Physiology of hearing

• Sound is alternating compression and decompression of medium through which the sound wave passes.
• Frequency is number of sound wave per second.
• Sound waves are passed through air in external ear through solid in middle ear and through liquid in inner ear. Sound wave do not passes from air medium to liquid medium. So these transition is important. During these transition energy is cost, however ear ossicles balances the energy loss.
•  Sound wave are pressure wave that enter the external ear. After crossing the external auditory meatus, the wave reaches the tympanic membrane.

• The air molecule under pressure causes vibration of tympanic membrane. Low frequency sound wave causes slow vibration while high frequency wave causes rapid vibration.
• The vibration of tympanic membrane moves the malleus in middle ear.
• The vibrating malleus produce vibration to incus and vibrating incus moves stapes in and out of oval window causing vibration of perilymph in scala vestibuli.
• Vibration of perilymph are transmitted across the vestibular membrane to endolymph in scala media (cochlear duct) and also up the scala vestibuli and down the scala tympani.
• The vibration of scala tympani are dissipated out of cochlea through round window into Eustachian tube.
• During transmission of vibration from perilymph to endolymph in scala media, the basilar membrane ripples. This ripple is concern with frequency and intensity of sound.
• The vibration causes bending of receptor of hair cells of organ of corti to generate potential.
• These potential excites the cochlear nerves to generate action potential.
• When the hair or microvilli of hair cells are displaced toward the basal body, hair cells get excited and when the hair are displaced away from basal body hair cells are inhibited.
• The nerve impulse from cochlear nerve are conveyed to auditory area of CNS via common vestibule-cochlear nerve. The auditory area is located in temporal lobe where sound is perceived.

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Student of rajiv educational society's gandhi ayurvedic medical college Ron 582201