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Necessary structures for simulating basilar
membrane (actually, entire organ of Corti)
movement:
•Scala vestibuli & scala tympani (only one is
required) are fluid filled; scala media is not
included (Reissner's membrane is acoustically
transparent).
•Pressure release (inward stapes movements
cause outward bulging of the round window)
occurs via the helicotrema.
•A flexible basilar membrane with exponential stiffness gradient (uncoiled) is generated by the triangular shaped cavity
filled with latex.
•A source is needed for delivering sinusoidal input to perilymphatic fluid at the oval window (window need not be oval,
and need not be in this location).
•Waves travel from base to apex, regardless of window location (hi Z to lo Z) — this is necessary for explaining hearing
by bone conduction where size asymmetry between scala vestibuli and scala tympani generates force on basilar
membrane.
•Remove one scala, and use a forearm as a "nerve supply" to "feel" the spectrum of the sound. The organ of Corti is
analgous to a touch receptor: You hear a pitch because the organ of Corti is touched in a certain place by a frequency.
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Concepts related to basilar membrane (BM)
movement
•the shape of BM is related to its stiffness
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•changes in stiffness influence the resonant frequency
fo = 1/(2π)*(k/m)^.5
•regions of high stiffness relate to high frequencies, low stiffness relate to low frequencies
•stiffness affects speed of wave propagation. The stiffer the partition, the faster the wave propagates.
•changes in speed of wave affect the wavelength (f= c)
 = c/f
•for any given frequency, as the speed decreases, wavelength decreases
•all of these factors mean that different portions of the BM will respond better (with a higher amplitude) to certain
frequencies
•the basilar membrane performs a mechanical spectral analysis
•The brain infers which frequency is delivered to the ear by noting where, along the organ of Corti, it is being "touched"
— much like how the body surface is mapped by the somatosensory cortex.
This is the place principle (theory), tonotopic organization of BM (and rest of auditory system)
•Theories of hearing, place and temporal coding
◊place theory
◊frequency theory
◊volley theory
The Traveling Wave
•movement of the footplate of the stapes imparts energy to perilymph, which is then transmitted to the BM
•the wave travels through the medium of the organ of Corti
•the wave begins (always) at the basal end with a…
◊velocity, based upon stiffness of the partition (high velocity at basal end)
◊wavelength, based on frequency of stapes motion (high frequencies begin with a shorter wavelength)
◊amplitude, which is initially small, but builds as the wave moves to the apex
•as the wave travels to the apex…
◊the velocity decreases as the stiffness decreases
◊the wavelength decreases because the velocity is decreasing
◊the amplitude is increasing
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•this process continues until the wavelength decreases to a critical value
wavelength = 2L
where L is the cross sectional depth of the bony labyrinth (see next card)
•at this point the wave crests, energy in the wave is dissipated (into the organ of Corti), and amplitude rapidly drops to
zero
Points to consider
•higher frequencies begin with a shorter wavelength, so they are decreased to 2L sooner, and crest more basally
•lower frequencies travel further, requiring a greater distance for the wavelength to decrease to 2L
•maximum stimulation (shearing force) is delivered to hair cells at the point where the wave crests, but prior regions are
stimulated also
•intense low frequency stimuli will cause basal hair cell stimulation; low frequency temporal information (including the
envelope of the wave) is coded in "high" frequency regions of the cochlea — and lost with high frequency hearing loss
•intense low frequency stimulation will cause masking of higher frequencies (upward spread of masking)
◊Most hearing aid processing strategies reduce the low frequency response in the presence of high level sound
•outer hair cells receive greater stimulation than IHC, because of their location
•IHC carry the bulk of afferent information in the radial fibers. Neural firings in the radial fibers reflect stimulation at
very specific locations on the BM
•BM movements cause electrical activity within cochlea (more about this later)
•OHCs exhibit motility (they change in length) in the presence of electrical potentials
◊As OHC stereocilia are bent, they change in length: Depolarization shortens the cell, hyperpolarization lengthens it.
•OHC motility has no appreciable time delay and is probably not dependent upon efferent processes (which would be
much slower)
•OHC motility adds mechanical energy to BM, enhancing its amplitude of motion (primarily at crest)
•OHC motility acts as a mechanical amplifier, probably…
◊increasing sensitivity of IHCs
◊sharpening neural tuning curves (NTC)
◊causing cochlear emissions (OAEs)
◊enhances our auditory sensitivity in the 0 to 45-50 dB HL range
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•Mead Killion suggests that the OHC amplifier adds "compressive nonlinearity" to the cochlea — similar to a WDRC
hearing aid with a 2.3:1 compression ratio (the K-AMP has a 2.1:1 compression ratio). This cochlear amplifier consumes
about 50 µWatts. A WDRC hearing aid requires roughly 230 µWatts.
•slower acting efferent activity (the crossed and uncrossed olivocochlear bundle, originating from the superior olive) is
probably neural in origin. It may have a general "damping" effect upon BM motion. Lack of this central suppression may
be related to hypersensitivity to sound.
The general concept is that the higher centers of the CNS control or suppress lower centers. Removal of this control
results in exaggerated output from lower centers, as with the more pronounced patellar reflex on the side paralyzed by a
hemispheric stroke.
Reduced DLs for intensity have been observed in the ear contralateral to Heschl's gyrus ablation.
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Neural Tuning Curves
•An experimental animal is anesthetized, the
VIII N. is exposed, and a system is in place for
varying the frequency and intensity of pure
tones delivered to the ear canal
•A micro electrode is inserted into a single
unit (single cochlear neuron)
◊Assume that the neuron is afferent, and
synapses with an inner hair cell (i.e., a radial fiber)
◊The inner hair cell is located at a place along the BM corresponding to the "characteristic frequency" or "best frequency"
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◊The best frequency is found by determining the frequency that will give the greatest firing rate for the least dB SPL
•An iso-rate contour is generated by selecting an appropriate target firing rate ("n" neural spikes per second), usually a
just observable increase over the spontaneous rate.
◊For each frequency, the SPL is varied until the target rate is achieved
◊The shape of the contour is thought to be determined largely by the amplitude of BM displacement
◊The sharp tuning at the best frequency is believed to be due to an additional "mechanical amplifier"
◊The motile action of the outer hair cells (OHCs) increases the amplitude of BM displacement, causing greater
stimulation of the inner hair cells (IHCs) and increased firing rate of the neuron
•Destruction of the OHCs results in poorer frequency resolution (greater frequency difference limens)
◊probably wider critical bands, greater susceptability to masking noise, less loudness summation and
◊most likely reduced speech intelligibility, especially in noise
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•CF1 represents the NTC for a neuron
(afferent neuron) synapsing with an IHC at a
basal location
•CF2 is the same, except it is located a bit
more basally, at a higher frequency, than CF1
•CF3 would represent a neuron associated
with an IHC at a location even more basal
than CF2
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Afferent System
•nerve fibers associated with transmitting
information from the end organ to the CNS
•roughly 35,000 fibers total
•The neurochemical transmitter is glutamate
•Radial fibers
◊synapse with the IHC
◊95% of the afferent supply
◊about 8 to 10 fibers per IHC
◊a "many to one" connection
•Outer spiral (OS) fibers
◊synapse with OHC
◊5% of the neural supply
◊each OS innervates 10 OHCs
◊a "one to many" connection
◊each travels about .6 mm basally
Efferent System
•transmits information from CNS to the end organ
•generally, the efferent system exerts a suppressive effect, modulating afferent output
•there are only about 600 fibers total, originating from the superior olive (medial olivocochlear bundle), either ipsi
(uncrossed olivo-cochlear bundle: UCOCB) or contra (crossed OCB)
•Acetylcholine (Ach) seems to be the main NCT which binds to receptor sites on the OHC (a nicotinic receptor, found in
the adrenal medulla and skeletal muscle).
◊GABA (g-aminobutyric acid), a major inhibitory NCT, is also present. So is adenosine triphosphate (ATP).
◊Ach, GABA, and ATP open ion channels that can change the polarization of the OHC, the cell's turgor, thus changing
the cell's length.
•Tunnel radial fibers
◊synapse with the OHC
◊synapses with hair cell base
◊80% of the efferent supply
◊originate from COCB
◊each fiber innervates about 10 OHC, locally, in a "grape like cluster"
•Inner spiral bundle
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◊synapse with IHC, indirectly
◊synapse with radial fibers
◊20% of efferent supply
◊originate from UCOCB
◊they travel apically
Comments…
•IHCs are extremely important for transmitting sensory information
•OHCs play a dominant role in efferent processes
•OAEs can be suppressed (about 3-5 dB) by contralateral stimulation, an objective test of brainstem integrity
card fld "OHC Motility"
The Cochlear Amplifier (from Boys Town National Research Hospital's web site, www.boystown.org)
The following animated cartoons illustrate the possible influence of outer hair cell (OHC) motility on cochlear
micromechanics. These three scenes show the OHC situated within a cross-section of the cochlea.
An MPEG player is required to view the video clips referenced below. For best results, set your MPEG player to "loop".
ohc0.mpg
coch0.mpg
In the first scene (MPEG:53k), three OHCs are shown embedded in the Organ of Corti (OC) as they would be in vivo. The
long lever at the bottom represents the basilar membrane (BM) which is deflected by pressure gradients in the
surrounding fluid. The hair bundles at the top of the OHCs are deflected by the shear displacement between the recticular
lamina (RL) at the top of the OC and the tectorial membrane (TM). The hair bundle of the inner hair cells (IHC) is
deflected in the same manner as the OHCs. (The cell body of the inner hair cell is not shown). In this scene, the OHCs do
not change their length. The red dot (in the upper right corner) indicates the input-output relationship; its vertical motion
is proportional to the input (BM displacement) and its horizontal motion is proportional to the output (IHC
displacement). The blue box shows the range of these displacements with no OHC length change for comparison with the
next two scenes.
ohc1.mpg
coch1.mpg
In the second scene (MPEG:51k), the OHCs contract in-phase with upward deflection of the BM. Note that this phase of
contraction reduces the defection of the hair bundles. The BM deflection is absorbed by the OHC contraction, so there is
very little shear displacement between the RL and TM. The red dot has very little horizontal motion. At low frequencies,
when the receptor voltage and current are in-phase, the OHCs will contract in phase with BM displacement. When this
happens, the amplitude of IHC deflection is smaller than the amplitudeof BM displacement.
ohc2.mpg
coch2.mpg
In the third scene (MPEG:57k), the OHC contraction lags BM displacement by 90 degrees. Note that the hair bundle
deflection now exceeds what was observed with no contraction. The red dots goes beyond the limits of the blue box. More
imporant is the fact that forces exterted by the basilar membrane in this phase become negative damping forces and
pump energy into the mechanical system in the same way that one does when "rocking a boat" or "pumping a swing". The
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energy contributed by OHCs will improve the sensitvity of the cochlea to low-level sounds. This is the basis for the
cochlear amplifier theory of cochlear mechanics
Last modified: 6-Mar-95 neely@boystown.org
The following are simulations of traveling waves at 250 Hz, 1 kHz, 4 kHz, and for a click (100/sec)
f250.mpg
f1k.mpg
f4k.mpg
click.mpg
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Cochlear Potentials
•Endolymphatic Potential (EP)
◊+80 mV resting, DC potential generated by
oxidative phosphorylation (ATP) in the stria
vascularis
◊sensitive to oxygen deprivation,
interruption of cochlear blood supply
•Summating Potential (SP)
◊a DC shift in EP, caused by stimulus (i.e., tone burst, click), present for entire duration of stimulus
◊a voltage drop (i.e., brownout) caused by current being drawn from scala media, inability of stria to maintain potential
◊strial atrophy receiving more attention as a cause of hearing loss associated with aging
◊SP exaggerated in cases of active endolymphatic hydrops (Ménière's disease)
◊measured by electrocochleography (EcochG)
•cochlear microphonic (CM)
◊first reported by Wever & Bray in 1930
◊an AC analog of the acoustic waveform (e.g., a pure tone)
◊originates at the juncture of the cilia with the cuticular plate of the hair cell (reticular lamina)
◊directly reflects magnitude of BM amplitude
◊is a direct result of modulation of current flow into hair cell by shearing of cilia
◊has no observable threshold
•generator potential
◊a reduction in negativity of the interior of the neuron caused by depolarization
◊this potential is caused by secretion of neurochemical transmitter (NCT; i.e. glutamate) into synaptic cleft
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◊it is a graded potential
◊when it reaches -40 mV it initiates the action potential
•Action Potential (AP)
◊all-or-none depolarization of the nerve fiber
◊peak magnitude is 40 mV
◊compound AP (CAP) is the sum of all fibers activated (Wave I of the ABR)
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Resting conditions
•Potassium rich endolymph is at a +80 mV potential
•the interior of the hair cell is at a -80 mV potential
•there is +160 mV potential across the cuticular plate (like the steady potential from a battery)
•the intracellular fluid which bathes the hair cell and neuron is rich in Na+
•a Na+ pump mechanism removes Na+ from the interior of the neuron
•the interior of the neuron is negative (due to large, negatively charged organic compounds); it is polarized
Generating the neural impulse
•The cilia are sheared
◊bending the cilia move tip links opening K+ channel; micropores in the cuticular plate (or cilia) are opened (i.e., like
opening a valve)
◊K+ ion flows into the hair cell in direct proportion to the amount that the cilia are bent
◊the K+ ions flow down an electrical gradient from the +EP to the negative intracellular potential of the hair cell
•current flowing into the hair cell generates a magnetic field
•the magnetic field forces the vesicles of NCT (probably glutamate) to migrate to the synaptic cleft
◊the NCT is dumped into the synapse where it migrates to the neural membrane
•the NCT causes the neural membrane to lose its integrity (to depolarize)
◊Na+ begins to flow into the neuron, reducing its negativity
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◊this is the generator potential
•when the neuron's threshold is reached (-40 mV)…
◊an avalanche of Na+ ion enters the neuron
◊polarity briefly goes positive, this is the AP
•the neuron's Na+ pump mechanism begins to remove Na+ from the cell
◊the cell's polarity is restored in this location
◊in adjacent locations, the depolarization continues, causing the impulse to propagate
◊while the potential is above -40 mV, the fiber cannot fire (the absolute refractory period)
◊while the potential is above -80 mV, the fiber can fire with additional stimulation (the relative refractory period)
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Encoding Intensity
As intensity increases…
•the rate of single unit firing increases
◊the increase plateaus after only a 40 dB
range
•but, more neurons fire than just one…
◊afferent fibers innervating outermost row of OHCs stimulated first, then 2nd row, 3rd row, then IHCs
◊as intensity increases, increased OHC motility increases IHC stereocilia shear, causing more of the 8 to 10 radial fibers
to fire, and causes each fiber to fire at a faster rate
•width of the crest of the traveling wave increases with intensity
◊spreading excitation along adjacent areas of the BM
◊encompassing more hair cells and nerve fibers
•Intensity is encoded as the total number of neural responses per unit time (i.e., neural spike density)
Encoding Frequency
1. place principle (2 to 6 kHz and above)
•width of the traveling wave is too broad to account for frequency resolution for lower frequencies
2. Temporal encoding (below 2 to 6 kHz)
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•Frequency Theory (Rutherford's telephone theory, late 1800s)
◊this phenomenon operates for frequencies below 400 Hz (approximately)
◊single units fire in synchrony (time locked, phase locked) with each cycle of tone
◊for a frequency, F, time interval between successive firings is T, where
T = 1/F
◊auditory system analyzes frequency by measuring time interval
◊limitation with absolute & relative refractory period
◊1000 to 2000 Hz phase locking at the very best, 400 Hz more realistic
•Volley Theory (EG Wever, 1949)
◊this phenomenon operates for frequencies from 400 Hz to 2-6 kHz (approx)
◊not single units, but GROUPS of nerve fibers are time locked to stimulus waveform
◊individual neurons "take turns" firing in volleys
◊neuron A fires, and goes into refractory
◊neuron B fires before A can recover, neuron B goes into refractory
◊neuron A has now recovered and can fire again (B is still recovering)
◊neurons A+B can fire at twice the rate of either alone (see PST Histogram.JPG)
◊sensorineural hearing loss and "thinning" of hair cell and neural supply produces poorer frequency resolution
◊poorer frequency resolution implies poorer ability to perform tasks such as tracking changing formant frequencies of
speech
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A model illustrating neural firing in the
cochlear nerve
Note: The simulation is computationally
intensive. Once it starts sit back and give it
some time to run. Try not to be impatient.
The model illustrates periodicity in VIII nerve
firing patterns (i.e., Wever's volley theory).
This periodicity is shown by neural impulses
clustering in time intervals related to the
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period of the stimulus frequency.
Assumptions
•a segment of the basilar membrane is responding to a tone of a given frequency
◊Change the stimulus frequency by clicking on the frequency
•this segment of the BM innervates a population of 100 neurons
◊the trigger threshold varies randomly (over a range) for the population
◊threshold varies to simulate varying sensitivity of OHCs, IHCs
◊the basilar membrane segment moves in a sinusoidal pattern as it responds to the pure-tone stimulus
◊when the segment is clicked, the simulation begins, the time that has expired since stimulus onset is indicated in a field
•the refractory period of each neuron can be varied from 1 to 2.5 msec
•the number of spikes occurring are collected in "bins"
◊there are 32 bins total
◊each bin is .25 msec in duration (4 msec/16), or .125 msec (2 msec/16) depending upon the value selected by clicking
upon the time base field
•for each portion of the cycle of BM motion
◊the neural population is scanned
◊a neuron fires providing that two criteria are met:
-its threshold must be exceeded. It is assumed that the probability of exceeding this threshold is directly proportional
to the instantaneous amplitude of BM displacement.
-and the neuron is not in refractory
The display…
•The display is a histogram showing the number of spikes within each bin as a function of time
•The time window is fixed at either 4 or 8 msec
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•Auditory neurophathy generally presents as
a behavioral hearing loss which varies in
degree, but may be profound. Word
recognition scores are poorer than would be
expected from the pure-tone audiogram.
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Acoustic reflexes are absent (ipsilaterally and contralaterally). MLDs are absent.
•Otoacoustic emissions are usually present, indicating normally functioning OHCs. The emissions do not suppress with
contralateral noise.
•The ABR is typically absent, which could be due to poor synchrony in firing of the radial fibers which synapse with the
IHCs.
This card and the next illustrate what might happen to normal neural firing patterns in the presence of demyelinating
conditions. Schwann cells are variable in length and are occasionally absent. This causes changes in velocity of neural
impulses — producing a lack of synchrony and, consequently, no observable ABR.
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