Download inner ear physiology and pathology

Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 1 of 23
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 (LRC)
Inner ear case

Differential diagnosis and treatment of hearing loss
Issacson and Vora paper
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
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 2 of 23
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.
There are two systems responsible for vestibulospinal reflexes. 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 with respect to their afferent source,
vestibular nucleus connections, efferent projections, how they produce
their effect on neck and trunk muscles and their control mechanisms.
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).
10. The STRIA VASCULARIS produces the endolymph (high K+) and the
endocochlear potential (+80 mV). Many of the ion transporters of the
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 3 of 23
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).
13. The caloric test can be 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  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.
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
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html

Winter 2009
Inner Ear Physiology and Pathology
Page 4 of 23
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:
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 (Dr. Stauffer)
8.
Neurological exam: Rinne and Weber tests
stereocilia, kinocilium, cuticular plate, reticular lamina,
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 5 of 23
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 [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 6 of 23
HAIR CELL TRANSDUCTION
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.
Structure/Function Relationships
TYPE I
TYPE II
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 7 of 23
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
Adaptation
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 8 of 23
BALANCE
Components
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.
Semicircular Canals
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 9 of 23
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).
Otolith Organs
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Vestibular Reflexes
Vestibulo-Ocular Reflex
Winter 2009
Inner Ear Physiology and Pathology
Page 10 of 23
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 11 of 23
Vestibulospinal Reflexes
There are two systems responsible for vestibulospinal
reflexes. 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 with respect to
their afferent source, vestibular nucleus connections,
efferent projections, how they produce their effect on
neck and trunk muscles and their control mechanisms.
LATERAL VESTIBULOSPINAL
TRACT (LVST)
AFFERENT SOURCE
VESTIBULAR NUCLEUS

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

ipsilateral

bilateral

excitatory

excitatory and inhibitory
see Dr. Forbes lectures
EFFERENT
CONNECTIONS
EFFERENT EFFECT

CONTROL MECHANISM
see Dr. Stauffer's lecture
on Descending control
or
this review of spinal
reflexes
MEDIAL VESTIBULOSPINAL
TRACT (MVST)
adjustment of proximal limb and
trunk musculature by:
1. contraction of extensor
muscles via direct excitation of
alpha and gamma motor neurons
(mechanisms 1 and 4)
2. indirect relaxation of flexor
muscles via excitation of
inhibitory interneurons
(mechanism 2)
 relaxation of muscles of
upper back and neck
1. direct inhibition of alpha
motor neurons
(mechanism 1)
IT IS UNDOUBTEDLY
MUCH MORE COMPLICATED
THAN THIS!!!
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
HEARING
Acoustics
Middle Ear Function
Winter 2009
Inner Ear Physiology and Pathology
Page 12 of 23
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 13 of 23
Impedance matching
Acoustic impedance
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 14 of 23
Audiograms
Basilar Membrane Deflection
Sound waves pass through the cochlea INSTANTANEOUSLY.
Traveling Wave & Place Principle
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.
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 15 of 23
Passive Properties
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).
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 16 of 23
Otoacoustic 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).
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Central Processing
Pathways
Auditory Brainstem Responses
Winter 2009
Inner Ear Physiology and Pathology
Page 17 of 23
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 18 of 23
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.
Hearing Tests

for details on tympanometry, Rinne/Weber and audiograms (behavioural,
ABRs, OAEs), please look at my Website
Audiograms
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 19 of 23
INNER EAR PATHOLOGY
Vestibular only
Nystagmus
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).
The caloric test can be 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  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.
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 20 of 23
Benign positional vertigo
Labyrinthitis (A&V?)
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.
Genetics
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Conductive Hearing Loss
Sensorineural Hearing Loss
Cochlear Implant
Winter 2009
Inner Ear Physiology and Pathology
Page 21 of 23
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 22 of 23
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.
Central Processing Disorders (CAPD)
Tinnitus
Presbycusis
Med 6573 Nervous System
Dr. Janet Fitzakerley [email protected]
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Winter 2009
Inner Ear Physiology and Pathology
Page 23 of 23
Vestibular and Auditory
Acoustic Neuroma
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.