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CHAPTER 14.3
PAGES 456-461
Hearing
and
Equilibrium
HEARING AND EQUILIBRIUM
• the ear
• is associated with two separate functions:
• hearing
• equilibrium
• can be divided into three sections
• the outer ear
• the middle ear
• the inner ear
HEARING AND EQUILIBRIUM
HEARING AND EQUILIBRIUM
The Outer Ear
• comprised of the
• pinna
• the external ear flap
• collects the sound
• auditory canal
• carries sound to the eardrum.
• lined with specialized sweat glands that produce earwax.
• earwax traps foreign particles and prevents them entering
the ear.
HEARING AND EQUILIBRIUM
The Middle Ear
• begins at the tympanic membrane and extends toward
the oval and round windows.
HEARING AND EQUILIBRIUM
• the tympanic membrane is a thin layer of tissue that
receives sound vibrations, also known as the eardrum.
HEARING AND EQUILIBRIUM
• the air filled chamber of the middle ear contains three small
bones called ossicles, which include the:
• mallus (the hammer)
• incus (anvil)
• stapes (stirrup)
• the ossicles amplify
and carry sound in
the middle ear.
HEARING AND EQUILIBRIUM
• sound vibrations that strike the eardrum and are
first concentrated within the solid malleus.
• vibrations are then transmitted to the incus
and finally to the stapes
HEARING AND EQUILIBRIUM
• the stapes strikes the membrane covering the oval window
in the inner wall of the middle ear
• the oval window is an oval shaped hole in the vestibule
of the inner ear, covered by a thin layer of tissue
• sound is amplified by concentrating the sound energy
from the large tympanic membrane to the smaller oval
window.
HEARING AND EQUILIBRIUM
• the eustachian tube
• an air-filled tube of the middle ear that equalizes
pressure between the external and internal ear.
• approximately 40 mm in length and 3 mm in diameter.
• extends from the middle ear to the mouth and chambers
of the nose.
• equalizing your ears on a plane by yawning or
swallowing allows air to leave your middle ear
through the eustachian tube.
HEARING AND EQUILIBRIUM
The Inner Ear
• has three distinct structures, the:
• vestibule
• semicircular canals
• cochlea
HEARING AND EQUILIBRIUM
• the vestibule
• a chamber found at the base of the semicircular
canals that provides information about static
equilibrium
• involved in balance
• connected to the middle ear by the oval window.
• houses two sacs
• the utricle
• the saccule
HEARING AND EQUILIBRIUM
• the utricle and saccule contain granules called otoliths
that allow us to detect gravity (linear movement) and
head movement.
• three semicircular canals arranged at different angles helps
identify body movement
(3 canals for 3 axis of movement)
(Three semicircular canals contain fluid and allow us
to detect angular acceleration such as the turning of
the head)
Dynamic eq. vid: balance
HEARING AND EQUILIBRIUM
• The cochlea
• a coiled structure of the inner ear that responds to
various sound waves and converts them to nerve
impulses.
• shaped like a spiralling snail’s shell.
• contains rows of specialized hair cells that run the length
of the inner cannal.
• the hair cells respond to sound waves and convert them
into nerve impulses.
HEARING AND EQUILIBRIUM
Hearing
• sound like light must be converted into an electrical impulse
before you can interpret it.
• you ear is so sensitive that you can hear a mosquito
even though the sound energy reaching you ear is less
than one quadrillionth of watt. The average light in the
house uses a 60 watt bulb.
HEARING AND EQUILIBRIUM
• hearing begins when sound waves push against the
eardrum, or tympanic membrane.
• the vibrations of the eardrum are passed on to the three
bones of the middle ear: the malleus, the incus, and the
stapes
• arranged in a lever system the three bones are held
together by muscles and ligaments.
HEARING AND EQUILIBRIUM
• the bones concentrate and amplify the vibrations
received from the tympanic membrane (they can triple
the force)
• during excessive noise a protection reflex
mechanism goes into effect.
• the muscles that join the bones together contract
and restrict the movement of the malleus
reducing the intensity of movement.
• at the same time a second muscle contracts
pulling the stapes away from the oval window.
HEARING AND EQUILIBRIUM
• the oval window receives vibrations from the ossicles.
• as the oval window pushes inwards, the round window,
located immediately below the oval window moves
outward.
• this triggers waves of fluid within the inner ear.
• the cochlea receives the fluid waves and converts them
into electrical impulses, which you interpret as sound.
HEARING AND EQUILIBRIUM
• the hearing apparatus within the cochlea is known as the
organ of Corti.
• it comprises a single inner row and three outer rows
of specialized hair cells anchored to a basilar
membrane.
• the hair cells respond to vibrations of the basilar
membrane.
• vibrations in the fluid on either side of the basilar
membrane cause the membrane to move.
• the hairs on the cells bend as they brush against
the tectorial membrane.
HEARING AND EQUILIBRIUM
HEARING AND EQUILIBRIUM
• the movement of the hair cells stimulates sensory nerves in
the basilar membrane
• Auditory information is the sent to the temporal lobe of
the cerebrum via the auditory nerves.
HEARING AND EQUILIBRIUM
• The ear conveys information about:
• Volume, the amplitude of the sound wave
• Pitch, the frequency of the sound wave
• The cochlea can distinguish pitch because the
basilar membrane is not uniform along its length
• Each region vibrates most vigorously at a
particular frequency and leads to excitation of a
specific auditory area of the cerebral cortex
http://health.howstuffworks.com/adam200010.htm
Hearing test
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3) TREATMENTS FOR HEARING LOSS
• Conductive hearing loss: caused by wax build-up, middle ear
infection, punctured eardrum
• Sensorineural hearing loss: auditory nerve severed, or
cochlear hair cells damaged.
Treatments:
• Hearing aid: amplifies sound
and transmit to eardrum.
• Cochlear implants: converts sound (speech processor) to
electrical impulses that are sent to auditory nerve.
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Tinnitus – ringing in ear; “phantom sound”
• Malfunction of cochlea, transmitted to
brain
• Temporary, after a rock concert
• Hair cells are damaged, bent, or
destroyed
• Loss of hair cells as we age
• Other causes:
• TMJ, damage to inner ear by
trauma, ear infection, tumors, use
of ototoxic drugs (antibiotics,
anticancer drugs, or antimalarial
drugs) – damage to hair cells or
auditory nerve or both
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