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CHAPTER 6. Outer and Middle Ears. 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). Gotta Catch a Plane.

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chapter 6

CHAPTER 6

Outer and Middle Ears

which way
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)
gotta catch a plane
Gotta Catch a Plane

Sagittal- dividing right from left

Coronal(Frontal) -dividing front from back

Horizontal-dividing up from down

the outer ear consists of
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.
pinna landmarks
Pinna Landmarks
  • Helix
  • Antihelix
  • Concha
  • Tragus
  • Intertragal Notch
  • Antitragus
external auditory canal
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.
outer ear functions 1
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

outer ear functions 2
Outer Ear Functions 2
  • Protection

-- medial displacement of ear drum

-- curvature of canal

-- hairs

-- cerumen

-- skin migration

outer ear functions 3
Outer Ear Functions 3
  • Localization

-- The ability to identify the location of a sound source

-- (Will be covered more later)

the middle ear a cleft within the temporal bone
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
middle ear structures
Middle Ear Structures

1- Malleus

2- Incus --Ossicles

3- Stapes

4- Tympanic Membrane (Eardrum)

5- Round Window

6- Eustachian Tube

middle ear muscles 1 the stapedius
Middle Ear Muscles1. The Stapedius

Attaches to Stapes

Contracts in Response to Loud sounds, chewing, speaking

Innervated by the Facial (VIIth cranial) nerve

m e muscles 2 the tensor tympani
M.E. Muscles2. The Tensor Tympani
  • Attaches to Malleus manubrium, E-tube cartilage
  • Contraction draws TM medially
middle ear functions
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
middle ear function
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

reduction in area
Reduction in AREA
  • sound striking the (relatively large) tympanic membrane
  • is delivered to the (much smaller) stapes footplate
  • Areal Ratio = 18.6 to 1
increase in force
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
leverage
Leverage
  • Small force (baby’s weight) supports man
  • because of the difference in length on either side of the pivot point
increase in pressure
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)
other key middle ear function
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.
middle ear filtering
Middle Ear Filtering:
  • Band Pass filter
  • Resonant Frequency near 1kHz
  • Effect can be seen in Minimum Audibility Curves (MAP/MAF) (Figure 10.2)
tympanometry
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
compliance opposite of stiffness
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.
tympanogram
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
tympanogram types
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
the acoustic reflex
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.
clinical tests using acoustic reflexes
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?