Transduction and Cochlear Potentials

Purdue SLHS 302 Section 2, Lab 9

endocochlear potential (EP)

+80 mV, positive potential in the endolymph of the scala media

inner hair cell (IHC) receptor potential @rest

-40 mV, approximate resting potential inside IHC

Outer hair cell (OHC) receptor potential @ rest

-60 mV, approximate resting potential inside OHC

Hair cell resting potential (general)

about -60mV to -70mV, negative electrocal potential inside the cell

auditory nerve phase locking

4-5 kHz, approximate upper limit in auditory nerve fibers

auditory cortex phase locking

below 50kHz, much lower limit than in auditory nerve

The difference between in endolymph and the inside of the hair cells is ______________. The creates a driving force for ____________.

150mV, potassium (k+) to enter the hair cell when it’s transduction channels

Stria vascularis

produces endolymph, maintains endocochlear potential, supplies oxygen needed for cochlea’s basic metabolic control, and is essential for normal cochlear function. (Loss of blood supply can cause loss of cochlear function)

Direct Current (DC)

a baseline or sustained electrical potential that does not alternate with every sound vibration.

EX: summating potential

Alternating current (AC)

an electrical potential that varies with vibrating tissue during acoustic stimulation

EX: cochlear microphonic

5 electrical potentials

1. Endocochlear potentials

2. Hair cell receptor potential

3. Cochlear microphonic

4. Summating potential

5. Compound auditory action potential (NOT a true cochlear potential bc it reflects the neural output if the cochlea)

Endocochlear potential (EP)

postive resting potential of the endolymph in the scala media

Hair-cell receptor potential

electrical potential within a hair cell

cochlear microphonic (CM)

Alternating current responce primarily associated with mulitple outer hair cells

Summating potential (SP)

Stimulus related direct current responce produced by cochlear hair cell activity

compound auditory action potential

summed activity of many auditory nerve fibers firing nearly simutaneously

Hair cells convert

mechanical movement into electrical signals

How do hair cells convert mechanical movement into electrical signals?

Their stereosilia bend as the basilair membrane moves, changing the opening of the ion channels

Depolarization: potential becomes LESS negative

stereocilia bend toward the tallest row

Tip-length controlled ion channels open

K+ enters the cell

Neurotransmitter release increases

Afferent auditory nerve activity increases

hyperpolarization: potential becomes MORE negative

stereocilia bend away from the tallest row

ion channels close

the hair cell becomes more negative

neurotransmitter release decreases

afferet auditory nerve activity decreases

tip links

connect adjacent stereocilia and help control the opening and closing of mechanically senestive ion channels. When stereocilia defelct in the excitatory direction, channels open, and potassium enters.

Inner hair cells primary role

transduce sound into neural signals

Outer hair cells primary role

amplify cochlear vibrations

inner hair cells auditory nerve vibrations

most afferent auditory nerve fibers connect to IHC

outer hair cell auditory nerve connection

play an important role in cochlear amplification

inner hair cell stereocilia and tectorial membrane

not firmly attached to tectorial membrane

outer hair cell stereocilia and tectorial membrane

firmly attached to tectorial membrane

Inner hair cell key mechanism

release neurotransmitters to auditory nerve fibers

outer hair cell key mechanisms

change length in repsonse to stimulation

think of inner hair cells as

biological transducers

think of outer hair cells as

cochlear amplifiers

prestin

protien that allows OHC to rapidly change length (expanding and contracting in responce to sound)

Results of prestin

greater sensitivity to quiet sounds

sharper frequency selectivity

improved responses to small vibrations

what happens with outer hair cells are damaged?

reduces hearing sensitivity, broadens tuning curves, which results in poor frequency selectivity!

what happens when the inner hair cells are damaged

interfers with transmission of sound information to auditory nerve fibers, if IHC are not intact, auditory action potential cannot be sent normally

do human hair cells regenerate

NO

the sequence of auditory transduction

1. stapes pushes inward

2. cochlear fluid moves

3. stereocilia deflect

4. potassium enters and depolarizes the hair cell

5. neurotransmitter is released

6. auditory nerve action potential occurs

step 1: stapes pushes inward

the stapes moves at the oval window and causes movemnet within the cochlea

step 2: cochlear fluid moves

fluid movemnet displaces the basilar membrane

step 3: stereocilia deflect

in excitatory direction, stereocilia bend toward the tallest row opening transduction channels

step 4: potassium enters and depolarizes the hair cell

the cell becomes less negative

step 5: neurotransmitter is released

the inner hair cell releases the nuerotransmitter, primarly glutamate, across the synapse to the auditory nerve fiber

step 6: auditory nerve action potentials occurs

neural spikes carry information toward teh cochlear nucleus and the central auditory system

the complete auditory pathway

1. auditroy nerve: the vestibulocochlear nerve

2. cochlear nucleus

3. superior olivary complex

4. lateral lemnisus

5. inferior colliculus

6. medial geniculate body

7. auditory cortex

the auditory pathway…

carries information that contributes to the perception of frequencu (pitch) and intensity (loudness) and complex sounds such as speech and music

spontaneous rate

spontaneous activity is the firing of the auditory nerve fiber without acoustic stimulation

high spontaneous rate (SRH)

greater than 18 spikes/sec, low threshold, repsonds to quieter sounds

medium spontaneous rate (SRM)

0.5-18 spikes/sec, intermediate category

low spontaneous rate (SRL)

less than 0.5 spikes/sec, high threshold, require loud sounds in order to repsond

why do we have different thresholds?

helps the auditory nerve encode a wider range of sound levels

absolute refractory period

after an acion potential, another action potential cannot be generated for a breif period

relative refractory period

another action potential is possible, but it is less likely and harder to generate

neural threshold

the minimum sound level needed for a neuron to fire above its spontaneous discharge rate

rate-level-function

(input-outut function) describes how an auditory nerve fiber’s firing rate changes as sond intensity increases

dynamic range

the range of sound levels over which firing rates change appreciably

saturation

a point where further increases in sound intensity produce little or no increase in firing rate

rate coding

the nervous system uses changes in neural firing rate to represent changes in sound intensity

population coding

multiple neurons with different thresholds and response ranges work together to represent a wider range of intensities

why is population coding necessary?

an individual fiber has a limited dynamic range and eventually saturates. Additional fibers with different thresholds becom active as the souund grows louder, and excitation can spread across a wider region of teh basilar membrane

characteristic frequency

frequency at which a neuron has it’s lowest responce threshold

tuning curve

threshold as a function of stimulus frequency

isolevel/isointensity curve

neural response across frquencies at a fixed sound level

rate-level function

firing rate as a function of sound intensity

Q10

measure of tuning sharpness based on tuning-curve width 10dB above characteristic frequency threshold

narrow tuning

sharper frequency selectivity

repsonse most strongly within a narrower frequency range

larger Q10

broad tuning

poorer frequency selectivity

responds over a wider frequency range

smaller Q10

As stimulus intensity increases, tuning becomes

broader

auditory nerve fibers are organized according to

characteristic frequency (CF)

higher characteristic frequency means that auditory nerve fibers will be

toward the periphary of the auditory nerve bundle

lower characteristic frequency means that auditory nerve fibers will be

toward teh center of the auditory nerve bundle

high frequencies are asscoaited with the _______ of the basialr membrane

base

low frequencies are ssociated with the _________ of the basilar membrane

apex

TWO main ways the auditory system encodes frequency

place coding and temporal coding

place code

frequency is represented by the timing or periodicity of neural discharges

information conveyed by where MAXIMAL neural activity occurs

temporal code

frequency is represented by the timing or periodcity of neural discharges

phase locking

a neuron tends to fire at a particular phase of a sound wave

TIMING of one neurons spikes relative to a sound wave

volley principle

multiple fibers contribute spikes across cycles, collectively perserving timing information

timing INFORMATION represented collectiveky by multiple neurons

interval code

the time interval between spikes carry information about sound frequency

information converyed by the intervals BETWEEN spikes

frequency

a place of maximal excitation and at suitable frequencies, temporal pattern of firing

intensity

firing rate abnd the number/distribution of active neurons

histograms

show the auditory nerve responces to repeated presentations of stimulus

post-stimulus histogram

number of neural spikes at each moment in time relative to stimulus onset

period histogram

distribution of spikes relative to the phase of a repeating sound wave

onset response

increased neural firing near the beginning of a stimulus

adaptation

reducing in firing during sustained stimulation

phase locking limit

highest frequency at which neural firing can relaibly folow a particular stimulus phase

PSTH

x-axis=time

y-axis= number of responces or spikes

the stimulus is presented repeatedly to observe neurons repsonce pattern

a period histogram provides information about…

the relationship between spike timing and the sound wave’s cycle