8 Auditory Physiology

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Published on November 5, 2007

Author: Clarice

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Auditory Physiology of the Ear:  Auditory Physiology of the Ear James Saunders, MD FACS Dept of ORL, OUHSC King Richard, III Battle of Bosworth Field, 1485 :  King Richard, III Battle 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!! Acoustic systems must accommodate for lost energy between fluids:  Acoustic systems must accommodate for lost energy between fluids Most (97 – 99%) of Acoustic Energy is Reflected from Water:  Most (97 – 99%) of Acoustic Energy is Reflected from Water IMPEDANCE:  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 Middle Ear Transmits Energy by Two Pathways::  Middle Ear Transmits Energy by Two Pathways: Coupling Mechanisms are Frequency Dependent:  Coupling Mechanisms are Frequency Dependent MECHANISMS OF MIDDLE-EAR GAIN:  MECHANISMS OF MIDDLE-EAR GAIN Acoustic Coupling Ossicular Coupling Area Difference (TM to footplate) Lever Action (Malleus to Incus) MECHANICAL LIMITATIONS OF ME STRUCTURES:  TM, OSSICLES AND OSSICULAR LIGAMENTS RESISTANCES, MASSES AND STIFFNESS OPPOSE MIDDLE EAR MOTION MECHANICAL LIMITATIONS OF ME STRUCTURES Total Impedance is the sum of: Resistance (R), Effective Mass (Xm), and Effective Compliance (Xc):  Total Impedance is the sum of: Resistance (R), Effective Mass (Xm), and Effective Compliance (Xc) Mass (m) is inversely proportional to frequency (f):  Mass (m) is inversely proportional to frequency (f) Therefore, 1/f = 2πfm / Xm or f = Xm / 2πfm Effective Stiffness (Xs) is opposite of compliance and is inversely proportional to frequency:  Effective Stiffness (Xs) is opposite of compliance and is inversely proportional to frequency Therefore, f = S/2πXs PHASIC COMPONENTS OF IMPEDANCE:  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. PHASIC RELATIONSHIPS RESISTANCE & REACTANCE:  PHASIC RELATIONSHIPS RESISTANCE & REACTANCE The Uncoiled Cochlea:  The Uncoiled Cochlea Tonotopic Mapping:  Tonotopic Mapping EARLY BIOPHYSICAL CONCEPTS:  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 doesn’t move along membrane Von Bekesy Traveling Waves:  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 Traveling Wave:  Traveling Wave Traveling Wave:  Traveling Wave Electrical Activity Matches Traveling Wave:  Electrical Activity Matches Traveling Wave Gregor Von Békésy Nobel Prize Physiology 1961:  Gregor Von Békésy Nobel Prize Physiology 1961 Bekesy Mechanical Tuning Curve are Broad Compared to Neural Responses:  Bekesy Mechanical Tuning Curve are Broad Compared to Neural Responses Actual Tuning Curve Analytical Coding Theories of Hearing:  Analytical Coding Theories of Hearing Place Theory frequency information is place in cochlea of diplacement Effective at high frequency (>5000 Hz) Frequency (Temporal) Theory Modification of telephone theory Volley Principle Effective at low frequencies (15 – 400 Hz) Transition Zone Both methods (400 – 5000 Hz) Volley Principle:  Volley Principle Analytical Coding Theories of Hearing:  Analytical Coding Theories of Hearing Place Coding frequency information is place in cochlea of diplacement Effective at high frequency (>5000 Hz) Frequency (Temporal) Coding Modification of telephone theory Volley Principle Effective at low frequencies (15 – 400 Hz) Transition Zone Both methods (400 – 5000 Hz) The Inner Ear:  The Inner Ear Inner and Outer Hair Cells:  Inner and Outer Hair Cells Cochlear Hair Cells:  Cochlear Hair Cells Inner Hair Cells One row Contact w/ tectorial Outer hair Cells Three Rows “V” shape Connected to tectorial Hair cells are excited when stereocilia are displaced toward kinocilium:  Hair cells are excited when stereocilia are displaced toward kinocilium Hair Cell Properties:  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 Shearing Forces Created by Different Pivotal Points:  Shearing Forces Created by Different Pivotal Points Electrical Potentials of the Cochlea:  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 – allor none response of auditory nerve fibers Endocochlear Potential:  Endocochlear Potential Summating Potential and Cochlear Microphonic:  Summating Potential and Cochlear Microphonic Innervation of the Cochlea:  Innervation of the Cochlea Afferent Nerves Cell Bodies in Spiral Ganglion (Rosenthal’s Canal) Type I synapses with IHC (95%) Type II synapses with OHC (5%) Tonotopically Organized in Auditory Nerve Hair Cell → Efferent Transmitter is Glutamate Innervation of the Cochlea:  Innervation of the Cochlea Innervation of the Cochlea:  Innervation of the Cochlea Afferent: Each neuron goes to only one IHC Up to 8 neurons per IHC ~ 10 OHC, all basal to IHC Cochlear Nerve Afferent Responses:  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 Characteristic Frequency :  Characteristic Frequency Phase Locked Firing Pattern:  Phase Locked Firing Pattern Period (phase locked) Post Stimulus Time Histogram (note saturation of response):  Period (phase locked) Post Stimulus Time Histogram (note saturation of response) Intensity Coding:  Intensity Coding Innervation of the Cochlea:  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) Innervation of the Cochlea:  Innervation of the Cochlea Innervation of the Cochlea:  Innervation of the Cochlea Efferent: Each may go to multiple OHC or IHC Either basal or apical direction Now back to those sharp tuning curves…:  Now back to those sharp tuning curves… ACTIVE PROCESS FOR NARROW TUNING:  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) OHC Responsible for Sharp Tuning Curve:  OHC Responsible for Sharp Tuning Curve OHC Damage equency OHC Electromotility:  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) OHC Contain Actin (contractile protein):  OHC Contain Actin (contractile protein) OHC Reduce Length with Depolarization:  OHC Reduce Length with Depolarization OHC MOTILITY:  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) PASSIVE MECHANICS:  PASSIVE MECHANICS Factor at ≥ 50-60 dB Direct Movement Of Cochlear Partition IHC Sterocilia Move → Transduction Channels Open Depolarization → glutamate → Aps ACTIVE MECHANICS (LOW LEVEL):  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 Tuning Curve Become Broader At High Intensities:  Tuning Curve Become Broader At High Intensities EFFERENT CONTROL OHCs:  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 Otoacoustic Emissions (OAE):  Otoacoustic Emissions (OAE) Spontaneous OAE:  Spontaneous OAE Distortion Product OAE (DPOAE):  Distortion Product OAE (DPOAE) TOTAL BONE CONDUCTION RESPONSE:  TOTAL BONE CONDUCTION RESPONSE Compressional Inertial Osseotympanic INNER EAR (Compressional):  INNER EAR (Compressional) Distortion of Bony Cochlea MIDDLE EAR (Inertial):  MIDDLE EAR (Inertial) Most effective at Low and High Frequencies EXTERNAL EAR (Osseotympanic):  EXTERNAL EAR (Osseotympanic) Sound Energy Radiated Bony EAC Mandibular contribution Slide79:  Happy Halloween!!

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