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Physiology

Explore the intricacies of middle ear physiology, from impedance to sound transmission mechanisms and protective functions. Learn about reactance and resistance, Eustachian tube dynamics, ossicular lever action, and the acoustic reflex. Discover how the middle ear compensates for impedance mismatches and enhances sound transmission to the inner ear.

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Physiology

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  1. Physiology • Middle ear space stiffness dominated: High-frequency emphasis

  2. Impedance Opposition to vibration Two forms: Reactance (X) Resistance (R) Remains same across frequencies

  3. Reactance (X) Two components Due to mass (Xm) Greatest at high frequencies Due to stiffness (Xs) Greatest at low frequencies

  4. Tympanic membrane has non-uniform displacement

  5. Ossicular chain • Stiffness dominated • Axis of movement depends on sound intensity

  6. Eustachian tube • Muscles: Tensor veli palatini and levator veli palatini • Normally closed, opens to equalize pressure

  7. Sound transmission to inner ear Impedance mismatch between air in middle ear and fluid in inner ear To compensate: Area difference between TM and footplate of the stapes (55/3.2 = 17) Ossicular lever action (1.3/1 = 1.3) Buckling movement of TM (2/1 = 2) Total pressure increase = 17 * 1.3 * 2 = 44.2 Total dB increase = 31-33 dB

  8. Possible methods of sound transmission into inner ear Bone conduction: Through bones of the skull Air in the middle ear cavity Through the ossicles: Most effective

  9. Functions of the middle ear Effective sound transmission to inner ear Shields the round window from direct air pressure variations Protective function: Acoustic reflex http://www.neurophys.wisc.edu/animations/

  10. Acoustic reflex Protective function Due to muscle contraction in response to intense sound Threshold of reflex: Around 80 dB SL (sensation level). Reflex results in attenuation of loud sounds by about 10-30 dB More effective at low frequencies (less than 2 kHz) Not very effective for short duration sounds: Latency about 10-150ms

  11. Measurement of acoustic reflex Middle ear muscles contract: Stiffness increases More sound reflected back Lack of acoustic reflex: Implies problem in middle ear muscles/part(s) of acoustic reflex pathway

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