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Hearing and Equilibrium

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Parts of the Ear

external ear middle ear inner ear

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Parts of the Ear

external ear

middle ear

inner ear

Eustachian tube

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Relationship of Ear to Mouth

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External Ear

collects sound waves and passes them inward into the external auditory canal

Includes:

- auricle

- external auditory canal

- tympanic membrane

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Auricle

flap of elastic cartilage, flared like a funnel

covered with thick skin helix - top lobule - bottom, earlobe attached to head by ligaments and

muscles

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External Auditory Canal

curved tube about 1 inch long skin-lined near exterior opening are ceruminous glands

- produce cerumen (wax) which helps to trap foreign material

lies in temporal bone extends from auricle to tympanic membrane

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Tympanic Membrane

thin, semitransparent membrane of fibrous connective tissue

lies between external auditory canal and middle ear

cone-shaped structure with apex directed medially

sound waves in auditory canal cause pressure changes that produce eardrum vibrations

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Middle Ear

air-filled cavity in temporal bone epithelium-lined contains auditory ossicles extends from eardrum to thin, bony partition

with two membrane covered openings called oval and round window

connected to mouth by Eustachian tube

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Auditory Ossicles

transfer vibrations from eardrum to oval window of internal ear

maleus is attached to the internal surface of eardrum at apex

incus is the intermediate bone stapes is attached by ligaments to the

membranous oval window

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Auditory Ossicles (cont.)

malleous vibrates with tympanic membrane; passes vibration to incus

incus causes stapes to vibrate on oval window oval window is pushed in and out, causing motion

in fluid within internal ear action activates receptor cells impulses travel to temporal lobe of cerebrum and

sound sensation results

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Inner Ear

complex series of interconnecting chambers

Includes:

- bony or osseous labyrinth

- membranous labyrinth

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Osseous Labyrinth

bony canal within the temporal bone lined with periosteum contains the fluid perilymph

- chemically similar to cerebrospinal fluid Divided into three areas:

- semicircular canals

- vestibule

- cochlea

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Membranous Labyrinth

surrounded by cerebrospinal fluid series of sacs following general shape of osseous

labyrinth lined with epithelium contains endolymph

- chemically similar to intracellular fluid portions within bony canals called semicircular

ducts communicate with utricle and vestibule

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Semicircular Canals

three canals; anterior, posterior, and lateral

each end enlarges into swelling called ampula

lie at right angles to each other contain receptors for equilibrium

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Vestibule

oval central portion of bony labyrinth contains two sacs called the utricle and

saccule

- connect to each other by small duct contains receptors for equilibrium

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Cochlea (koklea = snail shell)

coil-shaped cavity anterior to vestibule makes almost three turns central bony core called modiolus contains thin, bony shelf which divides

cochlea into upper and lower compartments and smaller cochlear duct

contains receptors for hearing

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Three Compartments of Cochlea

scala vestibuli scala tympani cochlear duct (scala media)

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Scala Vestibuli

above bony partition ends at oval window filled with perilymph

Scala Tympani below bony partition ends at round window contains perilymph

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Cochlear Duct (scala media)

portion of membranous labyrinth separated from scala vestibuli by vestibular

membrane separated from scala tympani by basilar

membrane organ of Corti (spiral organ) located on basilar

membrane within cochlear duct tectoral membrane projects over and in contact

with hair cells of spiral organ

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Organ of Corti

spiral organ lined with epithelial cells

- support cells and 16,000 hair cells

contain receptors for auditory sensations

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Hair Cells

Two types:

- inner

- outer processes at apical end extend into endolymph

of cochlear duct synapse with fibers of the cochlear branch of

the vestibulocochlear nerve (VIII) easily damaged by high intensity sounds

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Sound Waves

sound sensations are heard by vibrations transmitted through the air

result from alternate compression and decompression of air molecules

most audible sound vibrations to human ears are frequencies between 1000 and 4000 Hertz

entire audible range is 20 - 20,000 Hz

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Pitch

determined by frequency of vibrations the higher the frequency of vibrations,

the higher the pitch (musical high note)

Loudness determined by amplitude of sound

waves measured in decibels (dB)

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Decibels of Sounds

silence rustling leaves normal conversation crowd noise vacuum cleaner pneumatic drill uncomfortable sound painful sound

0 dB

15 dB

45 dB

60 dB

75 dB

90 dB

120 dB

140 dB

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Physiology of Hearing

auricle directs waves into external auditory canal

sound waves strike tympanic membrane alternate compression and decompression of air

cause membrane vibration movement of membrane depends on intensity

and frequency of sound waves low-frequency = slow membrane vibration high-frequency = rapid membrane vibration

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Physiology of Hearing (cont.)

malleus connects in central area of tympanic membrane

malleus vibrates, conducting vibration to incus and then stapes

stapes pushes membrane on oval window in and out

movement of oval window causes waves in perilymph of cochlea

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Physiology of Hearing (cont.)

inward movement of oval window pushes on perilymph of scala vestibuli to scala tympani to round window into middle ear

movement of perilymph exerts pressure on vestibular membrane

pressure in endolymph inside cochlea increases and decreases

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Physiology of Hearing (cont.)

pressure fluctuations move basilar membrane causing hair cells of spiral organ to move against tectorial membrane leading to generation of nerve impulses in cochlear nerve fibers

pathway extends into medulla oblongata through midbrain to thalamus and on to temporal lobes of cerebrum for interpretation

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Physiology of Equlibrium

when body movement occurs, organs detect motion and aid in maintaining balance

organs provide information on which way is up or down

Organs of equlibrium:

- utricle

- saccule

- semicircular ducts

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Two kinds of Equilibrium

static

- maintenance of posture in response to changes in body orientation relative to the ground

dynamic

- maintenance of body position, mainly the head, in response to sudden movements

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Deafness

conduction deafness

- impairment of structures that transmit vibrations

- punctured eardrum, otitis media, wax

buildup nerve deafness

- degeneration of receptors

- damage to receptor cells

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Ménière’s Syndrome

labyrinth disorder characterized by fluctuating loss of

hearing, vertigo, and tinnitus caused by an increased volume of

endolymph causing enlargement of the labyrinth

disease of cranial nerve VIII