anatomy and physiology of the ear §the temporal bone §outer ear §middle ear §inner ear...
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Anatomy and Physiology of the Ear
The Temporal BoneOuter Ear Middle EarInner EarCochlear Physiology
Which Way?
Anterior/Ventral = toward the front Posterior/Dorsal = toward the backLateral = toward the sideMedial = toward midlineSuperior = 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 LandmarksHelixAntihelixConchaTragus Intertragal NotchAntitragus
External Auditory Canal
lateral portion-cartilagemedial portion-osseous lined with epidermal (skin)
tissuehairs in lateral partcerumen (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
Lining is mucous membraneTympanic Membrane separates it from EACEustachian tube connects it to nasopharynxAlso 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
Middle Ear FunctionsImpedance 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
The malleus and incus act like a leverWhenever there is a pivot:Force x Length in = Force x Length outForce 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 manbecause of the difference in length on either side of
the pivot point
Increase in Pressure
Remember that Press. = Force/Areaforce is increased 1.3 timesarea 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 1kHzEffect 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 healthMade 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.
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 dPaShould see peak at point where pressures are
equal
Tympanogram types:
A: peak between +100 and -200 dPa: normal
C: peak beyond -200 dPa: neg pressureB: no peak flat tymp: effusionAs: peak but shallow: stiff: otosclerosis
Ad: peak off scale: floppy: dysarticulation
Tympanogram Types
The Acoustic ReflexStapedius contraction measured as change
in complianceReflex arc:
peripheral ear, VIIIth n. Cochlear nucleus superior olivary complex 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?
Two Halves:Vestibular--transduces motion and pull of gravityCochlear--transduces sound energy
(Both use Hair Cells)
INNER EAR
Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)
Cochlea is Divided into 3 “Scala”
Scala Vestibuli Reissner’s Membrane
Scala Media Basilar Membrane
Scala Tympani
Helicotrema - the opening between 2 outer Scala
Fluids filling the Inner Ear
Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV)
Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (85 mV)
Cross-Section of the Cochlea
Third Turn
Second Turn
First Turn
A Cross Section Shows the 3 Scala
Within S. Media is the Organ of Corti
I = Inner Hair Cells P = Pillar Cells
O = Outer Hair 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 acoustical-mechanical 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.
Mechanical Electro-chemicalOuter Hair Cells have both forward and
reverse transduction--
Mechanical Electro-chemical
Mechanical 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