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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