 times 13). A requirement for maximum sensitivity of the tympanic membrane to sound pressure is equality of pressure in the external auditory canal to that in the tympanic cavity. The eustachian tube is responsible for maintaining the middle ear pressure close to atmospheric pressure. This ensures that energy is not wasted in overcoming any pressure differentialacross the tympanic membrane.
The middle ear is a balanced system i.e. it moves about a fulcrum and the moments about that fulcrum are equal. The ossicles are suspended in the tympanic cavity by very tenuous ligaments. These provide minimal frictional forceand resistance to movement.Sound pressure that reaches the inner ear produces travelling waves in the liquid of the cochlear duct and eventual movement of the basilar membrane. The sensitive receptor hair cells lie in the organ of Corti on the basilar membrane. We can identify the frequency
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