Transcript The Ear - UniMAP Portal

The Ear
The Ear
Components of hearing mechanism
 - Outer Ear
 - Middle Ear
 - Inner Ear
 - Central Auditory Nervous System
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The Ear
The Ear
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Auricle (Pinna)
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Collects sound
Helps in sound
localization
Most efficient in
directing high
frequency sounds
to the eardrum
The Ear
External Auditory Canal
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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)
The Ear
Mastoid Process
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Bony ridge behind the
auricle
Provides support to the
external ear and
posterior wall of the
middle ear cavity
The Ear
Tympanic Membrane
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Thin membrane
Forms boundary
between outer and
middle ear
Vibrates in response to
sound
Changes acoustical
energy into mechanical
energy
The Ear
The Ossicular Chain
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A: Malleus
B: Incus
C: Stapes
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Ossicles are smallest
bones in the body
Act as a lever system
Footplate of stapes enters
oval window of the
cochlea
The Ear
Eustachian Tube
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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
The Ear
Stapedius Muscle
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Connects the stapes to the
middle ear wall
Contracts in response to loud
sounds; known as the Acoustic
Reflex
The Ear
Structure of The Inner Ear
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Cochlea - Snail-shaped organ
with a series of fluid-filled
tunnels; converts mechanical
energy into electrical energy
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
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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
The Ear
Organ of Corti
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The end organ of
hearing; contains
stereocilia and hair cells.
The Ear
Hair Cells
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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
The Ear
Vestibular System
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Consists of three semicircular canals
Shares fluid with the cochlea
Controls balance
No part in hearing process
The Ear
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

The Ear
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!
The Ear
Transduction of sound into an auditory
perception
 Sound is a propagating pressure wave.
 Perception of sound involves the electrical activity
of neurons in the auditory cortex of the brain.
 The transduction process is the means by which
the pressure waves in air (a mechanical stimulus)
is converted into neural activity (action potentials).
 This process involves a number of stages, some
of which involve conduction and impedance
matching.
The Ear
The path of sound
ear canal → vibrate tympanic membrane → vibrate ossicles
(3 bones: Malleus, Incus, Stapes) → vibrate oval window of
cochlea → create waves in cochlea fluid → create standing
waves in basilar membrane → movement of hair cells
generates electrical activity through mechanically
gated ionic channels → hair cells stimulate the
auditory nerve → series of action potentials up to
the auditory cortex.
The Ear
Cochlear Mechanics
Basilar membrane : The spectral analyser
• Basilar membrane (BM) is approx 33mm long in humans
• Apex of BM is wide and relatively loose
• Base of BM is thinner and more stiff
• Variations in length and stiffness provides BM with a continuum of resonant
frequencies along its length: low frequencies at apex and high frequencies at
base
• A wave with a particular frequency produces a maximum displacement at a
particular portion of the basilar membrane: tonotopic organization
• BM is heavily damped beyond the resonant frequency
• Travelling wave velocity is in range 1-20m/sec and is frequency dependent
(velocity is reduced apically for low frequencies)
• High frequency waves vibrate the basal part of the basilar membrane, dissipate
energy and then die out.
• Lower frequency waves travel further towards apex before dying out.
The Ear
The Ear
The Ear
Mechanism of Hearing by Organ of Coti
• Vibration of the basilar membrane produces shear
forces that bend the stereocilia (hairs protruding
from the hair cells) against the tectorial membrane
• Movement of the stereocilia either cause the hair
cell to depolarise or hyperpolarise, depending
upon the direction of movement
• Changes in the membrane potential of the hair cell
generate an AP in the nerve fibre attached to the
hair cell.
The Ear
Inner Hair Cells