ANATOMY AND PHYSIOLOGY OF THE EAR

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ANATOMY AND PHYSIOLOGY OF THE EAR
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Main Components of the Hearing Mechanism

• Outer Ear
• Middle Ear
• Inner Ear
• Central Auditory Nervous System

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Structures of the Outer Ear

• Auricle (Pinna)
• Collects sound
• Helps in sound localization
• Most efficient in directing high frequency sounds
  to the eardrum

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External Auditory Canal

• Approximately 1¼ inch in length
• S shaped
• Lined with cerumen glands
• Outer 1/3rd cartilage inner 2/3rds mastoid bone
• Increases sound pressure at the tympanic membrane
  by as much as 5-6 dB (due to acoustic resonance)

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Mastoid Process

• Bony ridge behind the auricle
• Provides support to the external ear and
  posterior wall of the middle ear cavity

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Tympanic Membrane

• Thin membrane
• Forms boundary between outer and middle ear
• Vibrates in response to sound
• Changes acoustical energy into mechanical energy

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The Ossicular Chain

• A Malleus
• B Incus
• C Stapes
• Ossicles are smallest bones in the body
• Act as a lever system
• Footplate of stapes enters oval window of the
  cochlea

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Eustachian Tube

• Lined with mucous membrane connects middle ear
  to back of the throat (nasopharynx)
• Equalizes air pressure
• Normally closed except during yawning or
  swallowing
• Not a part of the hearing process

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Stapedius Muscle

• Connects the stapes to the middle ear wall
• Contracts in response to loud sounds known as
  the Acoustic Reflex

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Structures of the Inner Ear

• Cochlea - Snail-shaped organ with a series of
  fluid-filled tunnels converts mechanical energy
  into electrical energy

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Structures of the Inner Ear (Cont.)

• Oval Window located at the footplate
• of the stapes when the footplate vibrates,
• the cochlear fluid is set into motion
• Round Window functions as the pressure relief
  port for the fluid set into motion initially by
  the movement of the stapes in the oval window

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Organ of Corti

• The end organ of hearing contains stereocilia
  and hair cells.

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Hair Cells

• Frequency-specific
• High pitch sounds base of cochlea
• Low pitch sounds apex of cochlea
• When the basilar membrane moves, a shearing
  action between the tectorial membrane and the
  organ of Corti causes hair cells to bend

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Vestibular System

• Consists of three semi-circular canals
• Shares fluid with the cochlea
• Controls balance
• No part in hearing process

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Central Auditory System

• 8th Cranial Nerve or Auditory Nerve carries
  signals from cochlea to brain
• Fibers of the auditory nerve are present in the
  hair cells of the inner ear
• Auditory Cortex Temporal lobe of
  the brain where sound is perceived
  and analyzed

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How Sound Travels Through The Ear...
Acoustic energy, in the form of sound waves, is
channeled into the ear canal by the pinna. Sound
waves strike the tympanic membrane, causing it to
vibrate like a drum, and changing it into
mechanical energy. The malleus, which is
attached to the tympanic membrane, starts the
ossicles into motion. (The middle ear components
mechanically amplify sound). The stapes moves in
and out of the oval window of the cochlea
creating a fluid motion. The fluid movement
within the cochlea causes membranes in the Organ
of Corti to shear against the hair cells. This
creates an electrical signal which is sent via
the Auditory Nerve to the brain, where sound is
interpreted!
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