Hearing

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

• 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

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

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

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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