AUD 6400 Physiology of the Cochlea & Sound Evoked Potentials

What are the two groups of sensory cells in the cochlea?

outer and inner hair cells

What do the outer hair cells do?

Act as motors to compensate or loss of energy in cochlea

What do the inner hair cells do?

convert vibrations of basilar membrane to neural code in individual nerve fibers of auditory nerve

What is frequency discrimination?

waves occur at different places on the basilar membrane in accordance to their frequency

What is the place hypothesis of frequency discrimination?

based on spectral analysis that occurs in cochlea, peak of traveling wave occurs at a different place on basilar membrane depending on frequency (considered to be tuned)

Where is the basilar membrane?

forms the floor of the scala media

What is the orientation of the basilar membrane?

Narrower and stiffer at the base and wider and looser at the apex

Where along the basilar membrane is most sensitive to low frequencies?

apex

Where along the basilar membrane is most sensitive to high frequencies?

base

What is the traveling wave?

displacement wave pattern that goes along the basilar membrane and activates hair cells as it goes

Where is the maximum area of displacement of the traveling wave?

at the frequency on the basilar membrane where it matches the acoustic stimuli

What are the research challenges of the traveling wave?

technology limitations due to the small amplitude of traveling wave and unable to perform research on live subjects

How does the basilar membrane move in response to various pitches?

vibration reaches peak amplitude at specific point depending on frequency of wave

What determines the distance a traveling wave moves along the basilar membrane?

The frequency and amplitude

Do lower or higher frequencies travel further along the basilar membrane before they diminish?

low frequencies

What is the role of the tectorial membrane for BM tuning?

tectorial membrane and stereocilia form a mass-stiffness resonator that is coupled with the BM for frequency selectivity along the cochlea

What is characteristic frequency(CF)?

frequency at which a specific neuron response to the softest intensity

What is a tuning curve?

graphical representation of a characteristic frequency (CF)

What is the role of outer hair cells in the BM?

Act as motors that compensate for the frictional loss of energy in the cochlea at low intensities, total loss of OHC reduces sound sensitivity by 50dB

What are the three mechanisms that contribute to cochlear frequency selectivity?

1. traveling wave motion of basilar membrane

2. active function of OHC that add energy to motion of basilar membrane

3. resonance of tectorial membrane and stereocilia

What is the excitation of hair cells?

how the hair cells bend change the intracellular potential of hair cells

What is the excitatory response of the hair cells?

deflections and bending of the hair cell stereocilia toward tallest stereocilia depolarizes cell

What is the inhibitory response of the hair cells?

deflections and bending in opposite direction toward smallest stereocilia hyperpolarizes cell

Is the compression phase an excitatory or inhibitory response?

Compression is inhibitory response that moves tympanic membrane inward and stapes moves inward which deflects basilar membrane toward the scala tympani

Is the rarefaction phase an excitatory or inhibitory response?

Rarefaction is an excitatory response that moves the tympanic membrane outward and pulls the stapes outward which deflects the basilar membrane toward the scala vestibuli

How common is congenital hearing loss?

every 1-3 per 1,000 births

What are connexins

group of proteins that use gap junctions to communicate, important for exchange of electrolytes, second messengers, and metabolites

What happens when connexin-26 is missing?

It disrupts fundamental processes in hair cells and may be involved in pathophysiology of some progressive adult deafness

Why do hair cells degenerate?

overstimulation or noise exposure, medications, aging, or idiopathic unexplainable reasons

Is regeneration of hair cells possible?

presently, stem cell research shows issues with regeneration because tonotopicity is unachievable, hair cells tend to migrate, and there is a lack of ability to connect with auditory nerve fibers

What is endocochlear potential?

potential difference between perilymphatic and endolymphatic spaces which is important for functioning and powering of hair cells

What is the role of stria vascularis?

Multilayered epithelium is part of barrier around endolymphatic space which is important in maintaining the endolymphatic potential and normal ionic composition

What is a result of abnormal endolymphatic volume?

Suspected Meneire’s Disease or sudden deafness

What are otoacoustic emissions?

low level sounds emitted by the cochlea related to the function of the cochlea’s OHC

Are otoacoustic emissions spontaneous or evoked?

Can be spontaneous or evoked, but evoked is more widely used in clinical settings

Is OAE testing subjective or objective?

Objective test which provides ear-specific information without behavioral responses

When are OAEs used?

infant screening (not the best measure), pediatric assessment, cochlear function monitoring (ototoxicity), differential diagnostics, malingering cases

What are the types of OAEs?

spontaneous and distortion product OAEs

What are spontaneous OAEs?

50-70% of all normal hearing ears will show results in spontaneous OAEs, absent in all ears with SNHL of over 30dB

What are distortion product OAEs?

two pure tone stimuli extremely close in frequency are presented simultaneously, they cause two distinct waves on basilar membrane which result in distortion product

What is normative data for DPOAEs?

HFH: DPOAE level >= -6dB AND signal-to-noise ratio level >= 6dB

How do you counsel based on OAE results?

you can rule out everything except mild hearing loss or auditory neuropathy

Where in the auditory system do OAEs target and test?

The cochlea

What are evoked potentials in human studies?

potentials recorded from electrodes placed on cochlear capsule or in ear canal near TM, electrocohleographic (ECoG)

What is near-field recording of ECoG?

electrode placed near source of evoked potential (tiptrodes)

What is far-field recording for evoked potentials?

electrode is placed farther from source of evoked potential (through TM), stronger response

What is “potential”?

electric tension; consequence of temporary change in membrane potential of a nerve or muscle upon excitation

What are the three kinds of sound-evoked potentials?

1. cochlear microphonic (CM)

2. Summating potential (SP)

3. Action potential (AP)

What is cochlear microphonic (CM)?

minute alternating-current electrical potential of hair cells, resembles input signal, same sound in same sound out, “microphone”

What is summating potential (SP)?

change in direct-current in cochlea as a result of acoustic stimulation, measured with ECoG

What is action potential (AP)?

compound action potential guaranteed by electrical stimulation of cochlea (extracochlear promontory or cochlear implant)

What is the presumed CM generator?

hair cell stereocilia

What is the presumed SP generator?

Hair cell body

What is the presumed AP generator?

distal portion of auditory nerve

How is the CM recorded?

it is the sum of potentials generated from large amount of OHCs in basal cochlea

What is the primary diagnostic use of ECoG?

Auditory neuropathy, monitoring during surgery, inner ear hydrops

When does SP occur?

At the same time as CM, looking to remove CM to assess difference between SP and AP

How do you remove CM from the ECoG?

Alternating polarity

What is action potential latency dependent on?

spectrum and intensity of stimulus because basilar membrane will show max deflection at the wave’s frequency (high frequency → max deflection of BM near base)

What is the stimulus of ECoG?

click: a broadband sound with abrupt onset and short duration, can evoke responses from many nerve fibers (BBN= spectrum of frequencies)

Why would it be negative to have a malleable nervous system?

neural plasticity sometimes overachieves and results in things like misophonia or hyperacusis