hearing
TRANSCRIPT
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Ears & Hearing
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• Sound waves funneled by pinna (auricle) into external auditory meatus
• External auditory meatus channels sound waves to tympanic membrane
Ears & Hearing - Outer Ear
Fig 10.1710-47
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• Malleus (hammer) is attached to tympanic membrane– Carries vibrations to incus (anvil)– Stapes (stirrup) receives vibrations from incus, transmits to
oval window
Ears & Hearing - Middle Ear continued
Fig 10.18
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• Stapedius muscle, attached to stapes, provides protection from loud noises – Can contract & dampen large vibrations– Prevents nerve damage in cochlea
Ears & Hearing - Middle Ear continued
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Fig 10.18
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Ears & Hearing - Cochlea
• Consists of a tube wound 3 turns & tapered so looks like snail shell
Fig 10.19
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Ears & Hearing - Cochlea continued
• Tube is divided into 3 fluid-filled chambers– Scala vestibuli,
cochlear duct, scala tympani
Fig 10.19
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Ears & Hearing - Cochlea continued
• Oval window attached to scala vestibuli (at base of cochlea)
• Vibrations at oval window induce pressure waves in perilymph fluid of scala vestibuli
• Scalas vestibuli & tympani are continuous at apex– So waves in vestibuli pass to tympani & displace round
window (at base of cochlea)• Necessary because fluids are incompressible & waves would not be
possible without round window
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Ears & Hearing - Cochlea continued
• Low frequencies can travel all way thru vestibuli & back in tympani
• As frequencies increase they travel less before passing directly thru vestibular & basilar membranes to tympani
Fig 10.20
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Ears & Hearing - Cochlea continued
• High frequencies produce maximum stimulation of Spiral Organ closer to base of cochlea & lower frequencies stimulate closer to apex
Fig 10.2010-55
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Spiral Organ (Organ of Corti)
• Is where sound is transduced
• Sensory hair cells located on the basilar membrane – 1 row of inner cells
extend length of basilar membrane
– Multiple rows of outer hair cells are embedded in tectorial membrane
Fig 10.22
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Spiral Organ (Organ of Corti)
• Pressure waves moving thru cochlear duct create shearing forces between basilar & tectorial membranes, moving & bending stereocilia– Causing ion channels to open, depolarizing hair
cells– The greater the displacement, the greater the
amount of NT released & APs produced
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Neural Pathway for Hearing
• Info from 8th nerve goes to medulla, then to inferior colliculus, then to thalamus, & on to auditory cortex
Fig 10.23
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Neural Pathways for Hearing
• Neurons in different regions of cochlea stimulate neurons in corresponding areas of auditory cortex– Each area of
cortex represents different part of cochlea & thus a different pitch
Fig 10.24
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Hearing Impairments
• Conduction deafness occurs when transmission of sound waves to oval window is impaired– Impacts all frequencies– Helped by hearing aids
• Sensorineural (perceptive) deafness is impaired transmission of nerve impulses– Often impacts some pitches more than others– Helped by cochlear implants
• Which stimulate fibers of 8th in response to sounds
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Vestibular Apparatus
• Provides sense of equilibrium– =orientation to gravity
• Vestibular apparatus & cochlea form inner ear
• V. apparatus consists of otolith organs (utricle & saccule) & semicircular canals
Fig 10.11 10-35
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Semicircular Canals
• Provide information about rotational acceleration
• Project in 3 different planes
• Each contains a semicircular duct
• At base is crista ampullaris where sensory hair cells are located
Fig 10.12
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• Utricle and saccule provide info about linear acceleration• Semicircular canals, oriented in 3 planes, give sense of angular
acceleration
Fig 10.12
Vestibular Apparatus continued
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• Hair cells are receptors for equilibrium– Each contains 20-50 hair-like extensions called stereocilia
• 1 of these is a kinociliumFig 10.13
Vestibular Apparatus continued
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• When stereocilia are bent toward kinocilium, hair cell depolarizes & releases NT that stimulates 8th nerve
• When bent away from kinocilium, hair cell hyperpolarizes– In this way, frequency of APs in hair cells carries information about
movement
Vestibular Apparatus continued
Fig 10.13
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Utricle & Saccule
• Have a macula containing hair cells– Hair cells embedded in gelatinous otolithic membrane
• Which contains calcium carbonate crystals (=otoliths) that resist change in movement
Fig 10.14
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Utricle & Saccule continued
• Utricle sensitive to horizontal acceleration– Hairs pushed
backward during forward acceleration
• Saccule sensitive to vertical acceleration
• Hairs pushed upward when person descends
Fig 10.14
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Semicircular Canals
• Provide information about rotational acceleration
• Project in 3 different planes
• Each contains a semicircular duct
• At base is crista ampullaris where sensory hair cells are located
Fig 10.12
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Semicircular Canals continued
• Hair cell processes are embedded in cupula of crista ampullaris
• When endolymph moves cupula moves– Sensory processes
bend in opposite direction of angular acceleration
Fig 10.15
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Neural Pathways for Equilibrium & Balance
Fig 10.16
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Nystagmus & Vertigo
• Vestibular nystagmus is involuntary oscillations of eyes that occurs when spinning person stops – Eyes continue to move in direction opposite to spin,
then jerk rapidly back to midline
• Vertigo is loss of equilibrium – Natural response of vestibular apparatus– Pathologically, may be caused by anything that
alters firing rate of 8th nerve• Often caused by viral infection
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