Transcript Hearing Part 1 - Pegasus @ UCF

Hearing
The Ability to Sense Vibrations
in the Air
Process known as
Mechanosensory Transduction
Mechanosensory Transduction
• The process by which mechanical energy in
the vibration of sound waves traveling
through air are converted into electrical
signals that the brain can process and
understand
Sound
• Audible variations in air pressure
• Objects that move make sound; as the
object moves toward a patch of air it
compresses (increases density of air
molecules)
• Object moving away from a patch of air it
rarefies (makes molecules less dense)
• Speed of sound travels at 343 m/sec 767
mph
Sensitivity to Sound
• At threshold of hearing, the air molecules
are moving 10 picometers.
• Hearing more sensitive than vision
Sound waves
• Sound waves are periodic changes in air
pressure
• Sound is a sine wave moving up and down
Sonic Boom
• aircraft traveling through the atmosphere continuously
produces air-pressure waves similar to the water waves
caused by a ship's bow.
• When the aircraft exceeds the speed of sound, these
pressure waves combine and form shock waves which
travel forward from the generation or "release" point.
• The sound heard on the ground as a "sonic boom" is
the sudden onset and release of pressure after the
buildup by the shock wave or "peak overpressure."
4 Features of Sound Waves
•
•
•
•
Waveform=amplitude vs time
Phase=completion of 1 cycle
Amplitude=intensity=loudness decibels dB
Frequency=cycles/second=pitch
Frequency of soundwave
• is the number of compressed/rarefied air
patches that move past ear/second.
• Audible range 20 Hz-20,000 Hz
• Frequency of sound wave determines the Pitch:
• Low organ note=20Hz; high piccolo note is 10K
Hz
• Double the frequency raise the pitch one octave
Frequency of Sound Wave
• Humans hear 20 cycles/sec= Hz to 20,000
Hz
• Greatest sensitivity is 1000-4000 Hz
• Each spiral ganglion sensory neuron having
a synapse with a hair cell is “tuned in” or
most sensitive to a particular frequency
WaveFrequency= Pitch
• Ultrasound = above 20KHz
• Infrasound = below 20 Hz
• Unheard sounds can have subconscious effects
causing dizziness, headache, nausea (carsick) due
to low frequency sound of car at high speed
• High intensity low frequency sound can damage
internal organs by resonating the body cavity
WaveHeight=
Intensity/Loudness=
• Difference in pressure between the peak
compressed and peak rarefied patch of air
• Determines loudness of sound expressed in
decibels
• Loud sounds have higher intensity
• To double loudness, intensity increases
10fold
Loudness-decibels
• Logarithmic scale
– 20dB=whisper
– 65 dB=normal speaking voice
– 100dB=near jet engine
– 120dB=pain
Is represented by the height amplitude sound wave
Encoded by number of neurons activated not height of
spike or number of spikes/time
Phase
• Used to locate sound in space by comparing
the in and out of phase waveforms
Anatomy of the EAR
• Mechanosensory cells=hair cells
• Located within the cochlea=a spiral shaped
bony enclosure filled with fluid.
• Air vibrations impact the tympanic
membrane stretched across the ear canal.
• Transmitted to cochlea through 3 bones in
middle ear
Ossicle Amplification
• Ossicles amplify the sound wave in air to produce
a force on the oval window 5 times greater than on
the tympanic membrane so that the fluid in the
cochlea is moved
• stapes transduces air waves into water waves since
the cochlea is a fluid filled chamber
• 1000 ft/sec sound through air
• 5000 ft/sec sound through water
Oval Window
• Oval window= connection of middle ear
stapes bone with opening of cochlea
• Is a flexible membrane
Cochlea
• Separated into 2 regions by
the basilar membrane.
• A pressure wave reaches
the oval window and
pushes it inward and
increases pressure above
the basilar membrane
• Basilar membrane moves
downward as pressure is
released by bulging out the
round window at base of
cochlea
Cochlear Compartments
• Scala vestibuli is connected to oval window
where sound waves enter cochlea
• Scale tympani is compartment connected to
round window
• Intervening compartment is scala media that
is bounded by basilar and vestibular
membranes
Basilar Membrane Architecture
• Narrow at base near oval window
• Wide at apex
• Hair cells sit along the basilar membrane, have
cilia will depolarize to different extents in
response to frequency of sound wave
• High frequency hair cells respond maximally to
high frequency sound with high frequency
oscillation of membrane potential
Basilar Membrane
• Moves up and down in response to waves
of pressure impinging on oval window and
transmitted through to round window.
• Hair cells sit atop the basilar membrane
• Hair cells connect to sensory neurons that
live in spiral ganglion inside cochlea
• Axon travels to CNS through auditory nerve
ie CN8
Organ of Corti
• Contains hair cells and rest on the basilar
membrane and move up and down with
sound waves
• Composed of outer and inner hair cells
• Three rows of outer hair cells, 1 row of
inner hair cells. Exist at ratio of 5:1 or 20K
to 4K
Hair Cells
• Mechanoreceptors for vibration
• Have cilia which are deflected by vibrations
• Deflection change membrane potential of
hair cells
George Van Bekesy
Nobel Prize in 1961
Ion Channels
• Changes in membrane potential of hair cells
is caused by movement of cilia that changes
ion permeability
• Cilia on hair cells are tethered to each other
at the tips by connecting filaments that act
like springs that transmit tension to cation
channels in membrane of cilia
NT release
• Potassium channels open
• Potassium comes in and depolarizes
membrane
• Voltage sensitive calcium channels open
• Increased calcium causes NT release onto
spiral ganglion neurite
Cochlear Fluid
• Perilymph: Same ionic
• In Scala vestibuli and
tympani
• Composition as CSF
– 7mM K
– 140mM Na
• Bathes the hair cells
• Endolymph:
• In Scala Media
• Hi K concentration
– 150mM K
– 1mM Na
• Bathes stereocilia of
hair cells
• Inward K+ flux leads
to depolarization
Outer Hair Cells
• Outer are larger with more cilia
• Embedded in overlying membrane ka
tectorial membrane
• Cilia are deflected by shearing forces
generated by movements of basilar and
tectorial membranes
Function of Outer Hair Cell
• To amplify movement of tectorial
membrane so that inner hair cell will
respond more strongly
• Outer hair cells do this by increasing and
decreasing their length thus amplifying
movement of basilar membrane at area that
matches the frequency of sound
Inner Hair Cells
• Are not directly connected to tectorial
membrane
• Cilia move in response to motion of fluid
within cochlea transmit caused by outer hair
cells
Functions of Inner Hair Cell
• Afferent cells that transmit information to
the sensory neuron
• 90% of all synapses with sensory neurons
occur with inner hair cells
• 1 inner hair cell can connect to 20 spiral
ganglion neurons
Sensory Neuron Connections
• Almost all spiral sensory ganglion neurons contact
inner hair cells
– 15K HC & 30K SGN
• 20:1 ratio of inner cells to outer cells contacted by
neurons
• 20 outer hair cells synapse with 1 neuron whereas
1 inner hair cell contact 5-10 neurons
• More information reaches CNS from inner hair
cell
Differences Between Outer and
Inner Hair Cells
• Outer HC are larger than Inner HC & have
more cilia that attach to tectorial membrane
above
• Inner hair cells do not directly touch the
tectorial membrane and fluid alone causes
cilia movement
Active Movements
• Hair cells elongate and shorten in height to
amplify basilar membrane movements
• Depolarization shortens the hair cell
• Hyperpolarization lengthens hair cell
• Involves changes in actin filament lengths
and is not well understood
Contractions of Hair Cells
• Amplifies movement of basilar membrane
Damage to Ear
• Mechanical or Neural
• Mechanical=damage to tympanic membrane
or ossification of middle ear bones
• Neural=shearing off or sticking of hair cell
cilia and damage to auditory nerve CN8
• Birds regenerate hair cells humans do not
END
Hearing Sound
• Sound is a sine wave moving up and down
• Frequency of the sine wave determines the
pitch of sound
• Each spiral ganglion nerve axon is tuned to
respond to a particular frequency maximally
and less well to higher and lower
frequencies
Afferent Connections
• Refer to hair cells sending info to spiral
ganglion neurons that bring info to the CNS