Post on 01-Apr-2015
Ch. 15Special Senses: Hearing
Slides mostly © Marieb & Hoehn 9th ed.Other slides by WCR
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The Ear: Hearing and Balance
• Three major areas of ear1. External (outer) ear – hearing only
2. Middle ear (tympanic cavity) – hearing only
3. Internal (inner) ear – hearing and equilibrium• Receptors for hearing and balance respond to
separate stimuli• Are activated independently
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Figure 15.24a Structure of the ear.
Externalear
Middleear
Internal ear(labyrinth)
Auricle(pinna)
Helix
Lobule
Externalacousticmeatus
Tympanicmembrane
Pharyngotympanic(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)
Semicircularcanals
Vestibule
Vestibularnerve
Cochlearnerve
Cochlea
Pharyngotympanic(auditory) tube
Entrance to mastoid antrum in the epitympanic recess
Auditoryossicles
Tympanic membrane
Round window
Stapes(stirrup)
Incus(anvil)
Malleus(hammer)
Middle and internal ear
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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
Anterior
Lateral
IncusMalleusEpitympanic
recess
Pharyngotym-panic tube
Tensortympanimuscle
Tympanicmembrane(medial view)
Stapes Stapediusmuscle
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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.
Temporalbone
Facial nerve
Vestibular nerve
Superior vestibularganglionInferior vestibularganglionCochlear nerveMaculaeSpiral organ
Cochlear ductin cochlea
Round windowStapes inoval window
Saccule investibule
Utricle investibule
Cristae ampullaresin the membranousampullae
LateralPosteriorAnterior
Semicircular ductsin semicircularcanals
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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.
Helicotremaat apex
Modiolus
Cochlear nerve,division of thevestibulocochlearnerve (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;containsendolymph)
Striavascularis
Spiral organ
Basilarmembrane
Scala vestibuli(containsperilymph)
Scala tympani(containsperilymph)
Osseous spiral lamina
Spiralganglion
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Tectorial membrane
Hairs (stereocilia)
Outer hair cells
Supporting cells
Inner hair cell
Afferent nervefibers
Fibers ofcochlearnerve
Basilarmembrane
Figure 15.27c Anatomy of the cochlea.
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Figure 15.27d Anatomy of the cochlea.
Innerhaircell
Outerhaircell
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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 ofhigh pressure(compressedmolecules)
Area oflow pressure(rarefaction)Wavelength
AmplitudeDistance
Air
pre
ssure
A struck tuning fork alternately compressesand rarefies the air molecules around it, creatingalternate zones of high and low pressure.
Sound waves radiateoutward in alldirections.
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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.
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
Pre
ssure
0.01 0.02 0.03Time (s)
Frequency is perceived as pitch.
High amplitude = loud
0.01 0.02 0.03Time (s)
Low amplitude = soft
Amplitude (size or intensity) is perceived as loudness.
Pre
ssure
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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
Tympanicmembrane
Roundwindow
Auditory ossicles
Ovalwindow
Cochlear nerve
Scala vestibuli
Scala tympani
Cochlear duct
Basilarmembrane
Route of sound waves through the ear
Malleus Incus Stapes
Helicotrema
3
4a
4b
Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.
Sound waves vibrate the tympanic membrane.
Auditory ossicles vibrate. Pressure is amplified.
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.
4a
4b321
1
2
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Figure 15.30b Pathway of sound waves and resonance of the basilar membrane.
Basilar membrane
High-frequency sounds displace thebasilar membrane near the base.
Medium-frequency sounds displace thebasilar membrane near the middle.
Low-frequency sounds displace thebasilar membrane near the apex.
Different sound frequencies cross the basilar membrane at different locations.
Apex(long,floppyfibers)
Fibers of basilar membrane
Base (short,stiff fibers)
2020,000Frequency (Hz)
2000 200
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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
Hairs (stereocilia)
Outer hair cells
Supporting cells
Inner hair cell
Afferent nervefibers
Fibers ofcochlearnerve
Basilarmembrane
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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
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/
Hair cell transduction by ion channel opening
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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 pathwayMedial geniculatenucleus of thalamus
Primary auditorycortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivarynucleus (pons-medulla junction)
Cochlear nuclei
Midbrain
Medulla
Vestibulocochlearnerve
Spiral ganglionof cochlear nerve
Bipolar cell
Spiral organ
Vibrations
Vibrations
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
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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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Normal organ of Corti, with tectorial membrane removed to show hair cells.
Ryan AF. Protection of auditory receptors and neurons: evidence for interactive damage. PNAS 97:6939-6940, 2000.
©2000 by National Academy of Sciences
Damaged organ of Corti.
Hair cells in healthy and damaged cochleas
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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