Transcript ch15_hearing_m&h9_wc..

Ch. 15
Special Senses: Hearing
Slides mostly © Marieb & Hoehn 9th ed.
Other slides by WCR
The Ear: Hearing and Balance
•
Three major areas of ear
1. External (outer) ear – hearing only
2. Middle ear (tympanic cavity) – hearing only
3. Internal (inner) ear – hearing and
equilibrium
•
•
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Receptors for hearing and balance respond to
separate stimuli
Are activated independently
Figure 15.24a Structure of the ear.
Middle Internal ear
External ear
(labyrinth)
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic Tympanic Pharyngotympanic
meatus membrane (auditory) tube
The three regions of the ear
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External Ear
• Auricle (pinna) & external acoustic meatus
(auditory canal)
– Funnel sound waves to eardrum
• Tympanic membrane (eardrum)
– Boundary between external and middle ears
– Connective tissue membrane that vibrates in
response to sound
– Transfers sound energy to bones of middle ear
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Middle Ear
• Air-filled (usually), mucosa-lined cavity in
temporal bone
– Flanked laterally by eardrum
– Remaining borders are formed by by temporal bone
– Oval window, round windows: covered connections to
the inner ear
• Contains 3 bones, 2 muscles
• Pharyngotympanic (auditory) tube
– Connects middle ear to nasopharynx
– Equalizes pressure in middle ear cavity with external
air pressure
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Ear Ossicles
• Three small bones in tympanic cavity: the
malleus, incus, and stapes
– Suspended by ligaments and joined by
synovial joints
– Transmit vibratory motion of eardrum to oval
window
– Tensor tympani and stapedius muscles
contract reflexively in response to loud
sounds to prevent damage to hearing
receptors
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Figure 15.24b Structure of the ear.
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Malleus
(hammer)
Incus
Auditory
(anvil)
ossicles
Stapes
(stirrup)
Tympanic membrane
Semicircular
canals
Vestibule
Vestibular
nerve
Cochlear
nerve
Cochlea
Round window
Middle and internal ear
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Pharyngotympanic
(auditory) tube
Otitis Media
• Middle ear inflammation
– Especially in children
• Shorter, more horizontal pharyngotympanic tubes
• Most frequent cause of hearing loss in children
– Most treated with antibiotics
– Myringotomy to relieve pressure if severe
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Figure 15.25 The three auditory ossicles and associated skeletal muscles.
View
Superior
Malleus
Incus Epitympanic recess
Lateral
Anterior
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Pharyngotym- Tensor
tympani
panic tube
muscle
Tympanic Stapes Stapedius
membrane
muscle
(medial view)
Two Major Divisions of Internal Ear
• Bony labyrinth
– Tortuous channels in temporal bone
– Three regions: vestibule, semicircular
canals, and cochlea
– Filled with perilymph – similar to CSF
• Membranous labyrinth
– Series of membranous sacs and ducts
– Filled with potassium-rich endolymph
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Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Anterior
Posterior
Lateral
Facial nerve
Vestibular nerve
Cristae ampullares
in the membranous
ampullae
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
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Stapes in
oval window
Round window
The Cochlea
• Spiral, conical, bony chamber
– Size of split pea, goes from base to apex
– Contains cochlear duct, which houses organ of Corti
(spiral organ)
• Cavity of cochlea divided into three chambers
– Scala vestibuli—abuts oval window & stapes,
contains perilymph
– Scala media (cochlear duct)—contains endolymph
– Scala tympani—terminates at round window; contains
perilymph
• Scala tympani, scala vestibuli connect with each
other at helicotrema (apex)
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The Cochlea
• Cochlear duct (scala media) is sandwiched
between scala vestibuli & scala tympani
• "Floor" of cochlear duct formed by basilar
membrane, which supports organ of Corti (spiral
organ)
• Cochlear branch of nerve VIII runs from cochlea
to brain
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Figure 15.27a Anatomy of the cochlea.
Helicotrema
at apex
Modiolus
Cochlear nerve,
division of the
vestibulocochlear
nerve (VIII)
Spiral ganglion
Osseous spiral lamina
Vestibular membrane
Cochlear duct
(scala media)
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Figure 15.27b Anatomy of the cochlea.
Vestibular membrane
Tectorial membrane
Cochlear duct
(scala media;
contains
endolymph)
Stria
vascularis
Spiral organ
Basilar
membrane
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Osseous spiral lamina
Scala
vestibuli
(contains
perilymph)
Scala
tympani
(contains
perilymph)
Spiral
ganglion
Figure 15.27c Anatomy of the cochlea.
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
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Figure 15.27d Anatomy of the cochlea.
Inner
hair
cell
Outer
hair
cell
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Properties of Sound
• Sound is
– Pressure disturbance (alternating areas of
high and low pressure) produced by vibrating
object
• Sound wave
– Moves outward in all directions
– Illustrated as an S-shaped curve or sine wave
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Figure 15.28 Sound: Source and propagation.
Area of
high pressure
(compressed
molecules)
Air pressure
Wavelength
Distance
Area of
low pressure
(rarefaction)
Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it, creating
alternate zones of high and low pressure.
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Sound waves radiate
outward in all
directions.
Properties of Sound Waves
• Frequency
– Number of waves that pass given point in given time
– Wavelength
• Distance between two consecutive crests
• Shorter wavelength = higher frequency of sound
– Frequency range of normal (healthy) hearing: 20 –
20,000 Hertz (Hz)
• Pitch
– Perception of frequency: higher frequency = higher
pitch
• Most sounds are mixtures of many different
frequencies simultaneously
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Properties of Sound
• Amplitude
– Height of crests
• Loudness = perception of amplitude
– Subjective interpretation of sound intensity
– Normal range is 0–120 decibels (dB)
– Severe hearing loss with prolonged exposure above
90 dB
– Loud music is 120 dB or more
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Figure 15.29 Frequency and amplitude of sound waves.
Pressure
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
0.01
0.02
Time (s)
0.03
Frequency is perceived as pitch.
Pressure
High amplitude = loud
Low amplitude = soft
0.01
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0.02
Time (s)
0.03
Amplitude (size or intensity) is perceived as loudness.
Transmission of Sound to the Internal Ear
• Sound waves vibrate tympanic membrane
• Ossicles vibrate and concentrate the energy
(amplify the pressure) at stapes footplate in oval
window
• Cochlear fluid set into wave motion
• Pressure waves move through perilymph of
scala vestibuli
• Basilar membrane is “mechanically tuned”:
different parts vibrate most (i.e. resonate) in
response to different frequencies
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Basilar Membrane Tuning (Resonance)
• Fibers near oval window short and stiff
– Resonate with high-frequency pressure waves
• Fibers near cochlear apex longer, more floppy
– Resonate with lower-frequency pressure waves
• Thus basilar membrane “maps” different
frequencies to different places along its length.
– The “place theory” of hearing is most true for
disciminating high frequencies.
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Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 1
Auditory ossicles
Malleus Incus
Stapes
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
4b
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
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vestibuli.
4a Sounds with
frequencies below hearing
travel through the
helicotrema and do not
excite hair cells.
4b Sounds in the hearing
range go through the
cochlear duct, vibrating the
basilar membrane and
deflecting hairs on inner hair
cells.
Figure 15.30b Pathway of sound waves and resonance of the basilar membrane.
Basilar membrane
High-frequency sounds displace the
basilar membrane near the base.
Medium-frequency sounds displace the
basilar membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Fibers of basilar membrane
Apex
(long,
floppy
fibers)
Base (short,
stiff fibers)
20,000
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2000
200
Frequency (Hz)
20
Different sound frequencies cross the basilar membrane at
different locations.
Excitation of Hair Cells in the Spiral Organ
• Cells of spiral organ
– Supporting cells
– Cochlear hair cells
• One row of inner hair cells
• Three rows of outer hair cells
• Have many stereocilia and one kinocilium
• Afferent fibers of cochlear nerve coil about
bases of hair cells
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Figure 15.27c Anatomy of the cochlea.
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
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Excitation of Hair Cells in the Spiral Organ
Stereocilia protrude from hair cells, some embed
in tectorial membrane above
• Passing pressure wave causes deflection of basilar
membrane
• Shearing action of basilar membrane and tectorial
membrane causes cilia to bend
• Opens mechanically gated ion channels via pull on tip
links
– Inward current causes graded potential and release of
neurotransmitter glutamate from hair cell onto sensory neuron
• Cochlear fibers transmit impulses to brain
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Hair cell transduction by ion channel opening
Source: Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th
edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors; 1999. Downloaded
2011-12-04 from http://www.ncbi.nlm.nih.gov/books/NBK28026/
Sensory pathway for hearing
1. Hair cells in specific area of basilar membrane become
stimulated
2. Sensory neuron axons (cell bodies in spiral ganglion) make
up cochlear branch of vestibulocochlear nerve (VIII)
3. Sensory neuron axons synapse onto neurons in cochlear
nucleus (medulla oblongata)
4. Information ascends bilaterally (often synapsing on the way)
to inferior colliculus (midbrain)
5. Inferior colliculus neurons synapse at medial geniculate
nucleus (thalamus)
6. Projection fibers from thalamus reach primary auditory
cortex (temporal lobe)
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Auditory pathway
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (ponsmedulla junction)
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear
nerve
Vibrations
Spiral ganglion
of cochlear nerve
Bipolar cell
Spiral organ
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Tonotopic organization
• Different frequency sounds excite different basilar membrane
regions (apex: low frequencies; base: high frequencies)
• Cochlear nucleus (first auditory area in CNS) has a “map” of
basilar membrane, i.e. frequency map: tonotopic map
• Tonotopic map seen in successive higher centers, up to &
including primary auditory cortex
Tonotopic organization of primary auditory cortex
Source: Lynch, downloaded 2011-12-04
http://www.colorado.edu/intphys/Class/IPHY3730/image/figure8-16.jpg
Localizing sounds
•
Most auditory information crosses over but
some doesn’t, so brainstem and cortical areas
get inputs from both ears
•
Right versus left arrival time difference
•
Right versus left intensity difference
•
Both are used to localize sounds
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Conduction deafness
•
Sound energy is not conducted from outside
world to the receptors, i.e. doesn’t make it to
inner ear
•
Causes include:
•
Water or excess cerumen in external ear
•
Scarring or perforation of tympanic membrane
•
Immobility of ear ossicles (fluid, pus, tumor;
otosclerosis)
Otosclerosis: abnormal bony growth
around stapes footplate prevents
normal stapes movement.
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Sensorineural (nerve) deafness
•
Most common cause of permanent deafness
•
Damage to hair cell receptors
•
•
Normal (young) range: 20–20,000 Hz; hearing
loss later, high frequencies go first
•
Loud noise, infection, some drugs
Damage to nerve or to central auditory
pathways
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Hair cells in healthy and damaged cochleas
Normal organ of
Corti, with tectorial
membrane
removed to show
hair cells.
Damaged organ of
Corti.
Ryan AF. Protection of auditory receptors and neurons:
evidence for interactive damage. PNAS 97:6939-6940, 2000.
©2000©by2011
National
Academy
of Sciences
Pearson
Education,
Inc.
Treating Sensorineural Deafness
• Cochlear implants for congenital or age/noise
cochlear damage
– Convert sound energy into electrical signals
– Inserted into drilled recess in temporal bone
– So effective that deaf children can learn to speak
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