3400-Test 4-CM 20-Phys.Cochlea

What should we picture the Cochlea as?

a Mechanical System on one side and a Neurological System on the other

The basilar membrane can "decompose" a sound into what?

a complex sound into its component frequencies.

The frequency-dependent maximum of membrane displacement is an indication that the cochlea performs what type of analysis?

a mechanical frequency analysis.(BM is "tuned" to react to certain frequencies at certain points)

As a longitudinal wave travels through the perilymph, what does it create?

a periodic pressure pattern which changes both in time and space, across the cochlear partition (scala media)....to the scala tympani, and finally to the round window, which in turn vibrates.

What does the inward motion of the stapes generate?

a pressure gradient on either side of the cochlear partition (scala media).

Because the membranes are hinged at different points, equal vertical displacement of the tectorial and basilar membranes results in what?

a shearing force, the magnitude of which is considerably greater than the vertical force that produces the up-and- down movements.

The wave in the cochlea oscillates at the frequency of stimulation but it's NOT a what?

a sinusoidal wave.

What is the response of the Basilar Membrane to sound?

a traveling wave

Bekesey noted that the motion of the basilar membrane was in the form of what?

a traveling wave,like the one that occurs when you flick a rope.

8th nerve fibers demonstrate what kind of refractory periods?

absolute and relative refractory

what did Bekesey use in the late 1950's-1970's for his research of the traveling wave?

actual cochleas and models

The Action Potential obeys what?

all-or-none law in that nerve fibers either fire at maximum limit, or they do not fire at all.

What does the traveling wave represent?

an energy exchange between the cochlear partition (scala media) and the surrounding perilymphatics

If points of maximum displacement of successive temporal sequences of instantaneous waves forms (traveling waves) are plotted, what is the result?

an envelope

The Basilar membrane is wider and more flaccid at the base or apex?

apex

Where is the PEAK of the envelope?

at the place where the traveling wave is BIGGEST.

The vibratory motion of the stapes is coupled to what?

basilar membrane.

What is the Envelope of the traveling wave?

curve that shows the amplitude of the traveling wave at each point along the basilar membrane

What is place theory?

each point along the basilar membrane develops a maximum of displacement that is associated with a specific frequency of excitation.

The point of maximum displacement is at different points on the B.M. for different...?

frequencies.

How does the Traveling Wave always travel?

from the base to the apex.

The CM can be measured only at the basal end for what?

high frequencies.

How does the stapes push in compression?

inward

The displacement patterns of the basilar membrane are due to its what?

its physical characteristics (Low freqs=Base, High freqs=Apex)

What is the helicotrema's true function?

keeps static balance in perilymphatic space of cochlea. Keeps pressure of perilymph in Scala Vestibuli and Scala Tympani EQUAL, as if they weren't being stimulated.

The shearing force of the BM causes the sterocilia to bend in what way?

laterally

Bekesy's curves showed that it doesn't really matter where the source of excitation is..?

located along the cochlea

The CM can be measured along the entire basilar membrane for what?

low frequencies

Since the B.M. is wider and flaccid at the apex what will the frequencies be like?

lower

Due to the shearing force of the BM, MINUTE force on the BM is transformed into a SHEARING force many times what?

many times greater.

Lower Frequencies =

maximum displacement toward the apex

Higher Frequencies =

maximum displacement toward the base

The length of time it takes for the displacement wave to propagate along the membrane can be calculated by doing what?

measuring phase relationship between stapes vibration & basilar membrane vibration at various locations for various frequencies of excitation.

What is Intracellular Polarization?

negative 80 millivoltelectrical charge or potential present in most body cells and tissues including the hair cells and cortilymph of organ of Corti

All of these characteristics of a traveling wave depend on what?

on the change in stiffness along the length of the basilar membrane.

The fluids are incompressible, so the displaced fluid forces the round window...?

outward

What opposite sides does this envelope have?

positive and negative side

What directions would the summating potential be in?

positive or negative depending on the intensity and the frequency of the input signal.

As a wave travels down the basilar membrane, it creates raised and lowered bulges towards where?

raised bulges toward the scala vestibuli. and downward bulges towards the scala tympani

What do the mechanical properties of the cochlea make it do in regards to its response to freqs.?

respond at different positions to different frequencies.

What happens to the stapes in rarefaction?

stapes moves away (out) of the oval window (motions are reversed)

lower frequencies displace larger segments of the BM which in turn explains why low frequencies can do what?

tend to mask high frequencies.

The slope of the envelope is LESS from the point of maximum displacement to where?

the base of the cochlear duct.

What do the changes Bekesy made WITHOUT changing the vibratory nature of the B.M. imply?

the characteristics of the B.M. are of utmost importance.

Tthe form of BM wave displacement is due to?

the dimensions and physical properties of the cochlear partition.(basilar membrane)

The motion of the stapes leads to what?

the displacement of the Basilar Membrane Remember: Compression/Rarefaction

It is not the instantaneous stimulus maximum that is of most importance, but the maximum point of displacement of the...?

the entire envelope.

What is the single most important property in BM wave displacement?

the gradual change in stiffness from the base to apex.

What motion affects the hair cells?

the horizontal and radial shearing motion of the BM

What is the Resting Potential?

the normal D/C charge of an area or structure under non-stimulation conditions.

What is another cause for shearing forces?

the pivot points for the tectorial and basilar membranes.

Which side of the Envelope do we generally talk about the most? Why?

the positive side, because that's where the stereocilia get pushed over in the right direction to get a neural response.

The process of the cochlea is greatly dependent upon what?

the properties of the BASILAR MEMBRANE.

What is the CM probably due to what action?

the radial bending or shearing of the stereocilia of cochlear hair cells.

As the BM is deformed across its WIDTH and bent towards the scala vestibuli and scala tympani, what happens?

the radius changes and hairs on BM have a sideways shearing force and are stimulated by the tectorial membrane.

Each time the stapes pushes in, where does the basilar membrane push towards?

the scala typmani; each time stapes moves out, the basilar membrane pulls away from the scala tympani.

What is the the TRAVELING WAVE PARADOX.

the wave always traveled from the base to the apex regardless of where the driving force is.

The CM is an alternating (AC) electrical response, the waveform of which is determined by what?

the waveform of the acoustic input

What else did Bekesy's curve show re: velocity and wavelength in relation to the stapes?

they both DECREASE with distance from the stapes

What is the general function of the cochlea?

to translate the mechanical vibrations of the stapes into neural responses in the auditory branch of the 8th nerve.

The Basilar Membrane is displaced or bent in what kind of fashion?

transverse, radial and longitudinal

What is the Summating Potential?

-A DC shift in the baseline of the cochlear microphonic in a negative direction -DC electrical response follows envelope pattern of acoustic wave rather than wave form pattern.

What principles account for the shearing mechanism associated with the cilia at the apexes of the OUTER hair cells?

-Changes in radii during compression and rarefaction -Off set pivot points between the tectorial membrane and the basilar membrane.

Bekesey found that vibratory models were not changed when what occurred?

-Elasticity of round window was changed -Length of cochlear canal changed -Altered position of the stapes -Changed the nature of the fluid

What do the Bekesy curves demonstrate that simple resonance CAN'T account for?

-basilar membrane displacement-(wave displacement is due to the nature of the BM itself)

Why is the stria vascularis thought to be responsible for the positive resting potential of endolymph?

-because this positive potential is non-existent in endolymph of vestibular system

The cochlear Action Potential is considered to be what type of pattern?

-discharge pattern, or action pattern of the entire cochlear portion of the 8th nerve -AP is a short, sudden change in charge moving at constant amplitude and rapidly along length of nerve

What is Positive Polarization?

-endolymphatic potential of plus 80millivolts which is unique to endolymph of scala media.

Any membrane or plate which is bent into a curve has produced on it two types of what?

-inside and an outside radius, -between which a large shearing force is generated.

What is Cochlear Microphonic?

-microphonic= serving to amplify sound. -cochlear microphonic= any of the electrical potentials generated in the hair cells of the organ of Corti in response to acoustic stimulation- -an electrical response to auditory stimulus - represents amplitude, frequency, timing characteristics of acoustic input stimulus. -example: hair cells in cochlea act as a transducer like an electroacoustic microphone

What is a spike?

-occur in a irregular pattern when electro-chemical status of neuron is suitable for "firing". (When timed stimulus like click is present, possible to observe spikes occurring at regular intervals corresponding to timing pattern of the click.)

At all frequencies where does an undulating wave begin?

-on basilar membrane nearest stapes, -increases in amplitude somewhere along the partition to reach a maximum - and beyond that max. point the displacement quickly reaches zero.

What is the most distinctive feature of the traveling wave?

-spatial relationship between the maximum and the excitatory frequency-in other words: - the locus (location) of maximum disturbance on basilar membrane is FREQUENCY dependent.

Since the inside radius is smaller than the outside radius, what must therefore happen?

-the substance between them must slide LATERALLY -This is one mechanism by which shearing forces can be produced.

What are the 4 Electrical Potentials for the Ear?

1. Cochlear Microphonic 2. Summating Potential 3. Resting Potential 4. Action Potential

How much intensity does it take to measure the summating potential?

20-30 dB MORE than is required to measure the cochlear microphonic (CM)

No matter where Bekesy placed the stapes into the cochlear partition, how did the wave travel?

ALWAYS TRAVELED FROM THE BASE TO THE APEX. It doesn't matter where the driving force is.

LOW frequencies have their peak amplitude at the BASE or APEX of the basilar membrane?

APEX (more apical postions)

In most natural situations where traveling waves are found what is the direction of the wave?

AWAY from the driving force (think of flicking a rope or a garden hose into a wave away from you) -NOT so in the cochlea

What is the first characteristic of a traveling wave?

Always starts at base of cochlea, moves toward apex.

HIGH frequencies have their peak amplitude at the BASE or APEX of the basilar membrane?

BASE

Different stimulus frequencies will cause a maximum vibration amplitude at different points along the what?

Basilar Membrane

Which has greater sensitivity- the CM or the SP?

Cochlear Microphonic (CM)

What are the 2 types of resting potential?

Intracellular polarization and Positive Polarization

What is the second characteristic of a traveling wave?

It's amplitude changes as it traverses length of cochlea.

The Summating Potential is due to what action of the hair cells?

LONGITUDINAL bending or shearing of primarily inner hair cells in direction of length of cochlear partition, as opposed to radial bending or shearing forces.

The CM can be measured along the ENTIRE BM for ______frequencies, but at the BASAL end only for _____frequencies.

LOW, HIGH

What is intiated when the cilia scrape back and forth across the tectorial membrane during a wave ?

NEURAL response because the pivot points of the Basilar Membrane and the Tectorial Membrane will be offset from each other

If the cochlear hair cells are destroyed, will the CM still be present?

NO

Do the sides of the membrane rise and fall as well?

NO, are attached and do not rise and fall with the rest of the BM

Is the helicotrema at the apex of the cochlea an OPEN or CLOSED space?

OPEN, but due to cochlear mechanics it ACTS as if it were closed- which is why it is NOT and alternative route for the pressures to transfer between the Scala V and Scala T

What are the 2 points of attachment for the basilar membrane?

Osseous Spiral Lamina, Spiral Ligament

What is the third characteristic of a traveling wave?

Position along basilar membrane where its amplitude is highest depends on frequency of stimulus.

What does the pressure gradient cause?

Reissner's Membrane and the Basilar Membrane to bend towards Scala Tympani.

The pressure gradient on either side of the cochlea is greater in the Scala Vestibuli than in the?

Scala Media

When single unit action potentials are measured, response from the nerve fiber is referred to as a?

Spike

If the VIII nerve is severed what will happen?

The Cochlear Microphonic will STILL occur.