HIS 120 The Basilar Membrane and the Traveling Wave

The Basilar Membrane & the Traveling
Wave
• Bekesy’s Traveling Wave Theory/Model
In 1961, Georg von Bekesy was awarded
the Nobel prize in medicine and
physiology. It was primarily due to his
contributions toward the understanding of
the physical mechanisms of excitation of
the cochlea.

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

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

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An illustration of his prize winning model is
on page 483 of Zemlin.
It demonstrated Pascal’s principle which
states that the creation of any pressure
point in a closed-fluid system (scala
media) will be transmitted to all other
points of that closed system.

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Perilymph has a viscosity similar to water.
The cochlear partition includes
endolymph, hair cells, the tectorial
membrane and the basilar membrane. It
is the consistency of gelatin.
There is no physical discontinuity between
the cochlear partition and the perilymph
fluid.

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Surface waves occur at the boundary
between the endolymph and perilymph.
Their only discontinuity is between the
physical properties of the perilymphatic
fluids and the cochlear partition.
As the waves travel through the perilymph,
the pressure pattern changes of both time
and space are created across the
cochlear partition.

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Since the bony portion of the labyrinth is
solid, the only release for fluid movement
is the round window.
If the round window was solid bone, the
stapes would be unable to move the fluid
from the oval window side.

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• Characteristics of the Basilar Membrane
It is about .1mm at its base increasing in
width to about .5mm at its apex.
Its stiffness is about one hundred times
greater at its base than its apex.
These characteristics become the
determinants of its frequency response
patterns.

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• Characteristics of the Basilar Membrane
There are transverse bands (side-to-side)
which are located along the length of the
basilar membrane. These transverse
bands vary in stiffness as they are spaced
along the basilar membrane.
Each band is (frequency) sensitive to the
various waves of energy received along
the traveling wave pathway.

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• Characteristics of the Traveling Wave
The spatial separation between the point
of initial stimulus and the maximum
amplitude of the traveling wave create
various frequency resolution.
After the maximum amplitude is reached,
the traveling wave reduces to virtually zero
displacement.

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

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The point of maximum amplitude for high
frequencies is close to the basal end;
while the maximum amplitude for low
frequencies is closer to the apical end.
(ref. Zemlin pg #483 figure 6-101)

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

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The velocity and therefore the wavelength
decrease as a function of distance from
the stapes.
This reduction of amplitude, velocity, and
wavelength is commonly found with any
sound transmission through a fluid.

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This frequency dependent maximum of
membrane displacement is a clear
indication that the cochlea performs a
mechanical frequency analysis.
This is defined as the Place Theory of
frequency resolution.

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

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

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

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• The Basilar Membrane and Hair Cells
Bekesy’s further study of the traveling
wave found eddy currents located at the
location of maximum membrane response.
These eddy currents created more
specific stimulation of the hair cells
associated in the region of maximum
amplitude of the traveling wave.

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

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

HIS 120 The Basilar Membrane and the Traveling Wave

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HIS 120 The Basilar Membrane and the Traveling Wave