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Anatomy and Physiology of the Ear

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Anatomy and Physiology of the Ear
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The Temporal Bone
Outer Ear
Middle Ear
Inner Ear
Cochlear Physiology
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 пЂЁ
2- Incus
пЂ¬-Ossicles
3- Stapes пЂ§
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
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
Curve (Figure 10.2)
Minimum Audibility Curve
(Figure 10.2)
 Plot of threshold of detection (in dB SPL)
for tones as a function of frequency.
 Shows:
best hearing around 1 kHz
poorer hearing below 500 Hz
and above 4000 Hz
Tympanometry
 Acoustic measures of middle ear health
 Made using an immittance (or impedance)
bridge:
пЃ¬
пЃ¬
пЃ¬
пЃ¬
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:
пЃ¬ peripheral ear,
пЃ¬ VIIIth n.
пЃ¬ Cochlear nucleus
пЃ¬ superior olivary complex
пЃ¬ 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?
INNER
EAR
Two Halves:
 Vestibular--transduces motion and pull of gravity
 Cochlear--transduces sound energy
(Both use Hair Cells)
Subdivision into spaces containing endolymph
(blue), and spaces containing perilymph (red)
Cochlea is Divided into 3 “Scala”
 Scala Vestibuli
пЃ¬
Reissner’s Membrane
 Scala Media
пЃ¬
Basilar Membrane
 Scala Tympani
 Helicotrema - the
opening between 2
outer Scala
Fluids filling the Inner Ear
 Perilymph- in S. Vestibuli and S. Tympani
пЃ¬
пЃ¬
High Sodium / Low Potassium concentrations
Low Voltage (0 to +5 mV)
 Endolymph- in S. Media
пЃ¬
пЃ¬
High Potassium / Low Sodium concentrations
High Positive Voltage (85 mV)
Cross-Section of the Cochlea
Third Turn
Second Turn
A Cross Section Shows the 3 Scala
Within S. Media is the Organ of Corti
I = Inner Hair Cells
O = Outer Hair Cells
P = Pillar Cells
D = Deiter’s Cells
The Stereocilia on IHCs and OHCs
 OHCs (at top)
 V or W shaped ranks
 IHC (at bottom)
 straight line ranks
Cochlear Functions
 Transduction- Converting acousticalmechanical energy into electro-chemical
energy.
 Frequency Analysis-Breaking sound up into
its component frequencies
Transduction Inner Hair Cells are the true sensory
transducers, converting motion of
stereocilia into neurotransmitter release.
Mechanicalпѓћ Electro-chemical
 Outer Hair Cells have both forward and
reverse transduction-Mechanical пѓћ Electro-chemical
Mechanicalпѓњ Electro-chemical
Frequency Analysis-the Traveling Wave
Bekesy studied cochleae from cadavers,
developed the Traveling Wave theory
1. Response always begins at the base
2. Amplitude grows as it travels apically
3. Reaches a peak at a point determined by
frequency of the sound
4. Vibration then dies out rapidly
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