Download 15 - Riverside City College

THE EAR: HEARING
AND BALANCE
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The Ear: Hearing and Balance
• Three parts of the ear
• The External (outer) Ear
• Pinna- Composed of the Helix (rim) and lobule
(earlobe)
• External acoustic canal (meatus)-short, curved tube
leading to eardrum
• Lined with ceruminous glands
• Tympanic membrane- eardrum; CT boundary btwn.
external and middle ear; vibrates in response to
sound
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External
ear
Middle Internal ear
ear
(labyrinth)
Auricle
(pinna)
Helix
Lobule
External
Tympanic Pharyngotympanic
acoustic
membrane (auditory) tube
meatus
(a) The three regions of the ear
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Figure 15.25a
Middle Ear
• The Middle Ear (tympanic cavity)
• A small, air-filled, mucosa-lined cavity in the
temporal bone; flanked laterally by the
eardrum and medially by the oval and round
windows
• Three bones (ossicles)- Incus, Malleus,Stapes
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Middle Ear
• Two tiny skeletal muscles prevent damage
due to large vibrations
• Tensor tympani-arises from wall of
pharyngotympanic tube and inserts on the
malleus
• Stapedius muscle-runs from posterior wall of
middle ear to the stapes
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Middle Ear
• Pharyngotympanic (auditory) tubes
• Connects middle ear to nasopharynx
• Normally flattened closed; opens when yawning or
swallowing
• Equalizes pressure in the middle ear cavity with the
external air pressure
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Malleus
Superior
Epitympanic
Incus
recess
Lateral
Anterior
View
Pharyngotympanic tube
Tensor
tympani
muscle
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Tympanic
membrane
(medial view)
Stapes
Stapedius
muscle
Figure 15.26
Inner Ear
• Consists of the bony (osseous) and membranous
labyrinth
• Bony labyrinth
• Tortuous channels in the temporal bone
• Filled with perilymph
• Three regions:
• Vestibule• Contains two sacs
• Saccule-continuous w/ the cochlear duct
• Utricle-continuous w/ the semicircular ducts
• Cochlea
• Semicircular canals
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The Inner Ear
• Membranous Labyrinth
• Series of membranous sacs and ducts
contained w/in the bony labyrinth
• Follows contours of bony labyrinth
• Filled with endolymph
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Superior vestibular ganglion
Inferior vestibular ganglion
Temporal
bone
Semicircular
ducts in
semicircular
canals
Facial nerve
Vestibular
nerve
Anterior
Posterior
Lateral
Cochlear
nerve
Maculae
Cristae ampullares
in the membranous
ampullae
Spiral organ
(of Corti)
Cochlear
duct
in cochlea
Utricle in
vestibule
Saccule in
vestibule
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Stapes in
oval window
Round
window
Figure 15.27
The Maculae and Static Equilibrium
• Maculae• Sensory receptors for static equilibrium (monitor the
position of head in space, respond to linear
acceleration)
• One in each saccule wall and one in each utricle wall
• Each maculae contains:
• ET hair cells and supporting cells
• Otolithic membrane- gel like membrane that overlies
hair cells
• Otoliths- calcium carbonate crystals; increase
weight and its inertia
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Kinocilium
Stereocilia
Otoliths
Otolithic
membrane
Hair bundle
Macula of
utricle
Macula of
saccule
Hair cells
Supporting
cells
Vestibular
nerve fibers
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Figure 15.34
Steps of linear movement
• Hair cells are always releasing neurotransmitter
• When hair cells bend towards kinocilium they
depolarize, neurotransmitter release is increased
• When hair cells bend away from kinocilium they
hyperpolarize, slowing release of
neurotransmitter
• This change in neurotransmitter release informs
the brain of the changing of the position of the
head in space
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Otolithic membrane
Kinocilium
Stereocilia
Hyperpolarization
Receptor
potential
Nerve impulses
generated in
vestibular fiber
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Depolarization
When hairs bend toward
the kinocilium, the hair
cell depolarizes, exciting
the nerve fiber, which
generates more frequent
action potentials.
When hairs bend away
from the kinocilium, the
hair cell hyperpolarizes,
inhibiting the nerve fiber,
and decreasing the action
potential frequency.
Figure 15.35
The Crista Ampullaris and Dynamic Equilibrium
• Dynamic Equilibrium
• Detected by:
• Crista Ampullaris –
• One in the ampulla of each semicircular
canal
• Major stimuli are rotatory movements
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The Crista Ampullaris and Dynamic Equilibrium
• Semicircular Canals
• Three canals are located in each ear:
• Located in all three planes of space
• Anterior, posterior and lateral
• Endolymph-fills the semicircular ducts
• Ampulla- swellling at end of semicircular duct
• Crista Ampullaris
• Composed of hair cells and supporting cells
• Structure and function of the crista ampullaris is basically
the same as the hair cells of cochlea and maculae
• Cupula – gelled mass that cilia of hair cells are embedded
in
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Cupula
Crista
ampullaris
Endolymph
Hair bundle (kinocilium
plus stereocilia)
Hair cell
Crista
Membranous
ampullaris
labyrinth
Fibers of vestibular nerve
(a) Anatomy of a crista ampullaris in a
semicircular canal
Supporting
cell
Cupula
(b) Scanning electron
micrograph of a
crista ampullaris
(200x)
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Figure 15.36a–b
The Crista Ampullaris and Dynamic
Equilibrium
• Steps of Rotational Movement
• At rest the cupula stands upright
• During rotational acceleration, hair cells are
bent, they depolarize and impulses reach the
brain faster
• As movement slows, endolymph keeps
moving, cilia are bent in opposite direction
causing hyperpolarization and reduction of
impulses to brain
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Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands
upright.
(c) Movement of the
cupula during
rotational
acceleration
and deceleration
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Flow of endolymph
During rotational acceleration,
endolymph moves inside the
semicircular canals in the
direction opposite the rotation
(it lags behind due to inertia).
Endolymph flow bends the
cupula and excites the hair
cells.
As rotational movement
slows, endolymph keeps
moving in the direction
of the rotation, bending
the cupula in the
opposite direction from
acceleration and
inhibiting the hair cells.
Figure 15.36c
Equilibrium Pathway to the Brain
• Vestibular nerve-Impulses travel to the
vestibular nuclei in the brain stem or the
cerebellum
• Pathways are complex and poorly traced
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Sound and the Cochlea
• Sound is detected by: the cochlea
• Cochlea
• The cochlea is A spiral, conical, bony chamber and contains the
cochlear duct
• Cochlear duct- houses the spiral organ (of Corti)
• Divides cochlea into three chambers:
• Scala vestibuli-superior to cochlear duct (contains
perilymph)
• Scala tympani-inferior to cochlear duct; terminates at
round window (contains perilymph)
• Scala media (cochlear duct) -middle cavity; (contains
endolymph)
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Modiolus
Cochlear nerve,
division of the
vestibulocochlear
nerve (VIII)
Spiral ganglion
Osseous spiral lamina
Vestibular membrane
Cochlear duct
(scala media)
(a)
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Helicotrema
Figure 15.28a
Vestibular membrane
Osseous spiral lamina
Tectorial membrane
Cochlear duct
(scala media;
contains
endolymph)
Spiral organ
(of Corti)
Basilar
membrane
Scala
vestibuli
(contains
perilymph)
Spiral
ganglion
Scala tympani
(contains
perilymph)
(b)
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Figure 15.28b
Sound and the Cochlea
• Oval window-an opening on the medial wall of the middle
ear (foot of stapes rests at oval window)
• Round window-an opening on the medial wall of the
middle ear (scala tympani terminates at round window)
• Vestibular membrane-roof of cochlear duct that separates
the scala media from scala vestibuli
• Basilar membrane- fibrous floor of cochlear duct
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Sound and the Cochlea
• Organ of Corti
• Runs through center of cochlea
• Has hair cells and supporting cells
• Tectorial membrane- gel-like mass that cilia of
hair cells are embedded in
• Bending of the cilia: excites hair cells
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Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
(c)
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Basilar
membrane
Figure 15.28c
Sound Transmission
• Transmission of Sound to the Inner Ear
• Sound waves enter the external acoustic canal
and cause tympanic membrane to vibrate
• Ossicles vibrate and amplify the pressure at
the oval window
• Pressure waves move through perilymph of
the scala vestibuli
• Sounds in the hearing range go through the
cochlear duct, vibrating the basilar membrane
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Auditory ossicles
Malleus Incus Stapes
Cochlear nerve
Scala vestibuli
Oval
window Helicotrema
2
3
Scala tympani
Cochlear duct
Basilar
membrane
1
Tympanic
Round
membrane
window
(a) Route of sound waves through the ear
1 Sound waves vibrate
the tympanic membrane.
2 Auditory ossicles vibrate.
Pressure is amplified.
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3 Pressure waves created by
the stapes pushing on the oval
window move through fluid in
the scala vestibuli.
Sounds with frequencies
below hearing travel through
the helicotrema and do not
excite hair cells.
Sounds in the hearing range
go through the cochlear duct,
vibrating the basilar membrane
and deflecting hairs on inner
hair cells.
Figure 15.31a
Sound Transmission
• Resonance of the Basilar Membrane - fibers of
the basilar membrane are “tuned” to a particular
sound frequency
• Vibrations of the basilar membrane causes cilia
of hair cells to bend
• Bending cilia towards kinocilium excites hair cells
(increase neurotransmitter release)
• Bending cilia away from kinocilium inhibits hair
cells (slow release of neurotransmitter)
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Sound Transmission
• Impulses from the cochlea pass via the spiral
ganglion to the cochlear nuclei of the medulla
• Eventually impulses are sent to the primary
auditory cortex (temporal lobe)
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Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear nerve
Vibrations
Spiral ganglion of cochlear nerve
Spiral organ (of Corti)
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Figure 15.33
Deafness
• Hearing loss can be temporary or permanent
• Common causes:
• Middle ear infections
• Conduction deafness
• Can be caused by:
• Impacted earwax
• Ruptured eardrum
• Middle ear inflammations
• Otosclerosis
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Deafness
• Nerve Deafness
• Can be caused by:
• Gradual loss of hair cells throughout life
• Single explosively loud noise
• Prolonged exposure to loud noise
• Degeneration of cochlear nerve, tumors in
auditory cortex, etc.
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Tinnitus
• Ringing or clicking sound in ears in the
absence of auditory stimuli
• One of the first symptoms of cochlear
degeneration
• Can be caused by middle ear inflammation
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Meniere’s Syndrome
• Labyrinth disorder
• Affects all three parts of the internal ear
• Symptoms are repeated attacks of vertigo,
nausea and vomiting
• Balance is severely disturbed and hearing is
ultimately lost
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