Hearing The Ability to Sense Vibrations in the Air

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Hearing The Ability to Sense Vibrations in the
Air

• Process known as
• Mechanosensory Transduction

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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

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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

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Sensitivity to Sound

• At threshold of hearing, the air molecules are
  moving 10 picometers.
• Hearing more sensitive than vision

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Sound waves

• Sound waves are periodic changes in air pressure
• Sound is a sine wave moving up and down

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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."

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4 Features of Sound Waves

• Waveformamplitude vs time
• Phasecompletion of 1 cycle
• Amplitudeintensityloudness decibels dB
• Frequencycycles/secondpitch

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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 note20Hz high piccolo note is 10K Hz
• Double the frequency raise the pitch one octave

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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

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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

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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

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Loudness-decibels

• Logarithmic scale
• 20dBwhisper
• 65 dBnormal speaking voice
• 100dBnear jet engine
• 120dBpain
• Is represented by the height amplitude sound wave
  
• Encoded by number of neurons activated not height
  of spike or number of spikes/time

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Phase

• Used to locate sound in space by comparing the in
  and out of phase waveforms

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Anatomy of the EAR

• Mechanosensory cellshair cells
• Located within the cochleaa 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

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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

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Oval Window

• Oval window connection of middle ear stapes bone
  with opening of cochlea
• Is a flexible membrane

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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

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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

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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

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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

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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 51 or 20K to 4K

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Hair Cells

• Mechanoreceptors for vibration
• Have cilia which are deflected by vibrations
• Deflection change membrane potential of hair cells

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George Van Bekesy

• Nobel Prize in 1961

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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

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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

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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

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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

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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

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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

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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

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Sensory Neuron Connections

• Almost all spiral sensory ganglion neurons
  contact inner hair cells
• 15K HC 30K SGN
• 201 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

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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

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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

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Contractions of Hair Cells

• Amplifies movement of basilar membrane

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Damage to Ear

• Mechanical or Neural
• Mechanicaldamage to tympanic membrane or
  ossification of middle ear bones
• Neuralshearing off or sticking of hair cell
  cilia and damage to auditory nerve CN8
• Birds regenerate hair cells humans do not

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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

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Afferent Connections

• Refer to hair cells sending info to spiral
  ganglion neurons that bring info to the CNS