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Harrison's Principles of Internal Medicine, 18e
| Part 2. Cardinal Manifestations and Presentation of Diseases > Section 4. Disorders of Eyes, Ears, Nose, and Throat > |
Chapter 30. Disorders of Hearing
Sections: Disorders of Hearing: Introduction, Further Readings.
Topics Discussed: hearing; hearing disorders.
Excerpt:"Hearing loss is one of the most common sensory disorders in humans and can present at any age. Nearly 10% of the adult population has some hearing loss, and one-third of individuals age >65 years have a hearing loss of sufficient magnitude to require a hearing aid.The function of the external and middle ear is to amplify sound to facilitate conversion of the mechanical energy of the sound wave into an electrical signal by the inner ear hair cells, a process called mechanotransduction (Fig. 30-1). Sound waves enter the external auditory canal and set the tympanic membrane in motion, which in turn moves the malleus, incus, and stapes of the middle ear. Movement of the footplate of the stapes causes pressure changes in the fluid-filled inner ear, eliciting a traveling wave in the basilar membrane of the cochlea. The tympanic membrane and the ossicular chain in the middle ear serve as an impedance-matching mechanism, improving the efficiency of energy transfer from air to the fluid-filled inner ear.The external ear, the external auditory canal, and the middle ear apparatus is designed to collect and amplify sound and efficiently transfer the mechanical energy of the sound wave to the fluid-filled cochlea. Factors that obstruct the transmission..."
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