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Understanding the Outer and Middle Ears: Anatomy and Functions

This chapter explores the anatomy and functions of the outer and middle ears. Topics covered include the structures of the outer ear, such as the pinna and external auditory canal, and the middle ear structures, including the ossicles and eustachian tube. The chapter also discusses the functions of the outer and middle ears, such as amplification, filtering, protection, and localization. Clinical tests like tympanometry and acoustic reflexes are also mentioned.

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Understanding the Outer and Middle Ears: Anatomy and Functions

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  1. CHAPTER 6 Outer and Middle Ears

  2. Which Way? • Anterior/Ventral = toward the front • Posterior/Dorsal = toward the back • Lateral = toward the side • Medial = toward midline • Superior = toward upper surface (rostral) • Inferior = toward lower surface (caudal)

  3. Gotta Catch a Plane Sagittal- dividing right from left Coronal(Frontal) -dividing front from back Horizontal-dividing up from down

  4. The Temporal Bone - Part of the Skull

  5. Temporal Bone:Lateral/Medial Views

  6. The Temporal Bone houses the “Ear”

  7. The Outer Ear Consists of: • The Pinna - cartilaginous, highly variable in appearance, some landmarks. • External Auditory Canal (or external auditory meatus) - 2.5 cm tube.

  8. Pinna Landmarks • Helix • Antihelix • Concha • Tragus • Intertragal Notch • Antitragus

  9. External Auditory Canal • lateral portion-cartilage • medial portion-osseous • lined with epidermal (skin) tissue • hairs in lateral part • cerumen (ear wax) secreted in lateral part.

  10. Outer Ear Functions 1 • Amplification / Filtering -- increases sounds between 1500 and 7000 Hz by 10 to 15 dB -- because of the resonance of Concha -- 5000 Hz E.A.Canal -- 2500 Hz

  11. Outer Ear Functions 2 • Protection -- medial displacement of ear drum -- curvature of canal -- hairs -- cerumen -- skin migration

  12. Outer Ear Functions 3 • Localization -- The ability to identify the location of a sound source -- (Will be covered more later)

  13. The Middle Ear:A cleft within the temporal bone • Lining is mucous membrane • Tympanic Membrane separates it from EAC • Eustachian tube connects it to nasopharynx • Also Connected to Mastoid Air Cells

  14. Middle Ear Structures 1- Malleus 2- Incus --Ossicles 3- Stapes 4- Tympanic Membrane (Eardrum) 5- Round Window 6- Eustachian Tube

  15. Middle Ear Muscles1. The Stapedius Attaches to Stapes Contracts in Response to Loud sounds, chewing, speaking Innervated by the Facial (VIIth cranial) nerve

  16. M.E. Muscles2. The Tensor Tympani • Attaches to Malleus manubrium, E-tube cartilage • Contraction draws TM medially

  17. Middle Ear Functions • Impedance Matching -- amplification of sounds to overcome difference in impedance between the air of EAC and the fluid of the inner ear. • Filtering -- resonant frequency is approximately 1000 Hz, functions as bandpass filter. • Acoustic Reflex -- Contraction of Stapedius muscle in response to loud sounds

  18. Middle Ear Function • Impedance Matching is accomplished through pressure increase produced by the middle ear. • From 2 main effects: Reduction in AREA Increase in FORCE

  19. Reduction in AREA • sound striking the (relatively large) tympanic membrane • is delivered to the (much smaller) stapes footplate • Areal Ratio = 18.6 to 1

  20. Increase in FORCE • The malleus and incus act like a lever • Whenever there is a pivot: • Force x Length in = Force x Length out • Force is greater on short side (Think of wheeled luggage) • Malleus manubrium = 1.3 times as long as Incus long process

  21. Leverage • Small force (baby’s weight) supports man • because of the difference in length on either side of the pivot point

  22. Increase in Pressure • Remember that Press. = Force/Area • force is increased 1.3 times • area is decreased 18.6 times • Pressure is increased 24.2 times (27.7 dB)

  23. Other Key Middle Ear Function • Oval Window Isolation-- Sound striking the tympanic membrane is delivered through the ossicular chain to the oval window • Without the middle ear, both the oval and round windows would receive sound energy and energy would cancel out.

  24. Middle Ear Filtering: • Band Pass filter • Resonant Frequency near 1kHz • Effect can be seen in Minimum Audibility Curves (MAP/MAF) (Figure 10.2)

  25. Tympanometry • Acoustic measures of middle ear health • Made using an immittance (or impedance) bridge: • PRESSURE PUMP/MANOMETER • MINIATURE SPEAKER • MICROPHONE • ALL CONNECTED THROUGH A SMALL PROBE INSERTED IN EAR CANAL

  26. Compliance: opposite of stiffness. • middle ear system is not massive, largely a stiffness-controlled system. • Changes in stiffness/compliance have large effects on functioning of system. • at point where air pressure in canal and middle ear are equal the most sound will be conducted through.

  27. Tympanogram: • A plot of middle ear compliance as a function of ear canal pressure • Pressure is swept from +200 to -200 or -400 dPa • Should see peak at point where pressures are equal

  28. Tympanogram types: • A: peak between +100 and -200 dPa: normal • C: peak beyond -200 dPa: neg pressure • B: no peak flat tymp: effusion • As: peak but shallow: stiff: otosclerosis • Ad: peak off scale: floppy: dysarticulation

  29. Tympanogram Types

  30. The Acoustic Reflex • Stapedius contraction measured as change in compliance • Reflex arc: • peripheral ear, • VIIIth n. • Cochlear nucleus • Superior olivary complexes • VIIth n. to the middle ear • Reflex is bilateral.

  31. Clinical Tests using Acoustic Reflexes: • A.R. Threshold: how intense sound must be to elicit the reflex? • A.R. Decay: Is the degree of a contraction maintained throughout a 10 second stimulus?

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