Hearing Science Exam Two

What are the four middle ear contributions to hearing?

Impedance Polarization Function

Ventilation and Pressurization Protection and Distortion Reduction

What is each middle ear contribution designed to do?

solve a problem

What problem is the middle ear polarization function trying to solve?

how to allow for motion of the cochlear liquids inside closed capsule (if no fluid motion then no travelling wave)

What is the solution for middle ear polarization function?

two windows to the inner ear, with mechanical stimulation at the oval window

What does the round window do?

the round window reciprocates response

-reciprocating allows for displacement of cochlear

liquids, causing wave action

Middle Ear Polarization Function

reciprocating action between the oval window (stimulated by rocking motion of footplate of stapes) and round window (uncovered membrane below the oval window)

What problem is the middle ear ventilation and pressurization trying to solve?

tympanic cavity is closed (air-filled cavity)

What is the solution for middle ear ventilation and pressurization?

periodically open the airway via the eustachian tube

When does the eustachian tube open to allow for an open airway?

during swallowing, yawning, and chewing

-both tensor veli pallatini and levator veli palatini muscles are used

-opening at torus tubarious in nasopharynx

-place TM in proper postition

-prevents unhygienic environment

What muscles are used during swallowing, yawning, and chewing?

tensor veli palatini and levator veli palatini muscles

Eponym

a proper noun (a person's name) that has become the name of a thing

Who is Bartolommeo Eustachio?

he discovered the tube that connects the middle ear to the nasopharynx in 1561

-Italian anatomist

Eustachian Dysfunction

the inability to equalize middle ear and atmospheric pressure is an important factor in the pathogenesis of middle ear disease

Pathogenesis

mechanism that causes disease

What does eustachian tube dysfunction lead to?

lack of middle ear aeration

What does a lack of middle ear aeration cause?

-Tympanic membrane retraction

-Chronic otitis media (chronic ear infections)

-Hearing loss with language delay in children

-Chronic tympanic membrane perforations

-Cholesteatoma

Cholesteatoma

an abnormal skin

growth in the middle ear, if allowed to

increase in size it may destroy the ossicular chain, and in rare cases it may also result in a permanent hearing loss, dizziness, and facial muscle paralysis

What problem is the middle ear protection and distortion reduction trying to solve?

protect inner ear from mechanical over-stimulation

What is the solution for middle ear protection and distortion reduction?

increase of ossicular chain in order to reduce the low-frequency sound intensity that reaches the cochlea

What mechanism is used for middle ear protection and distortion control function?

the acoustic reflex

Acoustic Reflex

stimulated by eating, talking, yelling, other vocalizations, and exposure to high level sounds

-primarily, the stapedius muscle is contracted which stiffens the ossicular chain and reduces the sound reaching the cochlea

Acoustic Reflex Pathway

What are the parts of the acoustic reflex pathway?

superior olivary complex (SOC)

ventral cochlear nucleus (VCN)

inner ear (IE)

facial nerve (FN)

stapedius

What is the descending path of the acoustic reflex pathway?

descending path from brainstem to stapedial muscle via VIIth (facial) nerve

What is the ascending path of the acoustic relfex pathway?

ascending path from cochlea to brainstem via VIIIth nerve

What happens when the muscle contracts?

stiffens up the whole ossicular chain (and sometimes the eardrum)

-reduces low frequency sounds

How does the acoustic reflex affect the transmission of sound from the ear canal to the cochlea?

reduces the low frequency sounds (low frequency sounds are attenuated)

What does the acoustic reflex pathway do?

reduce the amount of low frequency vibrations that the oval window is being beaten by the stapes

Measuring the Acoustic Reflex Response

as the stimulus increases, sound energy decreases flow into the middle ear

What is the y-axis of the acoustic reflex response?

admittance or the flow of sound energy into the middle ear

What is the x-axis of the acoustic reflex response?

time

Middle Ear Protection and Distortion Reduction

the acoustic reflex threshold is determined in order to measure the function of the acoustic reflex pathways

-it is also used in part to determine the site of lesion for auditory disorders

Which sounds are reduced more?

low-frequency sounds are attenuated (or reduced) more so than high-frequency sounds

If a sound is played at a high enough level...

it will reduce a low frequency sound and let high frequency sounds pass

What is the maximum benefit loss for low frequencies?

20-30 dB

Is the middle ear protection/distortion control function a form of hearin protection?

-fatigability of the stapedius reflex in industrial noise

-effect of stapedius reflex on the auditory system of rabbits following exposure to industrial noise

-reduced stapedius reflex during exercise

Fatigability of the Stapedius Reflex in Industrial Noise

11 normal hearing emloyees at a shipbuilding yard for 7 hours, noise level 89-103 dBA

-one ear exposed the other ear protected

-stapedius reflex was measured before and after noise exposure

Does the stapedius still function in response to the highest level sounds?

yes, in the shipbuilding yard at the end of the 7 hour work period

-stapedius reflex did not fatigue (give out)

-involves the stapedius muscle and nerves to the brain

Effect of stapedius reflex on the auditory system of rabbits following exposure to industrial noise

-Rabbits, one ear with and one ear without functioning middle ear muscles (MEMs) were exposed to industrial noise

-Following the exposure, the ear without functioning MEMs had more extensive damage than the ear with functional MEMs

Effect of the Stepdius on the Auditory System

-right ear: functioning middle ear muscles

-left ear: non-functioning middle ear muscles (greater hearing loss)

-non-functioning middle ear muscles = low frequency loss

"Hearing loss"...

estimated from auditory brainstem response thresholds following industrial exposure

Brain Stem Audiometry

electrodes on bunny's head, plays sound to see when the nerves fire, measures if the sound was transmitted at the level of the brain stem (reached the bunny's brain stem)

Reduced Stapedius Reflex (SR) During Exercise

-10 normal hearing males, aged 27-34

-SR measured before, during and after "submaximal" exercise (10 minutes on an ergometer cycle, 50% work capacity)

-Exercise depresses the stapedial reflex and increases the risk of a temporary threshold shift (temporary hearing loss)

Where are high frequency sounds received?

at the base of the cochlea

Where are low frequency sounds received?

at the apex of the cochlea

The cochlea turns around the...

modiolus

Where do the cochlear nerves connect?

the inner and outer hair cells

Where do the cochlear nerves go?

through the habenula perforata, and into the modiolus

What makes up the VIIIth nerve?

nerve fibers (when they come out of the modiolus, they are the VIIIth nerve)

What makes up the VIIIth nerve?

bipolar neurons

The VIIIth nerve is made up of bipolar neurons. Where does it start and end?

Base of hair cells, through the habenula perforata, into the modiolus (cell body is within/connects to cell body), becomes the VIIIth nerve, connects to cochlea nucleus

What happens to the nerve fibers?

the nerve fibers come out of base of hair cells, all go through habenula perforata, meet the cell bodies in the modiolus, forms the VIIIthe nerve, VIIIth nerve is connected to cochlea nucleus

What part of the cochlea is closest to the modiolus?

the medial aspect of the cochlea

Where is the tectorial membrane located?

on top of the stereocilia

What covers the tops of the hair cells?

reticular lamina (thin layer of connective tissue)

What is the modiolus closest to, inner or outer hair cells?

inner hair cells

Inner Hair Cell

-peak or flask shaped

-approx. 3,500

-single row

stereocilia not attached to the tectorial membrane

-stereocilia in cresent shape

-centralized nucleus

-organelles distributed throughout the cell body

More on the Inner Hair Cell

-95% of the afferent neurons

-many afferent neurons (inner radial fibers) connect to each IHC

-afferent neurons synapse with cell body, efferent neurons synapse with the afferent neurons

-no motility

Outer Hair Cell

-cylindrical shape

-approx. 12,000

-3 rows

-stereocilia attached under the tectorial membrane

-stereocilia in W or V shape

-nucleus found in base

-organelles found along the outer walls

More on the Outer Hair Cell

-5% of afferent neurons

-each afferent neuron (outer spiral fibers) connects to many OHCs

-efferent and afferent neurons synapse directly with cell body

-OHCs "stretch and shrink" this is called OHC motility

What is an afferent nerve fiber?

goes towards the brain

Afferent neurons synapse with...

inner hair cells

Efferent neurons synapse with...

afferent neurons

What synapses directly with the outer hair cells?

efferent and afferent neurons

What does each afferent neuron (outer spiral fibers) connect to?

many outer hair cells

Type II Fibers

associated with outer hair cells

-slow twitch

Type I

associated with inner hair cells

-fast twitch

Many Type I fibers synapse...

with one inner hair cell directly opposite their habenular opening

What makes the outer hair cells contract?

efferent nerve stimulation

Features of the Cochlea

-mechanical (vibratory motion)

-hydraulic (wave motion)

-chemo-electrical (nerve energy)

Membranous Labyrinth

filled with endolymph

What are the three major anatomical components of the inner ear?

semi-circular canals

vestibule (saccule and utricle)

cochlea

Semi-Circular Canals

sense movement of the head, both speed and direction

-three fluid-filled tubes

How many semi-circular canals are there?

three

Vestibule

utricle and saccule are used to detect the orientation of the head

Cochlea

closed, labyrinthine (maze-like) capsule, filled with fluid

-most anterior structure

-two 5/8th turns, large bundle of nerve cells enters the center (auditory branch of the VIIIth nerve)

Modiolus of the Cochlea

bony center of cochlea

-the bony canal turns around the modiolus

-"continuous left turn" (spiral staircase)

-walls are solid bone

The auditory nerve fibers from the hair cells...

enter the modiolus

Labyrinth of the Cochlea

-2 5/8th turns

-35 mm length base to apex

-ends at the helicotrema in the apex

Where is the base of the cochlea?

near the stapes footplate

Where is the apex of the cochlea?

other end of the bony labyrinth

Story through the Cochlea

-stapes footplate removed, you see oval window

-through oval window, you would be in the Scala vestibuli

-go through Scala vestibuli, you are in the helicotrema (at apex)

Helicotrema = where the scala vestibuli and scala tympani

-past helicotrema, you are now in lower level (scala tympani)

-down spiral staircase, to round window (the base)

Osseous Spiral Lamina

bony extension of the medial wall of the bony labyrinth

-runs continuously along the medial wall

-gets skinnier as it goes from the base to the apex

The width of the osseous spiral lamina _________ between the base and the apex of the cochlea.

decreases

Why is the change in width necessary?

-basilar membrane is narrow at the base because it is responsible for detecting high frequencies

-basilar membrane is wider at apex because it is responsible for detecting low frequencies

By the time the cochlea reaches its third turn, the osseous spiral lamina has nearly ___________.

disappeared

What are the three sections of the scala?

scala vestibuli

scala tympani

scala media

Scala Vestibuli

-bounded inferiorly by Reissner's membrane

-ends at helicotrema

-contains perilymph

Reissner's Membrane

-floor of scala vestibuli

-roof of scala media

Perilymph

fluid around scala media (fills scala vestibuli and scala tympani)

The scala vestibuli is bounded inferiorly by...

Reissner's membrane

Scala Tympani

-bounded superiorly by the basilar membrane

-ends at helicotrema

-contains perilymph

The width of the basilar membrane ___________ between base and apex of cochlea.

increases

Tonotopic

pertains to the way in which the primary auditory cortex is organized so that neurons that respond to particular frequencies are grouped together

The scala tympani is bounded superiorly by the...

basilar membrane

Scala Media

-bounded superiorly by Reissner's membrane

-bounded inferiorly by basilar membrane

-contains Organ of Corti

-contains endolymph

The scala media is bounded superiorly by...

Reissner's membrane

How do the scala vestibuli and scala tympani share perilymph?

at the helicotrema

What keeps the endolymph in the scala media?

reissner's membrane and basilar membrane

Gross Anatomy of Scala Media

-Reissner's Membrane

-Basilar Membrane

-Osseous Spiral Lamina

-Spiral Limbus

-Tectoral Membrane

-Organ of Corti

-Stria Vascularis