1. Special Senses: Hearing Slides mostly © Marieb & Hoehn 9th ed.
Ch. 15
Special Senses: Hearing
Slides mostly © Marieb & Hoehn 9th ed.
Other slides by WCR

2. The Ear: Hearing and Balance
Three major areas of ear
External (outer) ear – hearing only
Middle ear (tympanic cavity) – hearing only
Internal (inner) ear – hearing and equilibrium
Receptors for hearing and balance respond to separate stimuli
Are activated independently
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3. The three regions of the ear
Figure 15.24a Structure of the ear.
Middle
ear
Internal ear
(labyrinth)
External
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic
meatus
Tympanic
membrane
Pharyngotympanic
(auditory) tube
The three regions of the ear
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4. 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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5. 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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6. Three small bones in tympanic cavity: the malleus, incus, and stapes
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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7. Middle and internal ear
Figure 15.24b Structure of the ear.
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Semicircular
canals
Malleus
(hammer)
Vestibule
Incus
(anvil)
Auditory
ossicles
Vestibular
nerve
Stapes
(stirrup)
Cochlear
nerve
Tympanic membrane
Cochlea
Round window
Pharyngotympanic
(auditory) tube
Middle and internal ear
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8. Middle ear inflammation
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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9. View Superior Malleus Incus Epitympanic recess Lateral Anterior
Figure The three auditory ossicles and associated skeletal muscles.
View
Superior
Malleus
Incus
Epitympanic recess
Lateral
Anterior
Pharyngotym-
panic tube
Tensor
tympani
muscle
Tympanic
membrane
(medial view)
Stapes
Stapedius
muscle
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10. 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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11. Temporal bone Semicircular ducts in semicircular canals Facial nerve
Figure Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Facial nerve
Vestibular nerve
Superior vestibular
ganglion
Anterior
Posterior
Inferior vestibular
ganglion
Lateral
Cristae ampullares
in the membranous
ampullae
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
Stapes in
oval window
Round window
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12. 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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13. 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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14. Modiolus Spiral ganglion Osseous spiral lamina Vestibular membrane
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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15. Vestibular membrane Osseous spiral lamina Tectorial membrane Spiral
Figure 15.27b Anatomy of the cochlea.
Vestibular membrane
Osseous spiral lamina
Tectorial membrane
Spiral
ganglion
Scala
vestibuli
(contains
perilymph)
Cochlear duct
(scala media;
contains
endolymph)
Stria
vascularis
Spiral organ
Scala
tympani
(contains
perilymph)
Basilar
membrane
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16. Hairs (stereocilia) Supporting cells
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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17. Inner hair cell Outer hair cell Figure 15.27d Anatomy of the cochlea.
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18. Properties of Sound Sound is Sound wave
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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19. A struck tuning fork alternately compresses
Figure Sound: Source and propagation.
Area of
high pressure
(compressed
molecules)
Area of
low pressure
(rarefaction)
Wavelength
Air pressure
Distance
Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it, creating
alternate zones of high and low pressure.
Sound waves radiate
outward in all
directions.
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20. 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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21. Properties of Sound Amplitude Loudness = perception of 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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22. High frequency (short wavelength) = high pitch
Figure Frequency and amplitude of sound waves.
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
Pressure
0.01
0.02
0.03
Time (s)
Frequency is perceived as pitch.
High amplitude = loud
Low amplitude = soft
Pressure
0.01
0.02
0.03
Time (s)
Amplitude (size or intensity) is perceived as loudness.
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23. 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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24. 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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25. Route of sound waves through the ear
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 1
Auditory ossicles
Malleus
Incus
Stapes
Cochlear nerve
Scala vestibuli
Oval
window
Helicotrema 4a
Scala tympani
Cochlear duct 2 3
Basilar
membrane 4b 1
Sounds with frequencies below hearing travel through the
helicotrema and do not excite hair cells. 4a
Tympanic
membrane
Round
window
Route of sound waves through the ear
Sound waves vibrate the tympanic membrane. 1
Auditory ossicles vibrate. Pressure is amplified. 2
Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli. 3
Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and
deflecting hairs on inner hair cells. 4b
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26. High-frequency sounds displace the basilar membrane near the base.
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
2000
200 20
Frequency (Hz)
Different sound frequencies cross the basilar membrane at different locations.
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27. 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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28. Hairs (stereocilia) Supporting cells
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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29. 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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30. 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; Downloaded from

31. Sensory pathway for hearing
Hair cells in specific area of basilar membrane become stimulated
Sensory neuron axons (cell bodies in spiral ganglion) make up cochlear branch of vestibulocochlear nerve (VIII)
Sensory neuron axons synapse onto neurons in cochlear nucleus (medulla oblongata)
Information ascends bilaterally (often synapsing on the way) to inferior colliculus (midbrain)
Inferior colliculus neurons synapse at medial geniculate nucleus (thalamus)
Projection fibers from thalamus reach primary auditory cortex (temporal lobe)

32. Auditory pathway Medial geniculate nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (pons-
medulla junction)
Midbrain
Cochlear nuclei
Medulla
Vibrations
Vestibulocochlear
nerve
Vibrations
Spiral ganglion
of cochlear nerve
Bipolar cell
Spiral organ
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33. 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

34. Tonotopic organization of primary auditory cortex
Source: Lynch, downloaded

35. 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

36. 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.

37. 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

38. Hair cells in healthy and damaged cochleas
Normal organ of Corti, with tectorial membrane removed to show hair cells.
Damaged organ of Corti.
Scanning electron micrographs of the normal (a) and damaged (b) cochlear sensory epithelium. In the normal cochlea, the stereocilia of a single row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs) are present in an orderly array. In the damaged cochlea, hair cells are missing, and stereocilia are abnormal, leading to hearing loss. Duan et al. (1) present data indicating that hair cell damage can be prevented by protecting the postsynaptic structures of their associated sensory neurons. (Micrographs are courtesy of Elizabeth M. Keithley.)‏
Ryan AF. Protection of auditory receptors and neurons: evidence for interactive damage. PNAS 97: , 2000.
©2000 by National Academy of Sciences

39. 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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