Auditory Physiology of the Ear

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Auditory Physiology of the Ear

• James Saunders, MD FACS
• Dept of ORL, OUHSC

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King Richard, IIIBattle of Bosworth Field, 1485

• For want of a nail, the shoe was lost
• For want of a shoe, the horse was lost
• For want of a horse, the battle was lost
• For want of a battle, the kingdom was lost
• All for the want of a horseshoe nail
• A horse, a horse, my kingdom for a horse!!

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Acoustic systems must accommodate for lost energy
between fluids
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Most (97 99) of Acoustic Energy is Reflected
from Water
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TOTAL BONE CONDUCTION RESPONSE

• Compressional
• Inertial
• Osseotympanic

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INNER EAR (Compressional)

• Distortion of Bony Cochlea

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MIDDLE EAR (Inertial)

• Most effective at Low and High Frequencies

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EXTERNAL EAR (Osseotympanic)

• Sound Energy Radiated
• Bony EAC
• Mandibular contribution

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Middle Ear Transmits Energy by Two Pathways
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Round Window Effect in Acoustic Coupling
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Middle Ear Transmits Energy by Two Pathways
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Coupling Mechanisms are Frequency Dependent
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MECHANISMS OF MIDDLE-EAR GAIN

• Acoustic Coupling
• Ossicular Coupling
• Area Difference (TM to footplate)
• Lever Action (Malleus to Incus)

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IMPEDANCE

• Total opposition to motion
• Opposition of a system to the flow of energy into
  it and through it
• Inner Ear is fluid therefore, the Middle Ear must
  overcome or match the impedence

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MECHANICAL LIMITATIONS OF ME STRUCTURES

• TM, OSSICLES AND OSSICULAR LIGAMENTS
• RESISTANCES, MASSES AND STIFFNESS
• OPPOSE MIDDLE EAR MOTION

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Middle Ear Impedence Simplified

• Resonant frequency Stiffness / mass

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Total Impedance is the sum ofResistance (R),
Effective Mass (Xm), and Effective Compliance (Xc)
Xm mass x frequency
Xc compliance x frequency, or Xs stiffness /
frequency
stiffness is the opposite of compliance
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Mass (m) is directly proportional to frequency (f)
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Effective Stiffness (Xs) is opposite of
compliance and is inversely proportional to
frequency
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Effect of Stiffness/Mass on Auditory Threshold
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PHASIC COMPONENTS OF IMPEDANCE

• Resistance, which is independent of frequency, is
  in phase with velocity
• Compliance (elasticity), which is frequency
  dependent, lags resistance by 90
• Mass, which is proportional to acceleration and
  also frequency dependent, leads resistance by
  90 and it follows that
• Mass is 180 out of phase with compliance.

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PHASIC RELATIONSHIPSRESISTANCE REACTANCE
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The Uncoiled Cochlea
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Tonotopic Mapping
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EARLY BIOPHYSICAL CONCEPTS

• Resonance Theory - basilar membrane tuning
• Width differences of membrane
• Telephone Theory neurons respond to any freq.
• Not possible - maximal neural response (24 -1000
  Hz)
• Remember the Refractory period?
• Standing Waves movement of fixed rope
• Maximal displacement doesnt move along membrane

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Von Bekesy Traveling Waves

• Used closed cochlear model and cadaver studies
• Membrane displacement due to physical
  characteristics
• Stiffness increases from base to apex
• Maximal displacement correlate with frequency
• Increasing wave till max then drops quickly

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Traveling Wave
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Traveling Wave
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Electrical Activity Matches Traveling Wave
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Gregor Von BékésyNobel Prize Physiology 1961
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Bekesy Mechanical Tuning Curve are Broad Compared
to Neural Responses
Actual Tuning Curve
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Analytical Coding Theories of Hearing

• Place Theory
• frequency information is place in cochlea of
  diplacement
• Effective at high frequency (gt5000 Hz)
• Frequency (Temporal) Theory
• Modification of telephone theory
• Volley Principle
• Effective at low frequencies (15 400 Hz)
• Transition Zone
• Both methods (400 5000 Hz)

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Volley Principle
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Analytical Coding Theories of Hearing

• Place Coding
• frequency information is place in cochlea of
  diplacement
• Effective at high frequency (gt5000 Hz)
• Frequency (Temporal) Coding
• Modification of telephone theory
• Volley Principle
• Effective at low frequencies (15 400 Hz)
• Transition Zone
• Both methods (400 5000 Hz)

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The Inner Ear
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Inner and Outer Hair Cells
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Cochlear Hair Cells

• Inner Hair Cells
• One row
• Contact w/ tectorial
• Outer hair Cells
• Three Rows
• V shape
• Connected to tectorial

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Hair cells are excited when stereocilia are
displaced toward kinocilium
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Hair Cell Properties

• Kinocilia / Stereocilia Linked
• Displacement Opens K Channels
• Depolarization ? release of glutamate
• K flows through cell
• Glutamate ? increase spike rate in auditory nerve

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Shearing Forces Created by Different Pivotal
Points
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Electrical Potentials of the Cochlea

• Endocochlear Potential (EP) resting potential
  of the organ of Corti relative to the surrounding
  tissue
• Cochlear Microphonic - alternating currents due
  to hair cell depolarization
• Summating Potential change of EP in response to
  sound stimulation (DC current)
• Action Potential all or none response of
  auditory nerve fibers

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Endocochlear Potential
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Summating Potential and Cochlear Microphonic
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Innervation of the Cochlea

• Afferent Nerves
• Cell Bodies in Spiral Ganglion (Rosenthals
  Canal)
• Type I synapses with IHC (95)
• Type II synapses with OHC (5)
• Tonotopically Organized in Auditory Nerve
• Hair Cell ? Efferent Transmitter is Glutamate

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Innervation of the Cochlea
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Innervation of the Cochlea

• Afferent
• Each neuron goes to only one IHC
• Up to 8 neurons per IHC
• 10 OHC, all basal to IHC

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Cochlear Nerve Afferent Responses

• Resting discharge rate
• Threshold causes increase in firing rate
• Characteristic Frequency
• Phase locked below 1000 Hz
• Intensity function of rate increase and number
  of affected cells

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Characteristic Frequency
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Phase Locked Firing Pattern
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Period (phase locked) Post Stimulus Time
Histogram(note saturation of response)
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Intensity Coding
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Innervation of the Cochlea

• Efferent Nerves
• Cell Bodies in Superior Olive
• Medial and Lateral Olivocochlear Bundles
• MOC direct synapse to OHC (80)
• LOC en passant to INC Type I Afferent (20)
• Transmitter Acetylcholine (and others)

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Innervation of the Cochlea
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Efferent Olivocochlear Bundle
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Stapedial Reflex Arc
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Innervation of the Cochlea

• Efferent
• Each may go to multiple OHC or IHC
• Either basal or apical direction

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Now back to those sharp tuning curves
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ACTIVE PROCESS FOR NARROW TUNING

• Gold (1948) Postulated Narrower Mechanical
  Tuning required an Additional Supply Of Energy
• O2 Deprivation Degraded Sharp Tuning To Broad
  Tuning
• Evidence For Otoacoustic Emissions Sound
  Production By Inner Ear (Kemp 1978)

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OHC Responsible for Sharp Tuning Curve
OHC Damage
equency
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OHC Electromotility

• Electrical Stimulation OHC In Vitro Generate
  Length Change
• Elongate/Contract Depending On Polarity
• Hyperpolarize ? Free End Elongates
• Depolarize ? Free End Shortens
• Sound Generator Source (Brownell 1983)

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OHC Contain Actin (contractile protein)
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OHC Reduce Length with Depolarization
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OHC MOTILITY

• Source Of Energy
• In Vitro ? Applied Electrical Signal
• In Vivo ? Stria Vascularis ? EP 80
• OHC Electromotility (EM) Driven By Receptor
  Potential Modulation Of Standing/Silent Current
• EM Response Provides Positive Mechanical Feedback
  That Increases Movement Of Cochlear Partition
  Near Threshold (Low Level)

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PASSIVE MECHANICS

• Factor at 50-60 dB
• Direct Movement Of Cochlear Partition
• IHC Sterocilia Move ? Transduction Channels Open
• Depolarization ? glutamate ? Action Potential

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ACTIVE MECHANICS(LOW LEVEL)

• Low Level Sound Moves OHC Sterocilia
• Depolarization Decreases Length OHC
• Length Change Induces Additional Movement Of
  Cochlear Partition (CP)
• ? Length leads to ? Cp Motion ? Mechanical
  Amplification of Lower Signals
• OHCs Non Linear Amplifier

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Tuning Curve Become Broader At High Intensities
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EFFERENT CONTROL OHCs

• Contralateral, Ipsilateral, Binaural Sound
  Activate Olivocochlear Efferents
• Affect Activity Of OHCs via Acetylcholine
• OAE Amplitude With Sound Stimulation Suggests
  Activated Efferents Suppress Motor Activity Of Ohc

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Otoacoustic Emissions (OAE)
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Spontaneous OAE
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Distortion Product OAE (DPOAE)
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Stay Tuned