the ear: hearing and balance
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THE EAR: HEARING AND BALANCE. 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 - PowerPoint PPT PresentationTRANSCRIPT
Copyright © 2010 Pearson Education, Inc.
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
Copyright © 2010 Pearson Education, Inc. Figure 15.25a
Externalacousticmeatus
Auricle(pinna)
(a) The three regions of the ear
Helix
Lobule
Pharyngotympanic(auditory) tube
Tympanicmembrane
Externalear
Middleear
Internal ear(labyrinth)
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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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• 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
Middle Ear
Copyright © 2010 Pearson Education, Inc. Figure 15.26
Pharyngotym-panic tube
Tensortympanimuscle
Tympanicmembrane(medial view)
Stapes
Malleus
View
Superior
Anterior
Lateral
IncusEpitympanic
recess
Stapediusmuscle
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• 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
Inner Ear
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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
Copyright © 2010 Pearson Education, Inc. Figure 15.27
Anterior
Semicircularducts insemicircularcanals
PosteriorLateral
Cristae ampullaresin the membranousampullae
Utricle investibule
Saccule investibule Stapes in
oval window
Temporalbone
Facial nerve
Vestibularnerve
Superior vestibular ganglion
Inferior vestibular ganglion
Cochlearnerve
Maculae
Spiral organ(of Corti)Cochlearductin cochlea
Roundwindow
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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
Copyright © 2010 Pearson Education, Inc. Figure 15.34
Macula ofsaccule
Otoliths
Hair bundle
Kinocilium
StereociliaOtolithicmembrane
Vestibularnerve fibers
Hair cells
Supportingcells
Macula ofutricle
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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
Copyright © 2010 Pearson Education, Inc. Figure 15.35
Otolithic membrane
Kinocilium
Stereocilia
ReceptorpotentialNerve impulsesgenerated investibular fiber
When hairs bend towardthe kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials.
When hairs bend awayfrom the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.
DepolarizationHyperpolarization
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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
Copyright © 2010 Pearson Education, Inc. Figure 15.36a–b
Fibers of vestibular nerve
Hair bundle (kinociliumplus stereocilia)
Hair cell
Supportingcell
Membranouslabyrinth
Cristaampullaris
Cristaampullaris
Endolymph
Cupula
Cupula
(a) Anatomy of a crista ampullaris in a semicircular canal
(b) Scanning electron micrograph of a crista ampullaris (200x)
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• 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
The Crista Ampullaris and Dynamic Equilibrium
Copyright © 2010 Pearson Education, Inc. Figure 15.36c
Fibers ofvestibularnerve
At rest, the cupula standsupright.
Section ofampulla,filled withendolymph
(c) Movement of the cupula during rotational acceleration and deceleration
Cupula Flow of endolymph
During rotational acceleration,endolymph moves inside thesemicircular canals in thedirection opposite the rotation(it lags behind due to inertia).Endolymph flow bends thecupula and excites the haircells.
As rotational movementslows, endolymph keepsmoving in the directionof the rotation, bendingthe cupula in theopposite direction fromacceleration andinhibiting the hair cells.
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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)
Copyright © 2010 Pearson Education, Inc. Figure 15.28a
(a) Helicotrema
Modiolus Cochlear nerve,division of thevestibulocochlearnerve (VIII)
Cochlear duct(scala media)
Spiral ganglion
Osseous spiral laminaVestibular membrane
Copyright © 2010 Pearson Education, Inc. Figure 15.28b
(b)
Cochlear duct(scala media;containsendolymph)
Tectorial membrane
Vestibular membrane
Scalavestibuli(containsperilymph)
Scala tympani(containsperilymph)
Basilarmembrane
Spiral organ(of Corti)
Spiralganglion
Osseous spiral lamina
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• 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
Sound and the Cochlea
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• 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
Sound and the Cochlea
Copyright © 2010 Pearson Education, Inc. Figure 15.28c
(c)
Tectorial membrane Inner hair cell
Outer hair cells
Hairs (stereocilia) Afferent nervefibers
Basilarmembrane
Fibers ofcochlearnerve
Supporting cells
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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
Copyright © 2010 Pearson Education, Inc. Figure 15.31a
Scala tympani
Cochlear duct
Basilarmembrane
1 Sound waves vibratethe tympanic membrane. 2 Auditory ossicles vibrate.
Pressure is amplified.
3 Pressure waves created bythe stapes pushing on the oval window move through fluid in the scala vestibuli.
Sounds with frequenciesbelow 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.
Malleus Incus
Auditory ossicles
Stapes
Ovalwindow
Scala vestibuli
Helicotrema
Cochlear nerve
32
1
Roundwindow
Tympanicmembrane
(a) Route of sound waves through the ear
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• 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)
Sound Transmission
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• 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)
Sound Transmission
Copyright © 2010 Pearson Education, Inc. Figure 15.33
Medial geniculatenucleus of thalamus
Primary auditorycortex in temporal lobeInferior colliculus
Spiral organ (of Corti)
Spiral ganglion of cochlear nerve
Vestibulocochlear nerve
Medulla
Midbrain
Cochlear nuclei
Vibrations
Vibrations
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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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• 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.
Deafness
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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