Transcript Anatomy and Physiology of the Ear

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
Sagittal- dividing right
from left
Coronal (Frontal)
-
dividing front from
back
Horizontal -dividing
up from down
The Temporal Bone - Part of the Skull
Temporal Bone:Lateral/Medial Views
The Temporal Bone houses the “Ear”
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
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Helix
Antihelix
Concha
Tragus
Intertragal Notch
Antitragus
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
 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
 Protection
-- medial displacement of ear drum
-- curvature of canal
-- hairs
-- cerumen
-- skin migration
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
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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
1- Malleus
2- Incus
--Ossicles
3- Stapes
4- Tympanic Membrane
(Eardrum)
5- Round Window
6- Eustachian Tube
Middle Ear Muscles 1. 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
 Attaches to Malleus manubrium, E-tube cartilage
 Contraction draws TM medially
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
 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
 sound striking the (relatively large)
tympanic membrane
 is delivered to the (much smaller) stapes
footplate
 Areal Ratio = 18.6 to 1
Increase in FORCE
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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
 Small force (baby’s weight) supports man
 because of the difference in length on either side of
the pivot point
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
 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:
 Band Pass filter
 Resonant Frequency near 1kHz
 Effect can be seen in Minimum Audibility
Curves (MAP/MAF) (Figure 10.2)
Tympanometry
 Acoustic measures of middle ear health
 Made using an immittance (or impedance)
bridge:
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PRESSURE PUMP/MANOMETER
MINIATURE SPEAKER
MICROPHONE
ALL CONNECTED THROUGH A SMALL
PROBE INSERTED IN EAR CANAL
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:
 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:
 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
Tympanogram Types
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:
 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?