HIS 120 The Basilar Membrane and the Traveling Wave

The Basilar Membrane & the Traveling The Basilar Membrane & the Traveling WaveWave• Bekesy’s Traveling Wave Theory/ModelBekesy’s Traveling Wave Theory/Model

In 1961, Georg von Bekesy was In 1961, Georg von Bekesy was awarded the Nobel prize in medicine awarded the Nobel prize in medicine and physiology. It was primarily due and physiology. It was primarily due to his contributions toward the to his contributions toward the understanding of the physical understanding of the physical mechanisms of excitation of the mechanisms of excitation of the cochlea.cochlea.

The Basilar Membrane & the Traveling The Basilar Membrane & the Traveling WaveWave

•Bekesy’s InvestigationBekesy’s Investigation

He constructed a mechanical model He constructed a mechanical model of the cochlea. It’s basilar of the cochlea. It’s basilar membrane had much of the same membrane had much of the same stiffness characteristics as the fresh stiffness characteristics as the fresh human cadaver ones he had studied.human cadaver ones he had studied.


•Bekesy’s InvestigationBekesy’s Investigation

He found that there were consistent He found that there were consistent resonant patterns which were resonant patterns which were created with sound stimulation of the created with sound stimulation of the fluid-filled upper and lower scala.fluid-filled upper and lower scala.


• Bekesy’s InvestigationBekesy’s Investigation

An illustration of his prize winning An illustration of his prize winning model is on page 483 of Zemlin. model is on page 483 of Zemlin.

It demonstrated It demonstrated Pascal’s principlePascal’s principle which states that the creation of any which states that the creation of any pressure point in a closed-fluid system pressure point in a closed-fluid system (scala media) will be transmitted to all (scala media) will be transmitted to all other points of that closed system.other points of that closed system.


Perilymph has a viscosity similar to Perilymph has a viscosity similar to water. The cochlear partition includes water. The cochlear partition includes endolymph, hair cells, the tectorial endolymph, hair cells, the tectorial membrane and the basilar membrane. membrane and the basilar membrane. It is the consistency of gelatin. It is the consistency of gelatin.

There is no physical discontinuity There is no physical discontinuity between the cochlear partition and the between the cochlear partition and the perilymph fluid.perilymph fluid.


Surface waves occur at the boundary Surface waves occur at the boundary between the endolymph and between the endolymph and perilymph. Their only discontinuity is perilymph. Their only discontinuity is between the between the physical propertiesphysical properties of the of the perilymphatic fluids and the cochlear perilymphatic fluids and the cochlear partition. partition.

As the waves travel through the As the waves travel through the perilymph, the pressure pattern perilymph, the pressure pattern changes of changes of both time and spaceboth time and space are are created across the cochlear partition.created across the cochlear partition.


Since the bony portion of the Since the bony portion of the labyrinth is solid, the only release for labyrinth is solid, the only release for fluid movement is the round window. fluid movement is the round window.

If the round window was solid bone, If the round window was solid bone, the stapes would be unable to move the stapes would be unable to move the fluid from the oval window side.the fluid from the oval window side.


• Characteristics of the Basilar MembraneCharacteristics of the Basilar Membrane

It is about .1mm at its base increasing It is about .1mm at its base increasing in width to about .5mm at its apex. in width to about .5mm at its apex.

Its stiffness is about one hundred times Its stiffness is about one hundred times greater at its base than its apex. greater at its base than its apex.

These characteristics become the These characteristics become the determinants of its frequency response determinants of its frequency response patterns.patterns.

The Basilar Membrane & the Traveling The Basilar Membrane & the Traveling WaveWave• Characteristics of the Basilar MembraneCharacteristics of the Basilar Membrane

There are transverse bands (side-to-There are transverse bands (side-to-side) which are located along the length side) which are located along the length of the basilar membrane. These of the basilar membrane. These transverse bands vary in stiffness as transverse bands vary in stiffness as they are spaced along the basilar they are spaced along the basilar membrane. membrane.

Each band is (frequency) sensitive to the Each band is (frequency) sensitive to the various waves of energy received along various waves of energy received along the traveling wave pathway. the traveling wave pathway.


• Characteristics of the Traveling WaveCharacteristics of the Traveling Wave

The spatial separation between the The spatial separation between the point of initial stimulus and the point of initial stimulus and the maximum amplitude of the traveling maximum amplitude of the traveling wave create various frequency wave create various frequency resolution. resolution.

After the maximum amplitude is After the maximum amplitude is reached, the traveling wave reduces to reached, the traveling wave reduces to virtually zero displacement.virtually zero displacement.



The first portion of wave undulation The first portion of wave undulation is generally received close to the is generally received close to the stapes; the wave continues with stapes; the wave continues with increased undulation to a maximum increased undulation to a maximum point along the basilar membrane; point along the basilar membrane; this maximum amplitude point is this maximum amplitude point is dependant upon the frequency of the dependant upon the frequency of the stimulus. stimulus.



The point of maximum amplitude for The point of maximum amplitude for high frequencies is close to the basal high frequencies is close to the basal end; while the maximum amplitude end; while the maximum amplitude for low frequencies is closer to the for low frequencies is closer to the apical end.apical end.

(ref. Zemlin pg #483 figure 6-101)(ref. Zemlin pg #483 figure 6-101)



Because lower frequencies displace Because lower frequencies displace larger and larger segments of the larger and larger segments of the basilar membrane, we begin to see basilar membrane, we begin to see why low frequencies tend to mask why low frequencies tend to mask high frequencies.high frequencies.



The velocity and therefore the The velocity and therefore the wavelength decrease as a function of wavelength decrease as a function of distance from the stapes. distance from the stapes.

This reduction of amplitude, velocity, This reduction of amplitude, velocity, and wavelength is commonly found and wavelength is commonly found with any sound transmission through a with any sound transmission through a fluid.fluid.



This frequency dependent maximum This frequency dependent maximum of membrane displacement is a clear of membrane displacement is a clear indication that the cochlea performs indication that the cochlea performs a mechanical frequency analysis. a mechanical frequency analysis.

This is defined as the This is defined as the Place TheoryPlace Theory of of frequency resolution.frequency resolution.



• Place TheoryPlace Theory

It is when each point along the It is when each point along the basilar membrane develops a basilar membrane develops a maximum point of displacement maximum point of displacement (amplitude) associated with a (amplitude) associated with a specific frequency of stimulus.specific frequency of stimulus.



• Place TheoryPlace Theory

Thus, the single most important Thus, the single most important characteristic of the basilar characteristic of the basilar membrane would seem to be the membrane would seem to be the gradual changes in its stiffness from gradual changes in its stiffness from its base to its apex its base to its apex (almost one hundred (almost one hundred times stiffer at its base)times stiffer at its base)



The peaking of the traveling wave is The peaking of the traveling wave is not due to the mere resonance of the not due to the mere resonance of the basilar membrane but, to the energy basilar membrane but, to the energy exchange created between the exchange created between the basilar membrane and the cochlear basilar membrane and the cochlear fluids.fluids.


• The Basilar Membrane and Hair CellsThe Basilar Membrane and Hair Cells

Bekesy’s further study of the Bekesy’s further study of the traveling wave found traveling wave found eddy currentseddy currents located at the location of maximum located at the location of maximum membrane response. These eddy membrane response. These eddy currents created more specific currents created more specific stimulation of the hair cells stimulation of the hair cells associated in the region of maximum associated in the region of maximum amplitude of the traveling wave.amplitude of the traveling wave.



Bekesy concluded that these eddy Bekesy concluded that these eddy currents may be created by the motor currents may be created by the motor function of the outer hair cells function of the outer hair cells stimulated stimulated efferentlyefferently from the central from the central pathway. Thereby, providing further pathway. Thereby, providing further definition to the amplitude of the definition to the amplitude of the traveling wave.traveling wave.



As hair cells are destroyed or As hair cells are destroyed or become dysfunctional, frequency become dysfunctional, frequency resolution (pitch) as well as resolution (pitch) as well as amplitude intensity (loudness), amplitude intensity (loudness), become reduced--thus, creating become reduced--thus, creating sensorineural hearing loss.sensorineural hearing loss.

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HIS 120 The Basilar Membrane and the Traveling Wave