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External & Middle Ear


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Label the Peripheral Auditory System.
A: Temporal bone
B: Malleus
C: Incus
D: Stapes
E: Semi-circular canals
F: Vestibular Nerve
G: Facial Nerve
H: Internal Auditory Meatus
I: Cochlear Nerve
J: Cochlea
K: Eustachian Tube
L: Tympanic Cavity
M: Tympanic Membrane
N: External Auditory Meatus
O: Pinna

What does the pinna do?
-funnels sound to ear canal
-shape filters certain frequencies
-the irregular shape makes it possible to determine the elevation of a sound source.

Where is the...
-crus of helix
-concha (cavum conchae)

See first notes package
What part of the ear canal is osseous and which part is cartilage?
Outer 1/3: cartilage
Inner 2/3: Temporal bone
Functions of the outer ear pinna.
-Collect sound
-Sensitive earlobesss

How does the pinna collect sound?
BEcause of the spreading out of sound energy by inverse square law, a larger receiver will pick up more energy.

These structures amplify human hearing sensitivity by 6-10 dB.

If you lost the outer ear, what would you lost with regards to hearing?
The 20 dB gain for the specific frequency range.
What is the head-related transfer function?
Sound coming to left ear...a drop from high frequencies in right ear because of a head shadow effect.
1. Where does the middle ear begin?
2. What is it filled with?
3. What kind of mechanism does it contain?
4. What bones does it contain?

1. tympanic membrane
2. air
3. lever mechanism to bump up the SPL of certain frequencies.
4. malleus, incus & stapes

What is the middle ear made up of (bones, muscles, etc.)
Tympanic membrane, eustachian tube, malleus, incus, stapes, stapedius muscle, tensor tympani
Label the parts of the ossicular chain.
In first package of notes
How does the motion of the middle ear work?
Sound makes the TM vibrate, which vibrates the ossicles, then the sound waves in air turn into sound waves in fluid in the cochlea.
What is the function of the middle ear? Explain it.
The impedance-matching transformer.
-Water has higher impedance than air, and the cochlea is filled with fluid similar to water. Only 0.1% of sound energy is transmitted (99% is reflected.) and 30 dB are lost.
How does the middle ear overcome the low transmission of the sound energy transmitted through water?
Three ways.
1. Area Advantage: The tympanic membrane is a lot bigger than the oval window and because pressure = force/area, if we have the same force over a smaller area, then pressure will be hgher at output (about 15 to 20x more) 20log(17/1)=25dB = about a 25 dB gain

2. Lever System: Ossicles act as a lever system so output force is higher than the input force (1.3x, up to about 3x) (2.3 dB gain)

3. TM Buckling: The TM buckles when it moves adds 6 dB (2/1 = 6 dB)

Quickly summarize how the total gain across middle ear.
-area difference between TM and oval window (17x)
-Lever action of malleus and incus (1.3x)
-Buckling action of TM during movement (2x)

Total theoretical pressure increase across middle ear is 17 x 1.3 x 2 = 44.2 times

Gain in dB is 20log(44.2/1) = 33dB...but not for all frequencies

At what dB SPL does the stapedius muscle usually contract?
>75 dB SPL
What is the reflex latency of the acoustic reflex?
50 to 150 ms depending on the stimulus
What purpose does the acoustic reflex serve?
Reduces the level of sounds
What is the origin and insertion of the tensor tympani?
Origin: Cartilaginous and bony margins of the eustachian tube.

Insertion: Handle of the malleus

Tensor Tympani: What does it do and what nerve innervates it?
Protects and critically damps ossicular chain.

Trigeminal Nerve

What is the origin and insertion of the stapedius?
Origin: The pyramid (post wall of the middle ear)

Insertion: Neck of stapes

What does the stapedius do and what nerve innervates it?
Protects and critically damps ossicular chain.

Facial nerve (VII) in middle ear

What are the three functions of the eustachian tube?

(And where is it??)


(Joins the middle ear cavity with the back of the nasopharynx.)

What does tympanometry measure?
It measures the intensity of the probe tone in the ear canal. If the ear drum is working correctly, we should see the least amount of sound pressure in the ear canal.
Define impedance & admittance.
Impedance: a measure of the opposition to the flow of energy into a system.

Admittance: the reciprocal of impedance: a measure of the ease with which energy flows into a system. AKA compliance.

What might be the reason for a flat line on a tymp?
No change in air pressure in the ear canal...could be a result of fluid in the middle ear.
What could be the reason for a very high peak on a tymp?
A break in the ossicles.
What are the bone conduction mechanism? (Finish these points.)
a) radiation of sound into the _____
b) inertial movement of the ______
c) Skull vibration causes CSF pressure fluctuations which _____
d) Direct cyclic compression of the ____
a) canal, then via normal route
b) ossicles
c) is transfered to cochlear fluids (a relatively new discovery)
d) cochlear fluids by skull vibration

Does bone conduction behave linearly for audiologic frequencies and signal magnitudes?
If both air conduction and bone conduction are present, how does the cochlea respond?
Responds identically to either modality and sums the two.
How would congenital atresia of the external auditory canal affect hearing?
50 dB hearing loss in the example...because the pinna, concha and external canal added 50 dB
What is a tympanoplasty?
An operation to repair perforations in the tympanic membrane.
Label numbers 1, 2 & 3 on the diagram of the cochlea in the package on page 2
See answers in package
1. What is the vestibule connected to?
2. What does the vestibule contain?
1. The 3 semi-circular canals and the scala vestibuli of the cochlea.
2. The utricle and the saccule.
Label the cochlear ducts and the inner ear schematic on page 2.
Answers on page
Draw the part of the cross-section of the cochlea on page 3.
Look in package.
In the cochlea, how many membranes are there? And how many portions?
Two membranes and three portions.
What are the three portions of the cochlea?
Scala vestibuli, scale tympani & scala media
Label the diagram on page 3 with a star.
Which scala has the highest voltage?
Scala media: 80 mV
Label diagram of the organ of corti
Which part of the basilar membrane narrower and thicker?
the base
What part of the basilar membrane is wider and thinner?
How many are there of the two kinds of hair cells?
Inner hair cells: 3500
Outer hair cells: 12000
Which hair cells have stereocilia?
Both the outer and inner
What type of hair cells have more nerve fibres?
Inner hair cells
How many stereocilia on outer hair cells?
150 hairs in three+ rows.
How many stereocilia on inner hair cells?
40 in two+ rows
What are the two things that join the stereocilia mechanically?
cross bridges and tip links
What does the depolarization of inner hair cells activate?
Ascending afferent neurons
What type of hair cells are V-shaped?
Is the traveling wave motion along the BM sinusoidal?
What does the traveling wave envelope indicate?
The maximum displacement of the BM at each point along the basilar membrane.
On page 7, use the graph to figure out the traveling wave travel time for 200 Hz.
See slide
What is tonotopy (now called cochleotopy)?
Frequency-specific tuning.
On the BM, which tones (high or low) are spaced out and which are squished together?
Low tones = spread out.
High tones = squished together.
True or false?
Te higher the intensity level of the traveling wave, the wider the range of BM being stimulated.
On page 8, without reading my notes, what is this graph showing us?
Increasing the intensity of the stimulus means a larger stimulation at characteristic place and higher frequency places.
What are the differences between the apex and the base of the BM that cause the frequency selectivity?

Base: narrow and thick (stiff)
Apex: Wide & floppy
What is the formula that has to do with resonant frequency and mass and stiffness?
fR is equivalent to the square root of (stiffness/mass).
What would a travelling wave look like for a broadband stimulus, such as a square wave (click)? (At a Low-frequency)
The wave travels down the base to the apex, until the point where that frequency is represented, then dies off. It takes 3-4 msec. See page 10.
Explain the graph with the star on page 10.
Read the tiny print!
What do we mean by frequency selectivity?
Each sound frequency causes maximal displacement/vibration in only a small region of basilar membrane; each region of BM has its own characteristic frequency.
What is resonance?
An enhancement of the level of a sound that occurs when its frequency approaches the natural frequency of vibration of a mechano-acoustic system.
As the physical properties of the BM change from base to apex, so does the...
What is the main reason why the resonant frequency of the BM changes along its length?
Because of changes in stiffness.
The inner hair cells at a particular place are more stimulated when the basilar membrane motion is...
greater at that place.
Why did BM traveling wave envelopes become sharper and sharper over time?
Techniques improved over the years (resulting in healthier cochleas).
What difference do you see if you look at the basilar membrane traveling wave of a damaged vs. healthy cochlea.
The healthy (active) cochlea dramatically increases response for a given input!
What are the fluids in the three sections of the cochlea and what resting potentials do they have?
Scala vestibuli: perilymph: (+) 2-5 mV

Scala Media: endolymph: +80 mV

Scala Tympani: Perilymph (+) 0 mV

1) What is the hair cell intracellular resting potential?
2) Across the top of the hair cells is a potential difference of....
1) -70 mV
2) 80 - (-70) = 150 mV

What is the resting potential of inner vs outer hair cells?
IHC: -40 mV
OHC: -60 to -70 mV
What are the 4 different types of cochlear potentials?
1. resting potential, which is a dc (direct-current potential that exist without acoustic stimulation.)

2. summating potential, also a dc but only appears during acoustic stimulation and it goes in one direction.

3. the cochlear microphonic, which is an ac (alternating-current) potential difference but appears only during acoustic stimulation. (pos neg pos neg)

4. the action potential, which is also a ac potential difference, but is generated by the nerves rather than in the structurss of the inner ear. (8th nerve action potential or neural potential)

What two types of potentials are true COCHLEAR potentials.
The cochlear microphonic and the summating potential.
Is the summating potential dc (direct current) or ac (alternating current)?

Which direction does it go in?

What is the source?

dc (a constant shift in the baseline voltage)

can go in either direction

source is unknown

Is the cochlear microphonic an ac or dc?

What does it do?

What is the source?

What is the level of it?


follows stimulus cycle-by-cycle both in frequency and relative level for moderate levels.

source is outer hair cells near cilia

level is proportional to BM displacement -> can show traveling wave if measured at many points

The cochlear microphonic is proportional to...
BM displacement
The Whole Nerve action potential (AP) is the gross output of...
2) What it is made up of?
3)Where are the neurons that are more likely to fire in synchrony?

1) the cochlea!
2) The sum of the action potentials of many individual auditory neurons firing near simultaneously.
3) The bas of the cochlea and early in the stimulus. Also, if low intensity, a low-freq tone results in a compound action potential from apical region.

What is electrocochleography?
Clinical recording of cochlea/8th nerve potentials.
How does an IHC depolarize?
Relative movement of endolymph causes the IHC stereocilia to move. A movement towards the outer hair cells causes the tip links to stretch, opening ion channels so cell depolarizes.
Are OHCs more active for low-level or high-level sounds?
What are the 4 things that the active mechanism does?
-Increases sensitivity (lowers/improves thresholds)
-Leads to compressive loudness growth
-Increases frequency selectivity
-Produces otoacoustic emissions

What does compression mean?
Makes it not linear.
Unhealthy cochleas become (more or less?) linear?
More. They lose their non-linearity.
How much do OHCs change their length when stimulated, and who was this discovered by?

Have experiments demonstrated an upper limit of frequency?
Should OHCs be able to track 100kHz stimuli?


How do OHCs actively modify vibrations?
Their timely change in length amplifies movement of the BM at a particular place for its own characteristic frequency. (reduces friction so we can get a larger peak.)
What is the source of the active mechanism?
The OHCs
Depolarization of OHC causes ______, and application of _____ to surrounding structures.
1. cell motility
2. force
What are otoacoustic emissions?
Activity of OHCs produces vibration recordable in canal with a sensitive microphone.
Who discovered otoacoustic emissions?
Are otoacoustic emissions a linear or non-linear phenomenon?
What are otoacoustic emissions used for?
-Screen for hearing loss in infants
-Indicates status of OHCs and thus of cochlea.
-Determine whether a hearing loss is cochlear or more central.
-Monitor ototoxic medications
-Assess the effects of noise/music exposure in occupational settings.

What does TEOAE stand for?
transient evoked otoacoustic emission
Tallest stereocilia of OHCs are embedded in...
The underside of the tectorial membrane.
What does DPOAE stand for, and what are they?
Distortion Product Otoacoustic Emissions.
In-the-ear recordings of aural combination tones.
What stimuli are used in DPOAEs?
2. Where does emission occur?
1. f1 and f2 where f2 > f1
2. at fdp = 2f1-f2
If we play these two tones together 528Hz and 704 Hz, what tone will we hear?
352 (2x528 - 704)
What will happen if we play a pitch close to the distortion pitch?

And what will this do to the distortion pitch?


Beating will highlight our perception of the distortion pitch.

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