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CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008

CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008. The Middle Ear. The middle ear. Important Structures: Epitympanic recess Tympanic cavity Aditus ad antrum Mastoid air cells Ossicles. The middle ear. Schematic view of the middle ear boundaries and landmarks.

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CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008

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  1. CSD 3103anatomy of speech and hearing mechanismsHearing mechanismsFall 2008 The Middle Ear

  2. The middle ear Important Structures: • Epitympanic recess • Tympanic cavity • Aditus ad antrum • Mastoid air cells • Ossicles

  3. The middle ear Schematic view of the middle ear boundaries and landmarks

  4. The superior face • Tegmen Tympani

  5. The inferior face • Tympanic plate • Jugular fossa

  6. The medial face • Oval window • Footplate of the stapes • Round window • Promontory

  7. The lateral face • Eardrum

  8. The posterior face • Mastoid wall • Tympanic aditus • Pyramidal eminence • Chorda tympani nerve

  9. The anterior face • Carotid wall • Eustachian tube

  10. General structures of the middle ear • Eardrum • Ossicular chain • Eustachian tube • Middle ear muscles

  11. The ossicles • Malleus (hammer) • Incus (anvil) • Stapes (stirrup)

  12. The malleus • Manubrium • Neck • Head • Lateral process

  13. The malleus

  14. The incus • Short process • Long process • Lenticular process • Incudostapedial joint

  15. The incus

  16. The stapes • Head • Neck • Anterior crus • Posterior crus • Footplate

  17. The stapes

  18. Articulated ossicular chain

  19. Ossicles on a dime

  20. Ligaments of the ossicular chain • Superior malleal ligament • Anterior malleal ligament • Lateral malleal ligament • Posterior incudal ligament

  21. The ossicular chain in place

  22. Purpose of the ossicuar chain • Impedance matching • Protection

  23. Ossicular motion

  24. Vibratory motion of The stapes

  25. Impedance matching of the middle ear • a sound wave traveling in a medium of certain physical properties, namely density and elasticity, will not pass readily into a medium with different properties • the more different the characteristics of the two media are, the more sound energy will be reflected at the boundary

  26. Impedance matching of the middle ear • Acoustic resistance of air: 41.5 ohms • Acoustic resistance of cochlear fluid: 161,000 ohms • This represents a ratio of 3880:1 • Without the impedance matching capabilities of the middle ear, only 1/10 of 1% of the energy of an incoming sound wave would make it into the cochlea--99.9% of the energy would be reflected at the boundary

  27. Area advantage • The area of the tympanic membrane is 17x the oval window • As the area decreases, the pressure increases

  28. Impedance matching of the middle ear • Area advantage • Curved membrane buckling

  29. Curved membrane buckling Notice how the eardrum curves from its rim at both ends to its attachment with the malleus in the middle. This point of the eardrum (V1) doesn’t move as far. This causes an increase in force.

  30. Impedance matching of the middle ear • Area advantage • Curved membrane buckling • Lever action

  31. Lever action advantage • The advantage is increased in (B) when the fulcrum is moved closer to the mass to be lifted.

  32. Purpose of the ossicuar chain • Impedance matching • Protection

  33. Purpose of the ossicuar chain The acoustic reflex Tensor tympani muscle Stapedius muscle

  34. The tensor tympani • Larger of the two tympanic muscles • Tendon leaves the bony wall via the cochleariform process

  35. The stapedius • The smaller of the two tympanic muscles • Tendon leaves the bony wall via the apex of the pyramidal eminence

  36. The acoustic reflex • It is a reflex • Bilateral • Occurs in response to sound intensities delivered to either ear at 80-90 dB above threshold

  37. The eustachian tube • 35-38 mm long • Oriented downward, forward, medialward • Osseous portion • Cartilaginous portion • Isthmus • Tensor palatini muscle

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