The Middle Ear and Hearing

Key points

• • The middle ear is an air-filled cavity holding the three smallest bones in the body, the malleus, incus and stapes, which link the eardrum to the inner ear.
• • Sound vibrates the tympanic membrane (eardrum), and the ossicles carry and amplify the vibration to the oval window of the fluid-filled cochlea.
• • Because fluid is harder to move than air, the ear boosts the force by focusing the large eardrum onto the small oval window and by the lever action of the ossicles (impedance matching).
• • The Eustachian (auditory) tube connects the middle ear to the throat and equalises the air pressure on the two sides of the eardrum, for example during swallowing.
• • Two small muscles, the tensor tympani and the stapedius, tighten the ossicles in the acoustic reflex, reducing the transmission of sustained loud sounds, though they are too slow to guard against sudden explosive noise.
• • Blocked conduction through the ear canal or middle ear causes conductive hearing loss, whereas damage to the cochlea or nerve causes sensorineural hearing loss.