physiology of the cochlea mechanical response of cochlea in response to sound two major functions:...

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Physiology of the cochlea Physiology of the cochlea Mechanical response of cochlea in response Mechanical response of cochlea in response to sound to sound Two major functions: Two major functions: 1. 1. Analysis of sound into components: Analysis of sound into components: Frequency/Spectral analysis Frequency/Spectral analysis 2. 2. Transduction of sound: Transduction of sound: Converting mechanical energy into Converting mechanical energy into electrochemical/neural energy electrochemical/neural energy http://www.neurophys.wisc.edu/animations/

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Page 1: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Physiology of the cochleaPhysiology of the cochlea

Mechanical response of cochlea in response to Mechanical response of cochlea in response to soundsound

Two major functions:Two major functions:

1.1. Analysis of sound into components: Analysis of sound into components:

Frequency/Spectral analysisFrequency/Spectral analysis

2.2. Transduction of sound: Transduction of sound:

Converting mechanical energy into Converting mechanical energy into electrochemical/neural energyelectrochemical/neural energy

http://www.neurophys.wisc.edu/animations/

Page 2: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Mechanical response of cochlea to Mechanical response of cochlea to soundsound

Basilar membrane moves in response to stapes Basilar membrane moves in response to stapes vibrationvibration

This vibration takes the form of a ‘traveling wave’ This vibration takes the form of a ‘traveling wave’ (von Bekesy)(von Bekesy)

Wave motion starts at the base and moves Wave motion starts at the base and moves toward the apextoward the apex

http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm

Page 3: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

InstantaneousInstantaneous basilar membrane pattern basilar membrane patternandand

envelopeenvelope of traveling wave of traveling wave

Page 4: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Characteristics of traveling waveCharacteristics of traveling wave

Points of maximum displacement: PeakPoints of maximum displacement: Peak

Peak depends on the frequency of incoming Peak depends on the frequency of incoming soundsound

Each part of basilar membrane ‘tuned’ to a Each part of basilar membrane ‘tuned’ to a particular frequency called particular frequency called critical/center/characteristic frequency (CF)critical/center/characteristic frequency (CF)

Page 5: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Low frequencies: Localized toward the apexLow frequencies: Localized toward the apex

High frequencies: Localized toward the baseHigh frequencies: Localized toward the base

Page 6: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Frequency/Spectral analysisFrequency/Spectral analysis

Cochlea acts as a Cochlea acts as a series of bandpass series of bandpass filtersfilters

Incoming sound Incoming sound broken down into broken down into individual sinusoidal individual sinusoidal componentscomponents

Basilar membrane Basilar membrane vibrates in response to vibrates in response to each of these each of these componentscomponents

Page 7: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Basilar membrane mechanicsBasilar membrane mechanics

Greater the stimulus level, greater the Greater the stimulus level, greater the amount of basilar membrane displacementamount of basilar membrane displacement

Temporal pattern of basilar membrane Temporal pattern of basilar membrane vibration follows that of incoming soundvibration follows that of incoming sound

Page 8: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Input-output functionsInput-output functions

At mid-intensity levels, At mid-intensity levels, basilar membrane vibration basilar membrane vibration increases with intensity in a increases with intensity in a non-linearnon-linear or or compressive compressive fashionfashion

At very low (< 30 dB SPL) At very low (< 30 dB SPL) and very high (> 90 db and very high (> 90 db SPL), linear vibrationSPL), linear vibration

This is frequency This is frequency dependent: Only for the CFdependent: Only for the CF

Above and below the CF: Above and below the CF: Linear input-output Linear input-output functionsfunctions

Page 9: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Steps in transductionSteps in transduction

Mechanical vibrations translated into neural Mechanical vibrations translated into neural responses in the auditory nerveresponses in the auditory nerve

Stapes vibration sets inner ear fluid into Stapes vibration sets inner ear fluid into vibrationvibration

Basilar membrane vibratesBasilar membrane vibrates

Shearing motion of tectorial membraneShearing motion of tectorial membrane

Stereocilia of haircells bendStereocilia of haircells bendhttp://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm

Page 10: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Bending of stereociliaBending of stereocilia

http://www.iurc.montp.inserm.fr/cric/audition/english/corti/hcells/ohc/fohc.htm

http://www.iurc.montp.inserm.fr/cric/audition/english/corti/fcorti.htm

Page 11: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Transduction, Cont’d. Transduction, Cont’d. Increase in number of open ion channels at tip linksIncrease in number of open ion channels at tip links

Influx of positive pottassium and calcium ions into the cell Influx of positive pottassium and calcium ions into the cell bodybody

Resting membrane potential of endolymph: + 80 mvResting membrane potential of endolymph: + 80 mv

Intracellular resting potential:Intracellular resting potential:

OHC: -70mvOHC: -70mvIHC: - 45mvIHC: - 45mv

Change in membrane potentialChange in membrane potential

Release of neurotransmitterRelease of neurotransmitter

Neuron at the base of the haircell firesNeuron at the base of the haircell fires

Page 12: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Transduction, Cont’d. Transduction, Cont’d. Increase in number of open ion channels at tip linksIncrease in number of open ion channels at tip links

Influx of positive pottassium and calcium ions into the cell Influx of positive pottassium and calcium ions into the cell bodybody

Resting membrane potential of endolymph: + 80 mvResting membrane potential of endolymph: + 80 mv

Intracellular resting potential:Intracellular resting potential:

OHC: -70mvOHC: -70mvIHC: - 45mvIHC: - 45mv

Change in membrane potentialChange in membrane potential

Release of neurotransmitterRelease of neurotransmitter

Neuron at the base of the haircell firesNeuron at the base of the haircell fires

Page 13: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Differences between OHC and IHCDifferences between OHC and IHC

IHC: True sensory part of the cochleaIHC: True sensory part of the cochlea

Only forward transduction (Mechanical Only forward transduction (Mechanical to neural)to neural)

OHC: Both sensory and motor functionsOHC: Both sensory and motor functions

Forward and backward transduction Forward and backward transduction (Mechanical to neural, and neural to (Mechanical to neural, and neural to mechanical)mechanical)

Page 14: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Physiology of OHC and IHCPhysiology of OHC and IHC

OHC display ‘motility’ (change in length) in OHC display ‘motility’ (change in length) in response to stimulation. IHC do not. response to stimulation. IHC do not.

Because of muscular contractions within Because of muscular contractions within the cell body and efferent connections to the cell body and efferent connections to the OHC bodies.the OHC bodies.

Thought to be basis of Thought to be basis of ‘cochlear ‘cochlear amplifier’amplifier’ and and ‘active cochlea’‘active cochlea’

Page 15: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

What is the cochlear amplifier?What is the cochlear amplifier?

‘‘Feedback loop’ within the Organ of Corti, because of OHC Feedback loop’ within the Organ of Corti, because of OHC motility.motility.

Forward transduction: Basilar membrane moves, stereocilia Forward transduction: Basilar membrane moves, stereocilia are deflected, tranduction current flows into the OHC. This are deflected, tranduction current flows into the OHC. This transduction current causes a change in the receptor transduction current causes a change in the receptor potential (depolarization).potential (depolarization).

Reverse transduction: Depolarization triggers the motility of Reverse transduction: Depolarization triggers the motility of the OHC. This change in length exerts force on the basilar the OHC. This change in length exerts force on the basilar membrane, deflecting the stereocilia and so on. membrane, deflecting the stereocilia and so on.

OHC feed energy back into the basilar membraneOHC feed energy back into the basilar membrane!!!!

Page 16: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Evidence for Evidence for activeactive versus versus passivepassive cochleacochlea

History: von Bekesy measured broad traveling History: von Bekesy measured broad traveling waves in dead cochleae. However, finely-tuned waves in dead cochleae. However, finely-tuned tuning curves measured for auditory neurons. tuning curves measured for auditory neurons.

Fine tuning of basilar membrane is lost when OHC Fine tuning of basilar membrane is lost when OHC die.die.

Compressive non-linearity (as seen in input-Compressive non-linearity (as seen in input-output curve) lost when OHC are damaged.output curve) lost when OHC are damaged.

Most important evidence for active cochlea: Most important evidence for active cochlea: Oto-Oto-acoustic emissions.acoustic emissions.

Page 17: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Otoacoustic emissions (OAE)Otoacoustic emissions (OAE)

Responses that Responses that originateoriginate within the inner within the inner earear

May be spontaneous or evokedMay be spontaneous or evoked

Generated due toGenerated due toThe feedback system in the cochleaThe feedback system in the cochleaActive processes within the cochleaActive processes within the cochleaNon-linearities within the cochleaNon-linearities within the cochlea

Page 18: Physiology of the cochlea Mechanical response of cochlea in response to sound Two major functions: 1. Analysis of sound into components: Frequency/Spectral

Tuning in the basilar membraneTuning in the basilar membrane

Vibration of basilar Vibration of basilar membrane can be membrane can be described by a ‘tuning described by a ‘tuning curve’curve’

Amplitude required for Amplitude required for the basilar membrane the basilar membrane to vibrate at a certain to vibrate at a certain constant constant displacement, as a displacement, as a function of the function of the frequency of the input frequency of the input soundsound