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28 Cards in this Set
- Front
- Back
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What is the general function of the cochlea?
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The general function of the cochlea is to translate the mechanical vibrations of the stapes into neural responses in the auditory branch of the 8th nerve. This process is greatly dependant upon the properties of the BASILAR MEMBRANE (BM)
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What did Bekesy discover about the Cochlea?
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Bekesy used models and actual cochleae. He found vibratory responses of the B.M. was not changed when: – Elasticity of round window was changed
– Length of cochlear canal changed – Altered position of the stapes – Changed the nature of the fluid |
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What do the changes Bekesy made without changing the vibratory nature of the B.M. imply?
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implies the characteristics of the B.M. are of utmost importance.
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Describe the manner in which the motion of the stapes
leads to displacement of the B.M. |
1-Stapes pushes inward on compression
2-Pressure gradient develops on two sides of the cochlear partition 3-Fluids are incompressible 4- B.M. deflected towards Scala Tympani 5-Displaced fluid forces round window out 6-As stapes moves out- each of the above motions are reversed 7-Consequently, vibratory action of stapes is COUPLED to the basilar membrane |
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How does the helicotrema act and what is its function?
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Cochlear mechanics are such that the helicotrema acts as though it were CLOSED. The helicotrema functions as a static balance for perilymph
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Compare the width and tension on the basilar membrane and how does this impact maximum vibration amplitude?
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B.M. is wider at the apex AND more flaccid at the apex. Because of this fact, different stimulus frequencies will cause maximum vibration amplitude at different points along the B.M.
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What is the response of the BM?
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The response of the B.M. is a traveling wave
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What happens If points of maximum displacement of successive temporal sequences of instantaneous waves forms (traveling waves) are plotted...?
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The result is an ENVELOPE
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How is the BM attached and how does this impact the way it bends when displaced?
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B.M. is attached at its two edges (the O.S.L. on one side and the spiral ligament on the other side).
Therefore, it is displaced or bent In transverse or radial fashion As well as in a longitudinal fashion |
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What direction does the traveling wave travel?
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The traveling wave always travels from the base to the apex
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What is the TRAVELING WAVE PARADOX
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In most natural situations where traveling waves are found, the direction of the traveling wave is away from the driving force
NOT SO IN THE COCHLEA No matter where Bekesy placed the stapes into the cochlear partition, the wave ALWAYS TRAVELED FROM THE BASE TO THE APEX. |
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Describe the slope of the envelope
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The slope of the envelope is less from the point of maximum displacement to the base of the cochlear duct
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Describe how the point of maximum displacement is at different points on the B.M. for different frequencies.
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– Lower frequencies toward the apex – Higher frequencies toward the base
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If it is not the instantaneous maximum that is of ultimate importance, what is important?
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the maximum point of displacement of the envelope
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What Two principles account for the shearing mechanism associated with the cilia at the apexes of the outer hair cells?
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1 – Changes in radii during compression and rarefaction
2 – Off set pivot points between the tectorial membrane and the basilar membrane |
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What is the cochlear microphonic?
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The CM is an alternating current electrical response to an acoustic stimulus and represents quite accurately the amplitude, frequency, and timing characteristics of the acoustic input stimulus. In other words it faithfully represents the stimulus waveform.
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Explain what are receptor potentials? Then name all 4
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electrical signals which can be monitored by electrodes; cochlear microphonic, summating potential, resting potential and action potential
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How can the cochlear microphonic be measured?
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The CM can be measured along the entire basilar membrane for low frequencies, but can be measured only at the basal end for high frequencies. This is due to the fact that low frequencies activate the entire basilar membrane, whereas high frequencies activate only the basilar membrane at the basal end.
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What is the link between the hair cells and the cochlear microphonic?
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The CM is probably due to radial bending or shearing of the stereocilia of cochlear hair cells.
The CM does not cease to occur upon severing of the VIII nerve. However, if cochlear hair cells are destroyed, the CM is no longer present. |
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What is the summating potential?
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Deviation or shift in the DC baseline. It is not an A/C response to an acoustic input, but rather a shift in the baseline of the cochlear microphonic (CM)), usually in a negative direction. may be either in the positive direction, or in the negative direction depending on the intensity and the frequency of the input signal.
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What is the link between the Summating Potential and the Envelope of a wave?
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Summating potential is a DC electrical response which follows the envelope pattern of the acoustic wave rather than the wave form pattern.
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Which is more sensitive cochlear microphonic or summating potential?
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It takes about 20-30 dB more intensity to measure the summating potential (SP) than is required to measure the cochlear microphonic (CM). Thus, the sensitivity of the cochlear microphonic (CM)is greater than the sensitivity of the summating potential (SP)
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What causes Summating Potential?
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The SP is possibly due to longitudinal bending or shearing of primarily inner hair cells in the direction of the length of the cochlear partition, as opposed to radial bending or shearing forces.
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What is resting potential?
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This is the normal D/C charge of an area or structure under non- stimulation conditions.
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What are the 2 types of resting potential?
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1) Intracelluar polarization. This is a negative 80 millivoltelectrical charge or potential present in most body cells and tissues, including the hair cells and cortilymph of the organ of Corti.
2) Positive polarization or endolymphatic potential of plus 80millivolts which is unique to the endolymph of the scala media. It is thought that the stria vascularis is responsible for the positive resting potential of endolymph since this positive potential is non-existent in the endolymph of the vestibular system. |
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What is the action potential?
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The cochlear action potential is usually considered as the discharge pattern, or the over-all action pattern, of the entire cochlear portion of the VIII nerve.
The action potential (AP) in a nerve, including the cochlear branch of the VIII nerve, is a short sudden change in charge which moves at a constant amplitude along the length of the nerve at great speed. |
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What law does the action potential obey?
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The AP obeys the all-or-none law in that nerve fibers either fire at their maximum limit, or they do not fire at al. VIII nerve fibers also demonstrate absolute and relative refractory periods.
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What is a spike?
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When single unit action potentials are measured, the response from the nerve fiber is referred to as a spike.
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