Anatomy Physiology of The Ear

8/4/2019 Anatomy Physiology of The Ear

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ANATOMY ANDPHYSIOLOGY OF THEEAR


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Main Components of theHearing Mechanism:Divided into 4 parts (byfunction):

Outer Ear Middle Ear Inner Ear Central Auditory NervousSystem


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Structures of the OuterEar

Auricle (Pinna) Gathers

sound waves Aids inlocalization Amplifiessound

approx. 5-6dB


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External Auditory Canal: Approx. 1 inch long S shaped Outer 1/3 surrounded

by carti lage; inner 2/3by mastoid bone Allows air to warmbefore reaching TM Isolates TM fromphysical damage Cerumen glandsmoisten/soften skin Presence of somecerumen is normal


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Mastoid Process ofTemporal Bone Bony ridge behind theauricle Hardest bone in body,

protects cochlea andvestibular system Provides support to theexternal ear andposterior wall of themiddle ear cavity Contains air cavitieswhich can be reservoirfor infection


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Tympanic Membrane Thin membrane Forms boundary

between outer andmiddle ear Vibrates in responseto sound waves Changes acousticalenergy intomechanical energy

(From Merck Manual)


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The Ossicles Ossicular chain = malleus,incus & stapes Malleus

TM attaches at Umbo Incus

Connector function Stapes

Smallest bone in the body Footplate inserts in ovalwindow on medial wall

Focus/amplify vibration of TMto smaller area, enablesvibration of cochlear fluids


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Eustachian Tube (AKA: TheEqualizer) Mucous-lined, connectsmiddle ear cavity tonasopharynx Equalizes air pressure inmiddle ear Normally closed, opensunder certain condit ions May allow a pathway forinfection Children grow out of mostmiddle ear problems asthis tube lengthens andbecomes more vertical


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Stapedius Muscle Attaches to stapes Contracts in response to loud sounds;(the Acoustic Reflex) Changes stapes mode of vibration; makesit less efficient and reduce loudnessperceived Built-in earplugs! Absent acoustic reflex could signalconductive loss or marked

sensorineural loss


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Structures of the Inner Ear:The Cochlea Snail shaped cavity within mastoidbone 2 turns, 3 fluid-fi l led chambers Scala Media contains Organ of CortiConverts mechanical energy toelectrical energy


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Organ Of Corti The end organ of hearing

Contains stereocilia & receptor hair cells 3 rows OHC, 1 row IHC Tectorial and Basilar Membranes Cochlear fluids

(From Augustana College, Virtual Tour of the Ear)


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Hair Cells Frequency specific

High pitches= base of cochlea Low pitches= apex of cochlea Fluid movement causes

deflection of nerve endings Nerve impulses (electricalenergy) are generated andsent to the brain


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Vestibular System Consists of three semi-circularcanals Monitors the position of the

head in space Controls balance Shares f luid with the cochlea Cochlea & Vestibular systemcomprise the inner ear


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Central Auditory System VIIIth Cranial Nerve or Auditory Nerve

Bundle of nerve fibers (25-30K) Travels from cochlea through internal auditory

meatus to skul l cavity and brain stem Carry signals from cochlea to primary auditorycortex, with continuous processing along the way

Auditory Cortex Wernickes Area within Temporal Lobe of the brain Sounds interpreted based on experience/association


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Summary: How Soundummary: How SoundTravels Through The Earravels Through The EarAcoustic energy, in the form of sound waves, is channeled

into the earcanal by the pinna. Sound waves hit the

tympanic membrane and cause it to vibrate, like a drum,

changing it into mechanical energy. The malleus, which

is attached to the tympanic membrane, starts the ossicles

into motion. The stapes moves in and out of the oval

window of the cochlea creating a fluid motion, or

hydraulic energy. The fluid movement causesmembranes in the Organ of Corti to shear against the

hair cells. This creates an electrical signalwhich is sent

up the Auditory Nerve to the brain. The brain interprets

it as sound!


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Anatomy Physiology of The Ear