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How does the cochlea respond to different frequencies of sound?
Frequency of a sound refers to the number of cycles of sound waves in one second Humans the audible range is between 20Hz to 20000 Hz Ability to detect these different frequencies allows humans to distinguish between different pitches of sound with a higher pitched sound having a higher frequency Natural speech, sounds contain several different frequencies which vary in amplitude so ability to resolve individual frequency components are needed to understand speech
?????Key structure of the auditory system that plays a key role in distinguishing between different frequencies is the basilar membrane
?????Basilar membrane is found in the cochlea and separates the scala media from the scala tympani
?????Properties of this membrane were first discovered by von Bekesy who observed the motion of membrane by looking at the movement of apex of the cochlea of human cadavers in response to high intensity sounds using light microscopy. He observed the following things o Motion of the vibrating basilar membrane was in the form of a travelling wave with oscillations at the same frequency as the sound o The wave always starts at the base and propagates to the apex o Peak displacement of the membrane is related to the frequency of the sound
?????Observations are supported by the structure of membrane- which is made up of elastic fibres o Thick, floppy apex o Thin stiff base
?????From this von Bekesy concluded that the basilar membrane creates a topographical map across a spectrum of frequencies o Peak displacement for high frequencies occuing at the base o Peak displacements for low frequencies occur at the apex
?????So in complex sounds- each frequency component establishes a peak displacement at a point on the basilar membrane that is almost completely independent to the other components Hair cells
?????The hair cells of the auditory system are embedded in the basilar membrane and are of two types o Inner o Outer
?????Hair cells are also topographically organised as each hair cell is most sensitive to a particular frequency which is known as the characteristic frequency
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