Which cranial nerves are associated with hearing? - PowerPoint Presentation

Slide1

Which cranial nerves are associated with hearing?

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The Ear

The organ of hearing and equilibrium

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What is Sound?

Pressure disturbance originating from a vibrating objectCompressions and rarefactions of particles in a medium

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What sound looks like

http://www.youtube.com/watch?v=4shodbQMcmM

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Pitch

Frequency

Number of waves that pass a given point in a given time

Hearing range is 20 Hz to 20,000 Hz

Higher frequency = higher pitch

Lower frequency – lower pitch

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Which disturbance is a higher pitch?

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Loudness

Amplitude

Height of the wave (crest)

Intensity of the sound = energy

Humans: 0.1 decibels to 120 dB (pain threshold = 130dB)

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Which disturbance is louder?

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Hearing

Auditory area of temporal lobe cortex

Sound waves through the air must be perceived

What type of receptors

are involved?

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Intro video

http://www.youtube.com/watch?v=pCCcFDoyBxM

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Three Parts

External earMiddle earInner Ear

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External ear: Auricle/Pinna

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External ear: auditory canal

Lined with skin, bearing hairs, sebaceous glands, and modified sweat glands

Secretes sticky cerumen (earwax): purpose?

1 inch long

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Impacted eardrum

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Boundary between external and middle ear: tympanic membrane

Connective tissue, thin

Rich supply of nerves and blood vesselsCone shaped, apex points into earVibrates in response to sound waves entering the ear

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Middle ear

Small, air-filled chamberEardrum on one side, bone on other

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Otitis media

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Otitis media: middle ear inflammation

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Or this…

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Middle ear: eustachian tube

Linked to

nasopharynx

Closed by a membrane most of the time

Opens briefly when yawn or swallow

Equalizes middle ear pressure and external air pressure

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Middle ear: ossicles

Smallest bones in body

Suspended by tiny ligaments, linked together by joints

Malleus secured to ear drum

Stapes – base fits into oval window (entrance to cochlea)

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Middle ear: ossicles

Transmit vibratory motion of eardrum to oval window

Sets fluids of inner ear in motion

Eventually excites the hearing receptors

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Problems…

Chronic ear infections –

ossicles

can fuse

“glue ear”: stapes becomes fixed to the oval window

Protection:

Muscles help protect the eardrum and reduce sound transduction when accosted by loud noises

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Inner ear

Complicated structure

Behind eye socket

Secure site

Delicate receptor machinery

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Inner ear: two divisions

Bony labyrinth: the cavity – filled with perilymph (fluid)

Membranous labyrinth – inside the cavity, floating in perilymph, filled with endolymph

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Inner ear: vestibule

Equilibrium receptor regionOtoliths – increase sensitivityRespond to pull of gravityReport changes in head position

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Inner ear: semicircular canals

Oriented in 3 planes

AnteriorPosteriorLateralEquilibrium receptorsConducts to vestibular nerve of 8th cranial nerve

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Inner ear: cochlea

Converts the physical vibrations of sound waves and converts them into electrical impulses

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Inside the cochlea

http://www.youtube.com/watch?v=dyenMluFaUw

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Inner ear: cochlea

Lined with receptors for hearing

Stimulated by bending of hairs: mechanoreceptorsOrgan of corti: receptor organ for hearing

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Vestibulocochlear nerve (cranial nerve 8)

Cochlear branch

Conducts to primary auditory cortex of temporal lobe

Can damage this nerve when exposed to prolonged periods of loud noise

Drugs = ototoxic agents

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Hearing test

http://pedsohns.ucsf.edu/upload/medialibrary/2fa/2fa3732df5888aefd57ec0442570a239.jpg