Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 1 of 33
INNER EAR PHYSIOLOGY AND PATHOLOGY
Resources
Nolte The Human Brain: An introduction to its functional anatomy
Chapter 14 Hearing and Balance: The eighth cranial nerve
Auditory transduction animation
Inner ear case

Differential diagnosis and treatment of hearing loss
Issacson and Vora paper

Common causes of dizziness website
CRITICAL FACTS
(if med school is a Minnesota forest with millions of trees, these are the red pines).
1.
Inner ear receptors are divided into two types; both types
convert mechanical energy into receptor potentials. TYPE I (INNER
HAIR CELLS) are the true sensory receptors that convey information to
the brainstem. TYPE II (OUTER HAIR CELLS) function as biological
amplifiers, essentially acting as motor units.
2.
Inner ear transduction is DIRECTIONAL: displacement toward the
tallest stereocilia (positive deflection) results in DEPOLARIZATION. In
the cochlea, this occurs when the basilar membrane moves toward scala
vestibuli. Negative deflection (toward scala tympani) results in
HYPERPOLARIZATION.
3.
The SEMICIRCULAR
CANALS detect head
rotation (angular
acceleration). The OTOLITH ORGANS (UTRICLE and SACCULE) detect
gravity (linear acceleration). The vestibular system is involved in balance
and posture, co-ordination of head and body movements and in fixating the
visual image on the fovea.
4.
SEMICIRCULAR CANALS WORK IN PAIRS. HORIZONTAL CANALS:
depolarization occurs in the SAME direction as the head rotation. A/P
CANALS: depolarization occurs in the OPPOSITE direction as the head
tilt. The natural pairing is of LEFT ANTERIOR with RIGHT POSTERIOR
CANAL (and vice versa).
Med 6573 Nervous System
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http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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5.
The VESTIBULO-OCULAR REFLEX is a 3 neuron arc (hair cell/vestibular
nerve, vestibular nuclei, cranial nerve motor nuclei) that is used to adjust
eye position to compensate for changes in head position (i.e., it keeps
the visual image centred on the fovea). Remembering the pairings listed in
fact #4, there is depolarization / excitation / contraction in one of the
pathways of the pair, and hyperpolarization / inhibition / relaxation in
the other. Rotation of the head in one direction results in rotation of the
eyes in the opposite direction.
6.
Vestibulospinal reflexes coordinate the position of the head with the
trunk and body, with the goal of maintaining the head in an upright
position during movement. There are two systems.
The LATERAL
VESTIBULOSPINAL SYSTEM is responsible for postural changes to
compensate for tilts and movements of the body. The MEDIAL
VESTIBULOSPINAL SYSTEM stabilizes head position during walking.
The two systems differ in anatomical connections, function and the control
mechanisms that they use to affect alpha motor neuron function. The
function of the VST systems is evident during decerebrate rigidity.
7.
The middle ear transfer function determines the absolute threshold of
hearing at each frequency in normal individuals – the cochlea is so
sensitive, it can transduce any signal that reaches it. This implies that
anything that alters middle ear function (like an infection) will significantly
impact hearing thresholds.
8.
Sound waves pass through the cochlea INSTANTANEOUSLY. The
traveling wave pattern on the basilar membrane is established more
gradually and is INDEPENDENT of how the motion is initiated i.e., don't need
to deliver sound via the oval window --- can use bone!
The traveling wave
establishes a frequency vs. place relationship along the length of the
cochlea, with high frequencies being transduced in the base, and low
frequencies in the apex.
9.
Outer hair cells use their receptor potential to exert force on the basilar
membrane ---thereby generating a POSITIVE FEEDBACK MECHANISM
which amplifies the vibration of the membrane in a nonlinear, highly
frequency specific manner. This force produces its own fluid wave, which
is conducted back through the perilymph, vibrating the middle ear
apparatus and generating sounds that are emitted from the ear
(OTOACOUSTIC EMISSIONS).
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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10. The STRIA VASCULARIS produces the endolymph (high K+) and the
endocochlear potential (+80 mV). Many of the ion transporters of the
stria are the same as those in the kidney, so drugs that affect renal
function are often ototoxic – esp. loop diuretics (which affect the
Na+/K+/2Cl- transporter).
11.
Sounds are localized by the differences in timing and intensity between
the two ears.
Lateral superior olive (LSO) neurons localize high
frequency stimuli by comparing interaural intensity differences (IIDs);
medial superior olive (MSO) neurons use interaural timing differences
(ITDs) to localize low frequency stimuli.
12. NYSTAGMUS consists of a slow drift of the eyes in one direction
(PURSUIT) followed by a rapid recovery movement in the opposite
direction (SACCADE). The direction is named for the fast component
i.e., a RIGHTWARD NYSTAGMUS consists of slow movement of eyes to
the left, followed by fast recovery to the right. The PURSUIT is
controlled by vestibulo-ocular reflex; the SACCADE by higher centers
(e.g., cortex). Nystagmus can be observed in normal people following
stimulation of the vestibular system; in the absence of stimulation, it is a
sign of underlying pathology.
13. The caloric test is used to assess brain function. In a person with a
normally functioning cortex, injection of cool water into the right ear, will
produce a LEFTWARD NYSTAGMUS (COLD=OPPOSITE, WARM=SAME
 COWS). If the patient is COMATOSE, the SACCADE WILL BE
ABSENT (the VOR, which operates in the brainstem is still functional and
the pursuit will be intact). If the patient is BRAIN DEAD, both the
PURSUIT and SACCADE WILL BE ABSENT.
14. There are three types of hearing loss:
CONDUCTIVE: external or middle ear
SENSORINEURAL: inner ear, auditory nerve or cochlear nucleus
MIXED: conductive and sensorineural
60% of sensorineural hearing losses (the most severe type) are due to
genetic factors, and 40% to environmental factors. As the U.S. population
ages, this distribution will change.
Med 6573 Nervous System
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Winter 2010
Inner Ear Physiology and Pathology
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15. Because of the extensive bilateral connections of the auditory system,

the only way to have an ipsilateral hearing loss from a single lesion
is to have a peripheral defect i.e., at the cochlea, auditory nerve or
cochlear nucleus

bilateral hearing loss from a single lesion is invariably due to a
lesion located centrally
N.B. Noise exposure, ototoxic drugs and congenital malformations can cause
simultaneous damage bilaterally but these are considered to be multiple lesions.
16. Ménière’s disease is characterized by intermittent spells of severe
vertigo and nystagmus, fluctuating hearing loss and tinnitus. This
disease has an unknown etiology, and there is no universally successful
treatment.
ESSENTIAL MATERIAL FROM OTHER LECTURES
(i.e., things you should know before you get to this lecture)
1.
Anatomy of the external and middle ears: pinna, tympanic membrane, middle
ear ossicles (malleus, incus, stapes), middle ear muscles (tensor tympani,
stapedius), round and oval windows
2.
Histology of the cochlea: inner and outer hair cells, stereocilia, basilar
membrane; scalae vestibule, tympani and media; tectorial membrane, stria
vascularis
3.
Histology of the vestibular labyrinth: type I and type II hair cells, utricle,
saccule, semicircular canals, cupula, ampulla, cristae, striola, otoliths
4.
Hair cell ultrastructure: stereocilia, kinocilium, cuticular plate, reticular lamina,
subcellular cisternae
5.
Auditory pathways: auditory nerve, cochlear nucleus, superior olivary complex,
inferior colliculus
6.
Vestibular pathways: vestibular nerve, lateral, medial and superior vestibular
nuclei, III and VI cranial nerve nuclei
7.
Descending control of spinal reflexes
8.
Neurological exam: Rinne and Weber tests
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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LEARNING OBJECTIVES (i.e., the things that I will be testing you on!)
1.
Describe the structural features of hair cells that are critical to their function. Identify
key similarities and differences between type I and type II hair cells.
2.
Explain the tip link model of transduction. In particular, be able to describe the
generation of a biphasic receptor potential and adaptation.
3.
Contrast the semicircular canals and otolith organs with respect to: a) the mechanism
of stereocilia displacement, b) directionality and c) the type of effective stimulus.
Describe the natural pairing of semicircular canals, and be able to indicate which
canals are depolarized/hyperpolarized by specific head movements.
4.
List the steps in the vestibulo-ocular reflex and describe the changes in firing patterns
in each nucleus during head rotation. Identify the function of the VOR.
5.
Compare and contrast the anatomical and physiological aspects of the medial and
lateral vestibulospinal systems with respect to: overall function, afferent source,
vestibular nucleus, efferent projections and effect and control mechanism.
6.
Describe how the mass and stiffness characteristics of the middle ear affect sound
transmission. List the mechanisms used by the middle ear to minimize the impedance
mismatch between air and the cochlear fluids. Graph and be able to interpret the
audiograms generated in patients with normal hearing, a conductive hearing loss and
sensorineural hearing loss.
7.
Outline how a traveling wave is established on the basilar membrane in response to
an acoustic stimulus. Define the cochlear place code and describe how it is
established via the passive properties of the basilar membrane and organ of Corti.
8.
Diagram the active feedback mechanism invoked by contraction of the outer hair cells.
Identify the difference between otoacoustic emissions and tinnitus. Identify the
source of OAE, and describe how they can be used to derive an audiogram (e.g.,
during newborn hearing screenings).
9.
Describe the generation of the endocochlear potential (EP) and its function.
Understand the impact on hearing of interfering with the EP (e.g., with loop diuretics).
10. Compare the pathways and mechanism for localizing low and high frequency stimuli.
Be able to describe the physiology underlying the Rinne and Weber tests.
11. Define nystagmus, and understand its origins (i.e., which part is due to VOR, and which
to higher centers). Describe the differences in caloric nystagmus among a normal
individual, a comatose patient and in a person who is brain dead.
12. Be able to define the terms: prelingual and lingual deafness, conductive and
sensorineural hearing loss, central auditory processing disorder, presbycusis and
tinnitus.
13. Compare and contrast the symptoms and treatment of acoustic neuromas and
Ménière’s disease.
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 6 of 33
There are two types of inner ear receptors; both types convert
mechanical energy into receptor potentials. TYPE I (INNER
HAIR CELLS) are the true sensory receptors that convey
information to the brainstem. TYPE II (OUTER HAIR CELLS)
function as biological amplifiers, essentially acting as motor units.
HAIR CELL TRANSDUCTION
Structure/Function Relationships



there are two types of hair cells:
1. TYPE I: "true" receptors
2. TYPE II: amplifiers
in the cochlea, type I cells are called
INNER HAIR CELLS; type II cells are
called OUTER HAIR CELLS (based on
their relative position within the organ
of Corti)
there are 3 aspects of hair cell
structure that are critical to their
function:
1. Both types of hair cells are
mechanoreceptors.
 stairlike arrangement of stereocilia; longest near the kinocilium
2. Type I and type II hair cells have different afferent and efferent innervation.
TYPE I
TYPE II
90% of afferents
10% of afferents
1 Hair Cell/afferent
many Hair Cells/afferent
efferents terminate on
efferents have large, direct
afferent terminals under IHCs
contact with OHC soma
Med 6573 Nervous System
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http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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3. In the cochlea, OHCs have specialized lateral cisternae and other structural
adaptations that support their function as contractile units
Med 6573 Nervous System
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Winter 2010
Inner Ear Physiology and Pathology
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Transduction Mechanism
Inner ear transduction is DIRECTIONAL: displacement toward
the
tallest stereocilia (positive deflection) results in
DEPOLARIZATION. In the cochlea, this occurs when the basilar
membrane moves toward scala vestibuli. Negative deflection
(toward scala tympani) results in HYPERPOLARIZATION.
Tip Links & Transduction Channels
Hair cell transduction is DIRECTIONAL:
 displacement TOWARDS THE TALLEST ROW OF STEREOCILIA (positive
deflection) is DEPOLARIZING
o in the cochlea, this corresponds to movement towards scala vestibuli
 displacement TOWARDS THE SHORTEST ROW OF STEREOCILIA (negative
deflection) is HYPERPOLARIZING
o this corresponds to movement towards scala tympani
Adaptation




adaptation is the decrease in receptor potential in response to
a constant stimulus (more detailed explanation)
common to many sensory systems
o auditory hair cells exhibit extremely rapid adaptation
o vestibular hair cells (esp. in saccule and utricle) exhibit
slower adaptation
adaptation motor continuously moves up the actin core of the
tallest stereocilium, maintaining tension on the tiplink
o motor complex is thought to involve myosin 7A, a
protein only found in the inner ear and implicated in
one form of hereditary deafness
also defines a set point for the system, so that some
transduction channels are always open, causing slight depolarization of the hair cell and basal
release of neurotransmitter – this allows negative displacements to reduce auditory nerve firing
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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BALANCE
Components – have 10 different vestibular organs.
The SEMICIRCULAR CANALS detect head rotation (angular
acceleration). The OTOLITH ORGANS (UTRICLE and
SACCULE) detect gravity (linear acceleration). The vestibular
system is involved in balance and posture, co-ordination of head
and body movements and in fixating the visual image on the fovea.






two components:
o SEMICIRCULAR CANALS: motion detectors
o OTOLITH ORGANS (utricle and saccule): gravity detectors
vestibular system is involved in balance and posture, co-ordination of head and body
movements and in fixating the visual image on the fovea
transduction process is fundamentally similar to that of the cochlea, but it is less well
understood, particularly with respect to differences between type I and type II hair cells
outputs from the vestibular system are integrated with information from other sensory
receptors (proprioception=perception of body in space), and are not perceived as separate
however, when other sensory inputs are in conflict with the vestibular system, the
vestibular signal seems to act as the reference postural signal with which other sensory
inputs are compared
unexpected or conflicting inputs from the vestibular system can result in vertigo, nystagmus
and/or motion sickness
Semicircular Canals





in contrast to the otoliths, the semicircular canals detect the
rate of head rotation (angular acceleration), and are
therefore dynamic in function
when the head is initially moved (turning), the ampulla (and
therefore the hair cells) turns with it
1. the fluid in canal - endolymph remains in its initial position
due to inertia, causing movement of the stereocilia against
the cupula and altering the receptor potential (could be
depolarizing or hyperpolarizing, depending upon direction)
2. once the head is moving at a constant velocity, the duct
fluid moves at the same rate as the hair cells, and the
stereocilia are not deflected
3. when the head stops moving, the fluid keeps moving
(inertia again), and the receptor potential is again altered,
this time in the opposite direction to what occurred at the start of the rotation
Med 6573 Nervous System
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Winter 2010
Inner Ear Physiology and Pathology
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SEMICIRCULAR CANALS WORK IN PAIRS. HORIZONTAL
CANALS: depolarization occurs in the SAME direction as the head
rotation. A/P CANALS: depolarization occurs in the OPPOSITE
direction as the head tilt. The natural pairing is of LEFT
ANTERIOR with RIGHT POSTERIOR CANAL (and vice versa).
HORIZONTAL CANALS:
 deflection of the stereocilia TOWARD the utricle causes depolarization
 if head is going counterclockwise – stereocilia is going clockwise
BOTTOM LINE: depolarization occurs in the SAME direction as the head movement (LEFT head turn
produces depolarization in the LEFT horizontal canal)
ANTERIOR AND POSTERIOR CANALS
 anterior canals are located at ~90o to each other (41o to the sagittal plane)
 posterior canals are also located at ~90o to each other
 the directionality of the stereocilia is different in the anterior and posterior canals (in contrast to
the horizontal canals, depolarization occurs in a direction AWAY from the utricle in both cases)
o the anterior canals have their kinocilium anterior to the stereocilia
o the posterior canals have their kinocilium posterior to the stereocilia
 the natural pairing of A/P canals is:
LEFT ANTERIOR with RIGHT POSTERIOR
RIGHT ANTERIOR with LEFT POSTERIOR
o in order to produce a stimulation of one A/P pair of canals in isolation, the direction
of stimulation is a tilt (e.g., forward and left)
o i.e., the preferred angle for stimulation is OPPOSITE FOR THE ANTERIOR CANAL ON
ONE SIDE AND THE POSTERIOR CANAL ON THE OTHER
 therefore, tilting your head forward (without a tilt to the left or right) causes DEPOLARIZATION
of the POSTERIOR CANALS and HYPERPOLARIZATION of the ANTERIOR CANALS
o and…tilting head back  depolarize the anterior canals & hyperpolarize the
posterior canals
BOTTOM LINE: for anterior and posterior canals depolarization occurs in the OPPOSITE dirsection
as the head movement (FORWARD head tilt produces depolarization of the POSTERIOR canals)
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
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Otolith Organs

detect gravity (linear acceleration), and are
therefore static in function
o otoliths (small calcium carbonate particles)
drag on the stereocilia when the head
changes position
when the body is in anatomical position:
 the patch of hair cells in the UTRICLE is nearly horizontal, with the stereocilia oriented
vertically
 the sensory epithelium is vertical in the SACCULE, with the stereocilia oriented horizontally


in contrast with the semicircular canals (where directionality is inherent in the structure), in
the otolith organs, directionality is conferred solely by the orientation of the hair cell
stereocilia
orientation of the stereocilia within the sensory epithelium is determined by the STRIOLA, a
curved dividing ridge that runs through the middle of the MACULA – in the UTRICLE, the
kinocilia are oriented TOWARD the striola, and in the SACCULE they are oriented AWAY
from it
o in any position, some hair cells will be depolarized and others hyperpolarized in
both otolith organs
Vestibular Reflexes



pathways contain relatively few synapses (2-3)
very similar to spinal reflexes
3 primary reflexes:
1. vestibulo-ocular (VOR)
2. vestibulospinal
a. medial
b. lateral
vestibulocerebellar (discussed in cerebellum lectures)
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 12 of 33
Vestibulo-Ocular Reflex ‘dolls eyes’
Purpose: adjust eye position to
compensate for changes in head position
--Move your head one way and eyes will
move the opposite way!
1. The reflex sequence begins with a turn of
the head to the LEFT (top of diagram).
2. As described previously (and shown at the
bottom of the diagram), head rotation to
the LEFT causes:
o DEPOLARIZATION in the LEFT
HORIZONTAL CANAL
o HYPERPOLARIZATION in the
RIGHT HORIZONTAL CANAL.
3. This is followed by EXCITATION in the LEFT
VESTIBULAR NUCLEI, and INHIBITION in
the RIGHT VESTIBULAR NUCLEI.
4. Due to the organization of the pathway,
at the level of the extraocular motor
nuclei, there is:
o EXCITATION of cells in the LEFT
OCULOMOTOR and RIGHT
ABDUCENS nuclei
o INHIBITION of cells in the RIGHT
OCULOMOTOR and LEFT
ABDUCENS nuclei
5. This pattern of extraocular motor activity
results in:
o CONTRACTION of the LEFT
MEDIAL RECTUS and RIGHT
LATERAL RECTUS muscles.
o RELAXATION of the RIGHT
MEDIAL RECTUS and LEFT LATERAL RECTUS muscles.
6. Movement of the eyes to the RIGHT. (top of diagram)
7. PURPOSE: adjust eye position to compensate for head movements
o goal is to keep the visual image focused on the fovea
8. BOTTOM LINE: rotation of the head in one direction results in contraction of the extraocular
muscles to slowly rotate the eyes in the opposite direction
o in an awake, "normal" person, VOR is suppressed by voluntary eye movements
9. responsible for the pursuit phase (slow component) of nystagmus
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 13 of 33
Vestibulospinal Reflexes
Vestibulospinal reflexes coordinate the position of the head
with the trunk and body, with the goal of maintaining the
head in an upright position during movement. The two
systems differ in anatomical connections, function and the
control mechanisms that they use to affect alpha motor
neuron function. The function of the VST systems is evident
during decerebrate rigidity.
LATERAL VESTIBULOSPINAL
TRACT (LVST)

entire labyrinth
(motion and gravity)

semicircular canals
(motion)

lateral vestibular (Dieter's
nucleus) to medial aspects of
laminae VII and VIII

medial and descending
vestibular nuclei to MLF

postural changes to
compensate for tilts and
movements of the body

stabilize head position
during walking

ipsilateral

bilateral

excitatory

excitatory and inhibitory
AFFERENT SOURCE
VESTIBULAR NUCLEUS
see Dr. Forbes lectures
FUNCTION
EFFERENT
MEDIAL VESTIBULOSPINAL
TRACT (MVST)
CONNECTIONS
EFFERENT EFFECT

CONTROL MECHANISM
remember motor control
lecture: mechanisms to
influence motor neuron
and decerebrate rigidity
adjustment of proximal limb and
 relaxation of muscles of
trunk musculature by:
upper back and neck
1. contraction of extensor
1. direct inhibition of alpha
muscles via direct excitation of
motor neurons
alpha and gamma motor neurons
(mechanism 1)
(mechanisms 1 and 4)
2. indirect relaxation of flexor
IT IS UNDOUBTEDLY
muscles via excitation of
MUCH MORE COMPLICATED
inhibitory interneurons
(mechanism 2)
THAN THIS!!!
Med 6573 Nervous System
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http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 14 of 33
HEARING
Acoustics

INTENSITY: decibels: dB=20*log(P/P0)
o log scale: loudest sound detected by human ear is approx. 10 million times louder than the
quietest sound (huge dynamic range)
o ratio: various scales use different P0 reference values (relevant for the interpretation of
audiograms)
 dB SPL: relative to 2x10-5 N/m2 - same reference level for all frequencies - expresses
intensity in absolute terms
 dB HL (or SL): relative to the lowest sound pressure detected by normal individuals different reference level for each frequency - expresses intensity in relative terms
FREQUENCY
o complex sounds can be broken down into their
component frequencies via a practice called spectral
analysis
 this type of analysis is the key to understanding
cochlear frequency processing
o another fundamental principle is the concept of
linearity (i.e., "what you put in is what you get out")
 in a linear system such as the middle ear, if you
put in a signal that has two frequency
components (e.g., 2 kHz and 5 kHz), what you
get out is two frequency components (2 kHz
and 5 kHz)
 in a nonlinear system such as the cochlea, if
you put in two frequency components, what
you get out is multiple frequency components
(2, 5, 1, and 3 kHz)
 the added components are called DISTORTION
PRODUCTS - the distortion products created by the cochlea can be recorded as
otoacoustic emissions
Med 6573 Nervous System
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http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 15 of 33
Middle Ear Function



physiologically, the function of the middle ear is to minimize the loss of acoustic energy that
would result from the transition from air (external ear) to water (cochlea)
o think about sitting underwater in a pool: it's difficult to hear someone talking above
the water, because approximately 99.9% of the acoustic energy is reflected at the
water surface
o a loss of 99.9% corresponds to a decrease of 40-55 dB (depending upon frequency),
which is approximately the change that occurs with a conductive hearing loss
the energy loss is minimized by impedance matching
the improved air transmission that occurs is the basis of the Rinne test
Impedance matching

there are 3 aspects of middle ear movement that minimize energy loss because they amplify
the pressure delivered to the oval window relative to the pressure applied to the tympanic
membrane
o the effectiveness of the match is dependent on frequency, which results in the
relationship between hearing thresholds and frequency (described by an
audiogram)
AREAL RATIO

the area of the tympanic membrane is much larger
than the round window
LEVER RATIO

the length of the long arm of the malleus is much longer than the long
arm of the incus
BUCKLING ACTION

force is transmitted from the centre of the tympanic membrane (i.e., the
TM doesn't move as a plate)
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Winter 2010
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Acoustic impedance





Can’t get a perfect match.
Middle ear affects frequencies you can hear  natural resonance
the core concept is that the effectiveness of transmitting vibration is frequency dependent
i.e, every object has a resonant frequency, and that frequency is determined by an object's
acoustic impedance
o we manipulate acoustic impedance when we construct musical instruments
2 properties determine an object's resonant frequency:
o MASS: heavier objects vibrate at a lower frequencies
(i.e., mass limits high frequency transmission)
 this is the more intuitive concept - think of a xylophone: lower frequencies
use bigger keys
o STIFFNESS: less elastic objects vibrate at a higher frequencies
(i.e., stiffness limits low frequency transmission)
 think of a flute or recorder: changing the length of an air column changes its
elasticity; the shorter the column, the higher the resonant frequency
in the middle ear, the ossicles contribute mass and the volume of the middle ear space
affects stiffness
o alterations of middle ear impedance significantly impact hearing because the
incredible sensitivity of the cochlea means that any sounds reaching the cochlea can
be transduced
o two examples where changes in middle ear impedance cause hearing loss are
otosclerosis and otitis media
Otosclerosis:




Otitis Media:
abnormal growth of bone in the middle ear
resulting in immobilization of stapes
common cause of bilateral, gradual hearing loss in
adults
o 70% hereditary
o twice as common in females vs. males
o more common in Caucasians
(virtually nonexistent in Asian, Black and
American Indian populations)
high frequency sounds are affected first
(remember discussion of acoustic impedance)
repaired surgically

in the cochlea, the
establishment of gradients for
mass and stiffness along the
basilar membrane are
responsible for establishment
of the place principle







caused by a middle ear infection that produces pus, fluid and
inflammation
most common reason that children visit their physician
o 2/3 of kids under age 3 have at least 1 episode
o about 1/3 of kids will have at least 3 visits to the doctor
because of otitis media
o in the US in 1995, over 2 million tubes were put in children’s
ears, at a cost of over $2 billion
frequently associated with upper respiratory infection - may be
bacterial or viral - eustachian tube is shorter and less slanted in
children, facilitating entry
may cause TM rupture
hearing loss is usually temporary, but can have significant impact
on speech development
low frequency sounds (=speech) are affected first - significant
threshold elevations can be observed when fluid is not visible
through TM (remember discussion of acoustic impedance)
high frequency sounds are not affected until mass of middle ear
bones is increased - at that point, both low and high frequency
sounds would be reduced, resulting in the characteristic
audiogram of conductive hearing loss
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Audiograms


WHATEVER INPUT THE NORMAL COCHLEA RECEIVES, IT WILL DETECT
therefore, in the absence of any sensorineural hearing loss, any limits on hearing are
imposed by the acoustic impedance of the middle ear
Left side:
 this graph is plotted using absolute intensity levels (dB SPL)
 at birth, humans can hear frequencies between 100 Hz and 20 kHz
 at each frequency, it is possible to define the minimum intensity that is detectable (this
intensity will be different for different people) - this intensity is called THRESHOLD
o the lower black line on this graph represents the average threshold at each
frequency
o the threshold at 2 kHz was used to define 0 dB SPL
 the mass of the ossicles limits high frequency transmission, therefore frequencies above 5
kHz have higher thresholds than lower frequencies
 similarly, the stiffness of the air in the middle ear cavity limits low frequency transmission,
therefore frequencies below 1 kHz also have higher thresholds
 very intense sounds are painful; the pain threshold is typically between 110 and 140 dB SPL
(depending on frequency)
Right side:
 clinically, thresholds are typically reported in relative, rather than absolute, terms (dB HL)
 based on data averaged from many normal individuals, each audiology facility will
determine what their "normal" threshold is at each frequency (large blue dots on both
graphs), and this intensity is set at 0 dB HL
 of most concern to people (including clinicians) is the ability to perceive speech, and this
falls into a relatively restricted range of frequencies and intensities (shaded blue area) consonants have higher frequency components than vowels, which is an important
consideration in the discussion of otitis media and otosclerosis as well as sensorineural
hearing loss and presbycusis
 also shown on this graph are the relative intensities of various sounds if they were
presented at a very short distance from your external ear
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Assessing thresholds (audiograms)
 in an audiogram, thresholds are determined for both ears, and for bone and air
conduction
 in this example from a normal hearing individual,
o only data from the left ear shown
o air conduction = bone conduction
o both thresholds are ~0 HL at all tested frequencies
(i.e., there no difference between this person and normal mean
 different techniques can be used to determine thresholds
o 3 of the most common techniques are behavioural, otoacoustic emission (OAE) and
auditory brainstem response (ABR)
Conductive hearing loss
 decrease in sound transmission through the external and middle ears
 a conductive hearing loss decreases air transmission but not bone transmission
 as discussed under middle ear acoustic impedance, the frequency range
affected is determined by whether the conduction block is increasing the mass
(high frequencies) or the stiffness (low freqencies) properties or both (as shown
in this example)
 it can be detected using Rinne test, as well as seen in the audiogram (as shown
in this example; click here for more info re. audiogram interpretation)
 common causes of a conductive hearing loss include:
o ruptured eardrum
o intra-tympanic fluid (usually due to otitis media)
o otosclerosis
Sensorineural hearing loss
 increase in thresholds and/or loss of ability to transduce specific frequencies due to
damage to the cochlea, auditory nerve or cochlear nucleus
o i.e., anything between the middle ear and the bilateral outputs of the cochlear
nucleus
 most commonly used to refer to hair cell damage, and even more specifically, stereocilia
damage
 the base (high frequency region) of the cochlea is the most sensitive to damage
 common causes include:
o noise
 noise damage is not simply a function of intensity - length of exposure as
well as the frequency spectrum are also important - brief, high frequency
sounds (e.g., gunshots) have the most potential for damage
o genetic defects, especially damage to the system responsible for generating the
endocochlear potential
o ototoxic drugs, including
 aminoglycoside antibiotics (e.g., gentamicin, amikacin) which cause
stereocilia damage
 loop diuretics (e.g., furosemide) which block the Na+/K+/2Cl- transporter
responsible for the generation of the endocochlear potential
 sensorineural hearing loss affects both airbourne and bone-conducted stimuli, therefore
both curves exhibit a threshold shift in the affected frequency range on the audiogram
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Basilar Membrane Deflection
Traveling Wave & Place Principle
Sound waves pass through the cochlea INSTANTANEOUSLY.
The traveling wave pattern on the basilar membrane is established
more gradually and is INDEPENDENT of how the motion is
initiated i.e., don't need to deliver sound via the oval window --- can use bone!
The traveling wave establishes a frequency vs. place relationship
along the length of the cochlea, with high frequencies being
transduced in the base, and low frequencies in the apex.








transduction requires that the basilar membrane vibrate in order to induce stereocilia deflection
in the hair cells
hair cell transduction is DIRECTIONAL:
o movement toward SCALA VESTIBULI is depolarizing
o movement toward SCALA TYMPANI is hyperpolarizing
the pressure wave moves through the cochlea INSTANTANEOUSLY
(speed of sound in seawater is ~1500 m/s; length of the human cochlea is ~35 mm!)
establishment of the travelling wave vibration pattern is INDEPENDENT of how the motion is
initiated in the perilymph, i.e., the sound can be delivered either by the oval window or via bone
o this is the basis of the Rinne test
in response to a simple sinusoidal stimulus, the basilar
membrane resonates in a "traveling wave" that gradually
grows in amplitude as it moves along the cochlear ductaway
from the stapes (base), toward the helicotrema (apex)
o same direction of travel is observed if the sound is
introduced into the apex of the cochlea instead of the
base
it reaches a peak at a specific point that is determined by the resonant properties (acoustic
impedance) of the basilar membrane at each point
both passive (structural) and active (energy requiring) processes establish the location of the
peak vibration
PLACE PRINCIPLE: the peak of the travelling wave occurs at a different place along the basilar
membrane for each frequency (the basilar membrane movement is said to be “tuned”)
o low frequencies peak near the apex; high frequencies near the base
o as a result of this spectral analysis, complex stimuli produce vibration patterns where the
component frequencies are represented in different locations along the basilar mem
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The middle ear transfer function determines the absolute
threshold of hearing at each frequency in normal individuals –
the cochlea is so sensitive, it can transduce any signal
reaches it. This implies that anything that alters middle ear
function (like an infection) will significantly impact hearing
thresholds.
Passive Properties

anatomical characteristics
determine resonance properties
at individual points along the
cochlear partition:
o LOW FREQUENCIES produce
the largest deflections in the
APEX because the basilar
membrane is less stiff (fewer,
wider, less elastic fibres) and
has greater mass (more cells)
in the apex
o HIGH FREQUENCIES produce
the largest deflections in the BASE where the basilar membrane is more stiff and less heavy
 these passive properties determine the physiological response of a dead cochlea (such as those
used by von Bekesy to make his place principle hypothesis)
Active process
Outer hair cells use their receptor potential to exert force on the
basilar membrane ---thereby generating a POSITIVE FEEDBACK
MECHANISM which amplifies the vibration of the membrane in a
nonlinear, highly frequency specific manner. This force produces
its own fluid wave, which is conducted back through the perilymph,
vibrating the middle ear apparatus and generating sounds that are
emitted from the ear (OTOACOUSTIC EMISSIONS).



basilar membrane vibration patterns recorded from intact cochleae in
alive animals are much more narrow (frequency selective) and have much
lower thresholds than the passive responses observed von Bekesy
this enhanced frequency selectivity is due to an ACTIVE MECHANICAL
PROCESS that amplifies the basilar membrane movement, and increases
the sharpness of tuning
after years of intense investigation, it was determined that the contractile
properties of the outer hair cells are the source of the amplification
Med 6573 Nervous System
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
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think of a swing: if a person sitting on a swing (basilar membrane) pumps his legs (OHC),
the amplitude of the swing motion in response to a push (sound stimulus) is increased
Therefore, there are five steps in the cochlear transduction process:
1. Sound induces the travelling wave on the basilar membrane.
2. Basilar membrane vibration causes stereocilia deflection.
3. In both types of hair cells, the mechanical stimulus is transduced and receptor potentials
are generated. In IHCs, this leads to neurotransmitter release and AP generation in
underlying auditory nerve fibres.
4. However, in OHCs, receptor potentials result in contraction, which amplifies the basilar
membrane motion.
5. This amplification increases the movement of the basilar membrane (positive feedback).
 OHCs have been shown to be capable of contracting at rates up to 70 kHz
 efferent innervation provided to the OHC from the superior olivary complex is thought
to adjust the resting membrane potential of the OHCs, thereby adjusting the amount of
feedback provided to the basilar membrane (i.e., the CNS can change the sensitivity and
frequency selectivity of the cochlea)
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Otoacoustic emissions – sounds from your ear when it is working normally



sounds recorded from the external ear canal that are generated by the NORMAL cochlea as a
result of nonlinear processing caused by NORMAL OHC function
OAE can be spontaneous or evoked by sounds
o spontaneous OAE should NOT be confused with tinnitus
used clinically to test hearing in a non-invasive, non-behavioural manner
(i.e., can be used on people who cannot respond to a behavioural test, such as infants)
o can use either clicks (test all frequencies rapidly) or individual frequencies (use distortion
products to map individual frequencies/places more precisely) to evoke emissions
Endocochlear Potential & Stria Vascularis
The STRIA VASCULARIS produces the endolymph (high K+)
and the endocochlear potential (+80 mV). Many of the ion
transporters of the stria are the same as those in the
kidney, so drugs that affect renal function are often ototoxic –
esp. loop diuretics (which affect the Na+/K+/2Cl- transporter).



endolymph that bathes the apical
surfaces of mammalian hair cells is a
unique extracellular fluid
o 150 mM K+
o 1 mM Na+
scala media is also +80 mV relative to
scala tympani - this voltage difference is
called the endocochlear potential (EP)
hair cell transduction channels are nonselective cation channels
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
when the transduction channels open, K+ flows into the hair cells to cause depolarization
because there is a large electrochemical gradient between the endolymph and the hair cell
intracellular fluid
 concentration gradient: 150 vs. 120 mM
 electrical gradient: +80 vs. -40 mV
the reticular lamina, as well as an extensive tight junction system, enables this
electrochemical gradient to be established
there is also a K+ recycling system that allows supporting cells to take up K+ ions extruded
from the hair cells, and return them to the stria vascularis
o defects in the genes that code for the connexin proteins that form the gap junctions
are the most common inherited cause of sensorineural deafness
large driving force for K+ entry contributes to extremely low threshold of auditory hair cells
hair cells tend to be continuously slightly depolarized relative to the "normal" neuronal
potential of -70 mV
o allows for a receptor potential that follows the stimulus (depolarization and
hyperpolarization)
the stria vascularis, found on the lateral wall of the cochlea, produces the endolymph and is
responsible for the endocochlear potential
many of the ion transporters of the stria are the same as those in the kidney, so drugs that
affect renal function are often ototoxic
loop diuretics such as furosemide which blocks the Na+/K+/2Cl- transporter will decrease the
EP, and raise acoustic thresholds
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Central Processing
Pathways
IHC  spiral ganglion auditory nerve which bifurcates:
1. ventral cochlear nucleus  trapezoid body superior olivary complex (lateral and
medial superior olivary nuclei)  inferior colliculus
2. dorsal cochlear nucleus  lateral lemniscus  inferior colliculus
brachium of the inferior colliculus  medial geniculate nucleus of the thalamus  primary
auditory cortex
 all pathways central to the cochlear nucleus are BILATERAL - critical for understanding
sound localization
 auditory pathways are myelinated and have ion channel and receptor specializations to
ensure the precise timing of acoustic signals is preserved
Auditory Brainstem Responses





also called Brainstem Auditory Evoked
Responses (BAERs)
EEG recorded in response to repeated
presentations of sounds of specified
frequency and intensity – averaged many
times to filter background electrical noise
peaks in the waveform (I, II, III, etc.) correspond to successive stages in the auditory
pathway (auditory nerve, cochlear nucleus, superior olive, etc.)
ascribed to the nuclei, but likely are due to the nerve pathways
one way to assess hearing thresholds (determine an audiogram) without requiring
behavioral responses
Sound localization
Sounds are localized by the differences in timing and intensity between the two
ears. Lateral superior olive (LSO) neurons localize high frequency stimuli by
comparing interaural intensity differences (IIDs); medial superior olive (MSO)
neurons use interaural timing differences (ITDs) to localize low frequency stimuli.



superior olivary complex receives input from both cochlear nuclei (i.e., responses in these cells
are bilateral)
sounds are localized via two mechanisms:
o LOUDER IN ONE EAR: interaural intensity difference (IID)
– higher frequency
o REACHES ONE EAR FIRST: interaural timing differences
(ITD) – lower frequency
LATERAL SUPERIOR OLIVE is specialized for high frequencies
and cells measure IIDs by integrating ipsilateral excitatory
and contralateral inhibitory inputs
Med 6573 Nervous System
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

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MEDIAL SUPERIOR OLIVE is specialized for low frequencies and measures ITDs using excitatory
inputs from both sides
map of sounds in space  basis for the Weber test
Hearing Tests
Audiograms
Rinne test
 first described in 1855
PURPOSE: determination of a conductive hearing loss
 strike tuning fork and place on mastoid process
o physicians typically use tuning forks of either 256 or 512 Hz (i.e., low frequencies of
human hearing), which means that they are testing the stiffness component of the
middle ear's acoustic impedance, which is determined by the volume of the middle ear
space
 when person can no longer hear the sound (bone conduction threshold), place the tuning fork
near the ear canal
 ask when the person can no longer hear the sound (air conduction threshold)
 due to the amplification provided by the middle ear, in a normal person air threshold <= bone
threshold
o this is usually stated as air conduction is better than bone conduction (AC > BC)
o in a person with a conductive hearing loss, bone conduction is greater than air
conduction (BC > AC; bone threshold is less than air threshold)
o in a patient with unilateral sensorineural hearing loss, there may be a false negative
Rinne due to the response of the normal ear i.e., AC>BC in the affected ear, but both of
these thresholds are elevated relative to BC in the normal ear, so the patient reports
that bone conduction is louder (they're actually hearing the tone in their normal ear,
rather than their tested --- i.e., affected --- ear)
Weber test
PURPOSE: determination of a conductive vs. a sensorineural hearing loss
 strike tuning fork and place base in the centre of the forehead or the top of the head
 ask if the tone is louder in the left ear, the right ear or equally loud in both ears
 due to the sound localization process,
o in a patient with a unilateral conductive hearing loss, the sound will be louder in the
affected ear (airborne sounds mask bone conduction in the normal ear; conductive loss
prevents masking in affected ear è sound is perceived to be louder in affected ear)
o in a patient with unilateral sensorineural hearing loss, the sound is louder in the
normal ear (no signal is transduced by the cochlea on the affected side, therefore the
sound is louder on the normal side and is perceived to be coming from that side)
o in a normal person or a person with symmetrical hearing loss, it is equally loud in both
ears
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INNER EAR PATHOLOGY
Vestibular only
Nystagmus – can see in normal people if you stimulate vestibular system
NYSTAGMUS consists of a slow drift of the eyes in one direction (PURSUIT)
followed by a rapid recovery movement in the opposite direction (SACCADE).
The direction is named for the fast component i.e., a RIGHTWARD
NYSTAGMUS consists of slow movement of eyes to the left, followed by fast
recovery to the right. The PURSUIT is controlled by vestibulo-ocular reflex;
the SACCADE by higher centers (e.g., cortex). Nystagmus can be observed in
normal people following stimulation of the vestibular system; in the absence of
stimulation, it is a sign of underlying pathology.
DEFINITION: rhythmical oscillation of the eyeballs (click here for video)





at least 37 different kinds are recognized
commonly used to refer to JERKY NYSTAGMUS:
o slow drift of the eyes in one direction (PURSUIT) followed by a rapid recovery
movement in the opposite direction (SACCADE)
o direction is named for the fast component:
i.e, a rightward nystagmus consists of slow movement of eyes to the left, followed
by fast recovery to the right
can be induced in normal individuals by vestibular stimulation (see experiments listed
below) - nystagmus in the absence of vestibular stimulation indicates some kind of
pathology
PURSUIT is controlled by vestibulo-ocular reflex
SACCADE is controlled by higher centers (e.g., cortex)
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The caloric test is used to assess brain function. In a person with a
normally functioning cortex, injection of cool water into the right ear,
will produce a LEFTWARD
NYSTAGMUS
(COLD=OPPOSITE,
WARM=SAME  COWS). If the patient is COMATOSE, the SACCADE
WILL BE ABSENT (the VOR, which operates in the brainstem is still
functional and the pursuit will be intact). If the patient is BRAIN DEAD,
both the PURSUIT and SACCADE WILL BE ABSENT.
Caloric test

injection of cool or warm water into ear canal produces convection currents in
semicircular canals
o if the patient is lying down, and his/her head is tilted back about 60 degrees, the
HORIZONTAL semicircular canals will be affected





COOL WATER IN RIGHT EAR
clockwise current in right semicircular canal
stereocilia deflection to the right (same direction as a head movement to the left)
slow eye movement to the right,followedbyrapid eye movement to the left
LEFTBEATING NYSTAGMUS
COLD=OPPOSITE





WARM WATER IN RIGHT EAR
counterclockwise current in right semicircular canal
stereocilia deflection to the left (same direction as a head movement to the right)
slow eye movement to the left, followed by rapid eye movement to the right
RIGHTBEATING NYSTAGMUS
WARM=SAME
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Benign positional vertigo





age or trauma can cause the otoliths to detach from the utricle, and become lodged in a
semicircular canal (usually the posterior semicircular canal)
otoconia then move back and forth in response to changes in body position, creating movement
of endolymph
patient experiences vertigo and nystagmus when moving the head in the plane of the affected
canal
o patient will prefer to lie with the affected ear up as this minimizes symptoms
will often resolve on its own, however severe vertigo disrupts virtually every aspect of life, since
the patient loses the ability to do anything normally, especially when movement is involved
the Epley maneuver can return otoconia to the utricle
Labyrinthitis (A&V?)




inflammation of inner ear
usually caused by a viral infection
o 1-2 weeks after the flu or a cold
o very rarely follows a middle ear infection
primary symptom is vertigo (see BPV)
o may cause temporary hearing loss and/or tinnitus
no treatment (except in very rare cases of bacterial infection)
o antiemetic drugs may be used to control nausea and vomiting
Auditory only
 Hearing Loss
There are three types of hearing loss:
CONDUCTIVE: external or middle ear
SENSORINEURAL: inner ear, auditory nerve or cochlear nucleus
MIXED: both
60% of sensorineural hearing losses (the most severe type) are due
to genetic factors, and 40% to environmental factors. As the U.S.
population ages, this distribution will change.
 affects at least 20 million people in the U.S.
(numbers are increasing dramatically as the population ages)



1 in 1000 children is born profoundly deaf
1 in 500 children has a hearing impairment significant enough to impact speech and
language development, education or social development
temporally, can divide hearing loss into PRELINGUAL (before you learn how to speak) or
LINGUAL categories
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Inner Ear Physiology and Pathology
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Conductive Hearing Loss





decrease in sound transmission through the external and middle ears
a conductive hearing loss decreases air transmission but not bone transmission
as discussed under middle ear acoustic impedance, the frequency range affected is
determined by whether the conduction block is increasing the mass (high frequencies) or
the stiffness (low freqencies) properties or both (as shown in this example)
it can be detected using Rinne test, as well as seen in the audiogram (as shown in this
example; click here for more info re. audiogram interpretation)
common causes of a conductive hearing loss include:
o ruptured eardrum
o intra-tympanic fluid (usually due to otitis media)
o otosclerosis
Sensorineural Hearing Loss





increase in thresholds and/or loss of ability to transduce
specific frequencies due to damage to the cochlea, auditory
nerve or cochlear nucleus
o i.e., anything between the middle ear and the bilateral
outputs of the cochlear nucleus
most commonly used to refer to hair cell damage, and even
more specifically, stereocilia damage
the base (high frequency region) of the cochlea is the most
sensitive to damage
common causes include:
o noise
 noise damage is not simply a function of intensity - length of exposure as well as the
frequency spectrum are also important - brief, high frequency sounds (e.g., gunshots)
have the most potential for damage
o genetic defects, especially damage to the system responsible for generating the
endocochlear potential
o ototoxic drugs, including
 aminoglycoside antibiotics (e.g., gentamicin, amikacin) which cause stereocilia
damage
 loop diuretics (e.g., furosemide) which block the
Na+/K+/2Cl- transporter responsible for the
generation of the endocochlear potential
sensorineural hearing loss affects both airborne and boneconducted stimuli, therefore both curves exhibit a threshold
shift in the affected frequency range on the audiogram
Med 6573 Nervous System
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http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 30 of 33
Because of the extensive bilateral connections of the auditory system,

the only way to have an ipsilateral hearing loss from a single
lesion is to have a peripheral defect i.e., at the cochlea,
auditory nerve or cochlear nucleus

bilateral hearing loss from a single lesion is invariably due to
a lesion located centrally
N.B. Noise exposure, ototoxic drugs and congenital malformations can cause
simultaneous damage bilaterally but these are considered to be multiple lesions.
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Central Processing Disorders (CAPD)




central auditory processing involves:
o sound localization and lateralization
o signal to noise discrimination
 lose this filter with age
o pattern recognition
o the temporal aspects of sounds
o the ability to deal with degraded and competing acoustic signals ("cocktail party
effect") – can hear in quiet, but not in noisy environment
a deficiency in one or more of the above listed behaviors may constitute a Central Auditory
Processing Disorder (CAPD)
CAPD occurs when auditory centers of the brain are affected by injury, disease, tumor,
heredity or unknown causes
increases in incidence with age
o sensorineural hearing loss tends to synergize with CAPD in the elderly
Tinnitus
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DEFINITION: perception of sound when no external stimulus is present
"ringing in the ears" - may also be described as water rushing, buzzing, pulsing
o Pulsatile tinnitus is a rhythmic sound most often in time with the heartbeat. It can usually
-– but not always -- be heard objectively through a stethoscope on the patient's neck or
through a microphone placed inside the ear canal. It has some well-known causes:
hypertension, a heart murmur, Eustachian tube disorder, a glomus tumor, an abnormality
of a vein or artery, and others. This kind of tinnitus can be treated.
may be intermittent or constant; bilateral or unilateral
may be extremely debilitating (esp. in patients predisposed to depression)
may be of peripheral (cochlea) or central origin (i.e., we don't know - probably many different
causes)
things that make it worse:
o loud noises / alcohol, nicotine, caffeine, high sugar foods
o medications: anti-inflammatories, antibiotics, sedatives, antidepressants, and aspirin
(high doses)
o high blood pressure / stress/fatigue
no one treatment works:
o amplification (hearing aids)/masking/tinnitus retraining therapy
o biofeedback (reduced stress)
o cochlear implant (masking; electrical stimulation suppresses random auditory nerve
activity?)
o drug therapy (anti-anxiety, antidepressants, antihistamines, anaesthetics and, ironically,
aspirin)
o TMJ/dental treatment
Presbycusis
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age related hearing loss - affects nearly 1/3 of all people over the age of 65
features include sensorineural hearing loss (most common), CAPD and tinnitus
now recognized to have both peripheral and central causes
Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 32 of 33
Vestibular and Auditory
Acoustic Neuroma
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this is intended to be a brief overview; for more information, please check out the following
sites:
o U of M site for newly diagnosed acoustic neuroma patients
o SIU site for CT and MRI images (click on 1="brain" and 2="Acoustic neuroma")
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~3000 cases/year in US (incidence rate: 1/100,000)
highest incidence in 5th and 6th decades; sexes are equally affected
occurs occasionally as part of von Recklinghausen neurofibromatosis - two forms:
o type I: involves the 8th nerve, just as it might any other cranial nerve - bilateral acoustic
neuromas are rare in this form
o type II: autosomal dominant inheritance - characterized by bilateral acoustic neuromas
that practically always occur before age 21
almost always originate on the vestibular division of the 8th nerve, just within the internal
auditory canal
as it grows, the schwannoma will extend into the posterior fossa to occupy the cerebellopontine
angle, where it can compress the 7th, 5th and (less often) the 9th and 10th cranial nerves
at even later stages, it can displace and compress the pons and lateral medulla and obstruct CSF
circulation
earliest symptom is usually hearing loss or vertigo, but not all patients seek medical advice
immediately → more complicated clinical picture, which may follow this progression:
o vertigo: nausea, vomiting and pressure in the ear (similar to Ménière's disease, except
periods of normalcy are rare)
o hearing loss and tinnitus (most often a unilateral high-pitched ringing, roaring or hissing)
o unsteadiness, especially when rapidly changing position, gait disturbances
o facial weakness, cheek numbness, loss of taste
o signs of  ICP
treatment is surgical excision (radiation therapy is some limited cases)
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Med 6573 Nervous System
Dr. Janet Fitzakerley jfitzake@d.umn.edu
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2010
Inner Ear Physiology and Pathology
Page 33 of 33
Ménière’s Disease
Ménière’s disease is characterized by intermittent spells of
severe vertigo and nystagmus, fluctuating hearing loss and
tinnitus. This disease has an unknown etiology, and there is no
universally successful treatment.
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unknown  but think its b/c of defect in endolymph production
diagnosed by excluding other causes
variations in the nature of the symptoms is the rule rather than the exception
symptoms include (survivor's story):
o intermittent spells (including periods of remission) of severe vertigo and nystagmus
accompanied by nausea, vomiting, sweating and all the symptoms normally
associated with extreme motion sickness
o fluctuating hearing impairment and tinnitus - may be a low frequency or mixed low
and high frequency ("Pike's Peak") pattern. A documented fluctuating hearing loss,
especially in the low frequencies, is very helpful in making the diagnosis.
o a sensation of pressure or fullness in the involved ear that cannot be relieved by
swallowing
rapid onset in attacks, with the severe vertigo usually lasting for hours, followed by auditory
symptoms and unsteadiness that can last for several days
after the initial attacks, the hearing usually returns to normal but, ultimately, there is a
progressive permanent hearing loss - in 50% of patients, disease becomes bilateral
occasionally, the patient will present with only the vertigo or only the auditory symptoms
thought to be due to distention of the membranous labyrinth (endolymphatic hydrops)
o full blown attacks of Ménières are probably due to a break in the membrane
separating perilymph and endolymph - potassium rich endolymph then bathes the
vestibular nerve, leading to a depolarization block and transient loss of function
o theories of causes (remember, Ménière's is idiopathic!) include electrolyte
imbalance, autoimmune disease, allergies, vasomotor reactions, trauma, metabolic
disorders, infection (eg, viral, syphilitic), or hereditary factors
attacks typically begin when patients are in their 40s to 50s - extremely rare in children
therapy includes: (not effective for all)
o short term: antiemetics and sedatives
o long term: diuretics, with concurrent restriction of dietary sodium, caffeine, and
alcohol
o vasodilators, steroids, and other anti-inflammatory drugs may be helpful
o surgical procedures for patients who fail medical management include:
 endolymphatic sac procedures
 use of ototoxic antibiotics to ablate the affected vestibular system
 labyrinthectomy
 vestibular nerve section