Download The Special Senses The Ear External Ear Middle Ear

1/24/2016
The Ear
• The organ of hearing and equilibrium
The Special Senses
– Cranial nerve VIII - Vestibulocochlear
– Regions
• External ear
• Middle ear
• Internal ear (labyrinth)
Hearing and
Equilibrium
External Ear
External
ear
• Two parts
– Pinna or auricle (external structures)
– External auditory meatus (car canal)
• Site of cerumen (earwax) production
Middle Internal ear
ear
(labyrinth)
Auricle
(pinna)
– Waterproofing, protection
• Separated from the middle ear by the tympanic membrane
(eardrum)
Helix
– Vibrates in response to sound waves
Lobule
External
Tympanic
acoustic
membrane
meatus
(a) The three regions of the ear
Pharyngotympanic
(auditory) tube
Figure 15.25a
Middle Ear
Middle Ear
Malleus
Superior
Incus
Epitympanic
recess
Lateral
• Tympanic cavity
Anterior
– Air-filled chamber
– Openings
View
• Tympanic membrane – covers opening to outer ear
• Round and oval windows – openings to inner ear
• Epitympanic recess – dead-end cavity into temporal bone of unknown
function
• Auditory tube
– AKA Eustachian tube or pharyngotympanic tube
Pharyngotympanic tube
Tensor
tympani
muscle
Tympanic
membrane
(medial view)
Stapes
Stapedius
muscle
Figure 15.26
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Middle Ear
• Auditory tube (Eustachian tube)
Middle Ear
• Otitis Media
– Connects the middle ear to the nasopharynx
• Equalizes pressure
– Opens during swallowing and yawning
Middle Ear
• Contains auditory ossicles (bones)
– Malleus
– Incus
– Stapes
• Middle ear is air-filled; inner ear is fluid-filled
• Sound is mostly reflected from a liquid medium
Middle Ear
• Sound waves cause tympanic membrane to vibrate
• Ossicles help transmit vibrations into the inner ear
– Reduce the area where force is applied
– Increases the pressure of the force enough to transfer most of the
energy into the liquid
– Reflexive muscle action
restricts the movement of
the bones during loud
noises
Inner Ear
Superior vestibular ganglion
Inferior vestibular ganglion
• Contains functional organs for hearing & equilibrium
– Bony labyrinth - filled with perilymph
– Membranous labyrinth – functional component
• Filled with endolymph
• Location of various
inner ear receptors
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
Stapes in
oval window
Round
window
Figure 15.27
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Inner ear - Labyrinth
Superior vestibular ganglion
Inferior vestibular ganglion
• Labyrinth is modified to form 3 distinct regions
– Vestibule
• Gravity
• Head position
• Linear acceleration and deceleration (changes in speed)
– Semicircular canals
• Angular acceleration and deceleration (changes in direction)
– Cochlea
• Vibration
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
Stapes in
oval window
* Note: It is always the membranous labyrinth that contains the receptors
Round
window
Figure 15.27
Inner Ear
• The cochlea
– A spiral, conical, bony chamber
Cochlear nerve,
division of the
vestibulocochlear
nerve (VIII)
Modiolus
– Still 2 portions of bony labyrinth enclosing a portion of membranous
labyrinth
Bony
labyrinth
Spiral ganglion
Membranous
labyrinth
Osseous spiral lamina
Vestibular membrane
Bony
labyrinth
Cochlear duct
(scala media)
Helicotrema
(a)
Figure 15.28a
Inner ear
Vestibular membrane
Osseous spiral lamina
Tectorial membrane
• Cavity of the cochlea is divided into 3 chambers
– Vestibular canal (scala vestibuli)
• Vestibular membrane
– Cochlear duct (scala media)
• Basilar membrane supporting Organ of Corti
– Organ of hearing
– Tympanic canal (scala tympani)
Cochlear duct
(scala media;
contains
endolymph)
Scala
vestibuli
(contains
perilymph)
Spiral
ganglion
Stria
vascularis
Spiral organ
(of Corti)
Basilar
membrane
Scala tympani
(contains
perilymph)
(b)
Figure 15.28b
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Organ of Corti
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Organ of Corti
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
(c)
Figure 15.28c
Physiology of hearing
Physiology of Hearing in a Nutshell
Sounds set up vibrations in air that beat against
the eardrum that pushes a chain of tiny bones
that press fluid in the internal ear against
membranes that set up shearing forces that pull
on the tiny hair cells that stimulate nearby
neurons that give rise to the impulses that travel
to the brain – and you hear.
• Transduction of sound
Mechanical energy in middle ear
Fluid pressure wave in inner ear
Nerve impulse
Wavelength
Area of
high pressure
(compressed
molecules)
Area of
low pressure
(rarefaction)
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
Pressure
Air pressure
(This is from your textbook)
Crest
Trough
Time (s)
(a) Frequency is perceived as pitch.
Distance
Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it,
creating alternate zones of high and
low pressure.
Pressure
High amplitude = loud
Low amplitude = soft
(b) Sound waves
radiate outward
in all directions.
Figure 15.29
Time (s)
(b) Amplitude (size or intensity) is perceived as loudness.
Figure 15.30
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Incus
Malleus
Stapes vibrating
in oval window
Helicotrema
Auditory ossicles
Cochlea
Malleus
Incus
Stapes
Cochlear nerve
Sound waves
Perilymph
Scala vestibuli
Oval
window
3
7
4
Scala
tympani
Scala
vestibuli
Basilar
membrane
8
5
1
2
6
9
External auditory
canal
Helicotrema
Scala tympani
Cochlear duct
2
3
Basilar
membrane
1
8
Spiral organ
(organ of Corti)
Tectorial membrane
Sounds with frequencies
below hearing travel through
the helicotrema and do not
excite hair cells.
Vestibular membrane
Cochlear duct
(contains endolymph)
Tympanic
membrane
Secondary tympanic
membrane vibrating
in round window
Tympanic
Round
membrane
window
(a) Route of sound waves through the ear
3 Pressure waves created by
1 Sound waves vibrate
Middle ear
Auditory tube
the tympanic membrane.
2 Auditory ossicles vibrate.
Pressure is amplified.
the stapes pushing on the oval
window move through fluid in
the scala vestibuli.
Sounds in the hearing range
go through the cochlear duct,
vibrating the basilar membrane
and deflecting hairs on inner
hair cells.
Figure 15.31a
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Basilar membrane
Outer hair cells
High-frequency sounds displace
the basilar membrane near the base.
Supporting cells
Fibers of
cochlear
nerve
Fibers of basilar membrane
Medium-frequency sounds displace
the basilar membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Apex
(long,
floppy
fibers)
Base
(short,
stiff
fibers)
Frequency (Hz)
(b) Different sound frequencies cross the
basilar membrane at different locations.
Basilar
membrane
(c)
Figure 15.31b
•Movement of the basilar membrane bends the hair cells
Figure 15.28c
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary nucleus
(pons-medulla junction)
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear nerve
Vibrations
Spiral ganglion of cochlear nerve
Bipolar cell
Spiral organ (of Corti)
Figure 15.33
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Localization of Sound
• Timing comparison
Abnormalities of hearing
• Conductive deafness
– Interference in movement of middle ear bones
– Side nearest sound detects sound first
• Comparison of volume
– High pitched = blocked by head
• Impacted earwax, perforated eardrum or otosclerosis of the ossicles (overgrowth
of bone)
• Sensorineural deafness
– Damage to the neural structures
• Perceived as loudest in ear nearest the source
• Aging, prolonged exposure to loud sounds
– Low pitched = curve around head
• Perceived as equally loud in both ears
Abnormalities of hearing
• Otitis media
Abnormalities of hearing
• Otosclerosis
Abnormalities of hearing
• Meniere’s Disease
Episodes of vertigo
Progressive hearing loss
Tinnitus
Feeling of fullness or
pressure in ear
– Usually only in one ear
–
–
–
–
Abnormalities of hearing
• Tinnitus
–
–
–
–
Ringing, clicking, hissing, or roaring
Sometimes can be heard with a stethoscope (objective tinnitus)
May occur in the same rhythm as the heartbeat
Caused by noise-induced hearing loss, ear infections, diseases of blood
vessels, head injury, neurological problems, brain tumors, earwax…
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Vestibular Apparatus
Physiology of Equilibrium
Kinocilium
Stereocilia
• Information about position and movement of head
– Vestibular apparatus
Otoliths
Otolithic
membrane
Hair bundle
Macula of
utricle
Macula of
saccule
• Utricle
Vesibule
• Saccule
• Semi-circular canals
Hair cells
Maculae are
perpendicular
to one another
Supporting
cells
Vestibular
nerve fibers
Figure 15.34
Vestibule
Semi-Circular canals
– Crista ampullaris
• Macula = receptor
• Sensory receptor for dynamic equilibrium
–Utricle
– One in the ampulla of each semicircular canals
– Major stimuli are rotatory movements
• Horizontal movements
• Tilting the head side to side
–Saccule
• Vertical movements
Equilibrium
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
• Motion Sickness
– Conflicts between eye movements and equilibrium
• Nystagmus
• Physiologic
Supporting
cell
– Involuntary eye movement, part of a reflex
– Preserves clear vision during rotation of the head
• Pathologic
– Abnormal eye movement – eyes move as though the head is rotating
while the head is still
– Caused by damage to any part of the vestibular system
Cupula
(b) Scanning electron
micrograph of a
crista ampullaris
(200x)
Figure 15.36a–b
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