Unit 2 - Lecture 2 Flashcards
SGN send dendrites to the OC and the axons to the lower brainstem through the ____
internal auditory meatus
To form the auditory nerve bundle, low-f ANFs are ____, high-f ones are ____ located
inside, peripherally
SGN axons are sent through the center of the ____
modiolus
Two types of auditory afferent neurons
Type I Auditory Neurons
- Correspond to inner radial fibers connected to IHCs
- Bipolar
- Myelinated
Type II Auditory Neurons
- Correspond to outer spiral fibers connected to OHCs
- Pseudomonopolar
- Unmyelinated
Type I SGNs to IHCs
- Radial fibers
- Convergent innervation: >10 SGNs to one IHC, each SGN one synapse with one IHC,
Type II SGNs to OHCs
- Outer spiral fibers
- Divergent: one SGN to > 10 OHCs, each SGN synapse with many OHCs
Ribbon synapses specific in ____ and ____
cochlea, retina
On the pillar side there is a really ____ and a ____
big terminal, small synapse
On the modiolar side is a ____ and a ____
small terminal, large synapse
Function of ribbon synapse
high speed of NT releases - temporal coding, long-lasting release for continuous sound
Where are ribbon synapses located?
photo-receptor cells in retina, hair cells in vestibular organ and cochlea
How are ribbon synapses different from conventional synapses? (2)
Different in anatomy and function
Ribbons in retina cells shape like ____, those in IHCs shape like ____
horseshoe, American football
____ is critical for long lasting response
Recycling
____ holds vesicles containing NT closer to the zone to be released to ensure quick response
Ribbon synapses
Presynaptic Molecules (A, B, Bassoon)
Ribeye A: ribbon frame
Ribeye B: active components for holding vesicles
Bassoon: anchoring ribbon to active zoon
Special proteins for cochlear ribbon synapses:
- CaV1.3, specific L-type Ca2+ channel
- Otoferlin and adaptor protein
- Piccolino: a short version for Piccolo
- Several proteins common for conventional synapses are missing from ribbon synapses
____ and ____ are not seen in retina
Otoferlin, piccolino
What are the special mechanisms for NT?
the process of vesicle trafficking/replenishment, tethering, docking and fusion, and probably recycling (via endocytosis).
Neurotransmitter release is facilitated by (4)
- Large “Ready to release pool (RRP)” of vesicles hold by ribbons
- The number and distribution of CaV1.3 channels
- Special Mechanisms for exocytosis (related to otoferlin)
- Special mechanisms for vesicles replenishment and endocytosis (neurotransmitter recycle)
What is the neurotransmitter for IHC-SGN synapses?
Glutamate
Why is glutamate the main NT in IHC-SGN synapses?
- Glutamate is an amino acid existing every cell, rich in vesicle (criterion i)
- Glu can be released from synapse, agonism can activate action potentials in AN (ii).
- Action potential can be blocked by special blocker against AMPAR (criterion iii)
- It is not fully understood how the released glu can be removed (criterion iv): (1) by glial cell, and (2) by endocytosis
Bassoon Mutant mice
- In this mutation, <3% IHC-SGN synapses retained anchored ribbons
- AN has normal threshold, dynamic range, post-onset adaptation to tone bursts, phase lock
- Rate decrease (driven and spontaneous), increased variance of first-spike latencies
- In this mutation, less than 3% of IHC retain anchored ribbons (they mutated the bassoon and the ribbon also disappeared)
What 2 things does the bassoon do?
- Bassoon makes the synapse quickly respond to stimulation
- Bassoon holds the ribbon in place
What does delayed and reduced onset response in mutated mice suggest?
The role of ribbon in quick response
What is the order of signal transient?
click > pip > tone
What to code in signal
- Frequency
- Intensity
- Temporal pattern
How to code by neurons
- Rate change
- Place code
- Temporal coding (phase locking)
Tuning Curve
-what does it represent
-what are the 2 parameters
- Frequency Selectivity represented in tuning curve
- Concept of response area (any area above the running curve is the response area)
- Two parameters; intensity and frequency
What side is a TC sharper on?
High frequency side
Why tuning curve is sharper at high frequency side?
- When the BM vibration peak move slightly towards basal turn, the vibration at CF location drop quickly, because the envelope is sharp at low frequency side.
- Therefore, input signal must be much stronger to excite the fibers at this CF location up to their threshold
Traveling Wave Envelope Asymmetry and TC shape
- when is the threshold reached?
- where is the traveling wave sharpest?
- what do you need to do to see vibration?
- The threshold is reached if a certain amount of vibration is reached at CF location
- Traveling wave is sharp at low-f side of the peak
- Need to increase much larger intensity in order for the vibration to be seen.
Shifting of fre away from CF, vibration at CF will be ____ than threshold
lower
To reach the threshold at CF, sound level must be ____.
increased
Frequency selectivity
- what does each AN work as?
- what intensity is best?
- Each auditory nerve works as a bandpass filter
- Better selectivity at low intensity
- TC spreads to low frequency side as a tail at high intensity
- CF tip needs active mechanism of OHCs
What is the Q value formula?
Q10 dB = CF/bandwidth (BW) of TC
CF/freqeuncy range = Q value
What is the Q value telling us?
Standardized measurement to measure frequency selectivity
What is a bandpass filter?
Can only respond to frequencies in a given range
The more ____ the range, the more frequency selective the range is
narrow
TC becomes ____ at high intensity
wider
____ is better because that is how the cochlea is arranged along the BM
Logarithmic scale
How is frequency selectivity measured?
Quantitatively measured as Q value: e.g.
- Q10 dB = CF/bandwidth (BW) of TC
- The higher the Q value, the better the frequency selectivity
The cochlea is organized in ____
logarithms
____ is which frequency point you do the measurement
CF
Tuning curves with the same CF can have different ____
Bandwidth
Larger the ____ better the frequency selectivity
Q value
The range is more narrow showing that the range is more selective to certain frequencies
If bandwidth of TCs is measured in octave, it will decreased with ____.
CF
Q values are larger along ____
high frequencies
Cochlea is mapped by frequency in ____
Logarithm
Explain the cochlea and a linear scale (and why not to use one)
- what will it show
- where does JDD cover a shorter distance?
- how is frequency selectivity better measured?
- Bandwidth increases with CF in linear scale. This does not indicate poor frequency selectivity at higher frequencies (Because a linear scale will show that there is a wider bandwidth for high frequencies, which isn’t right)
- Just detectable difference (JDD) for frequency (in octave) covers a shorter distance at high frequency region.
- Frequency selectivity better measured with Q10, which is bigger at high frequency.
____ change along the cochlea
Q10
Idea about using Q.
- BW is inversely related to frequency selectivity.
- CF should also be considered.
- Q value is a ratio, putting CF into the consideration of frequency selectivity (similar to Weber’s fraction).
- Larger the Q, better the frequency selectivity
What is the absolute threshold similar too?
behavioural threshold (from audiogram)
Thresholds and distribution
- Most fibers have thresholds within 20-40 dB above the absolute threshold
- Spread to 60-80 dB above abs threshold only for less than 10% of fibers.
- Nerve fibers with high thresholds often have low spontaneous spike rate and often receive efferent inhibition on their terminals with IHCs
Most of our fibers are ____ SR fibers
high
Fibers with LSR are distributed at a ____ rate (but there is less of them)
wider
AN classification and Ribbon Synapses (low & high SR fibers, difference between the two)
- Low SR fibers innervating modiolar side of IHCs (large ribbon, small postsynaptic terminals)
- High SR fibers innervating lateral side of IHCs (small ribbon, large postsynaptic terminals)
- Functional differences: threshold, sensitivity to loud sound
On modiolar side: ____ ribbon, ____ terminal
Large ribbon, small terminal
On pillar/lateral side: ____ ribbon, ____ terminal
Small ribbon, large terminal, high SR, low threshold
ANFs synapse IHCs at modiolar side (explains 3 components of LSR fibers)
- low SR,
- high threshold
- larger dynamic range (encoding in noise)
- more sensitive to noise damage
What is dynamic range and how is it related to HSR fibers?
- The increase in intensity is our dynamic range
- Dynamic range = change in input results in change in output (typically around 20-30dB for high SR units)
The pattern of RLF changes with ____
Signal frequency
RLF is more linear away from the ____
CF
Saturated RLFs
- Seen from ANFs with high SR (HSR)
- It is called typical, because HSR ANFs are the majority (~90%)
- Those ANFs have low threshold, narrow dynamic range, most likely responsible for auditory sensitivity.
- They may not be able to code high level sound (not conclusive)
The impact of signal frequency vs CF on RLF of high-SR units (where is it and where is it not linear, where is the lower maximum)
- Nonlinear at CF (saturated)
- More linear at low/high F
- Lower maximum at high F
OHC active amplification is ____ and ____ limited
frequency, level
If you move lower or higher, you see the RLF become ____
linear
RLF is nonlinear at the CF because of the gain control of the ____
OHC
Higher SPL, RLF ____
Saturates
Lower SPL, there is ____
Larger gain (only for CF)
RLF comparison across SR groups
HSR: much more likely to saturate at high sound levels (even away from CF)
LSR: don’t saturate to begin with, so wont saturate as much at high sound level (more responsible for encoding high sound levels)
How can coding of temporal pattern be seen?
Using a bunch of different methods
1) PSTH (post/peri stimulus time histogram)
2) ISIH (interspike interval histogram)
3) PRH (period histogram)
Temporal pattern I: PSTH
- what is the name
- when is it measuring
- explain time bins
- what does the number of spikes in each time bin represent?
- Post (or peri) stimulus time histogram (PSTH)
- After the onset of the stimulus, count the number of spikes in each time bin
- Time bin- have equal time durations (i.e., 1-5 ms)
- The number of spikes in each bin represents the prevalence of neural firing with respect to the stimulus
- Add the response of hundreds of stimuli
PSTH 5 stages
- Onset peak
- Fast adaption
- Slow adaptation
- Offset depression
- Recovery
This is after the onset of the stimulis
Temporal processing: Concept of Phase locking
ANFs fire at certain phase of the sound. When the sound frequency is low, one ANF can fire to every cycle of the sound. With increasing frequency, firing will skip: not occuring in every cycle.
Neuron has a higher tendency to generate AP at a certain stimuli
____ is an important temporal feature of a neuron’s response
Phase locking
Phase locking is when the AP is in sync with the ____
Stimulus
Recording from one ANF usually need sweeps of ____ to show phase locking
Many times
Phase locking becomes more apparent with more sitmuli
Phase locking in Volley principle by Wever
- how many neurons do we look at?
- what does the sum of the AP line up with?
- what is the volley principle looking at?
- If we look at one neuron, we wont get a good picture of a stimulus, so add them together to get a better idea of what is happening
- The sum of the AP line up perfectly with the stimulus
- Volley principle = all neurons acting together
What is the response to low frequency tones? (what is the period)
Phase locking
The period is ~~2 ms re: 500 Hz
Phase locking is a temporal feature of ____ response
neuron
Interval between peaks in phase locking is the ____ of the sound
period
Temporal pattern II: ISIH
- name
- when are you measuring
- ISIH: Interspike interval histogram
- Time axis divided into equal time bins
- Does not refer to stimulus onset, but to intervals between spikes
- Due to phase locking, spike interval is not a random event, rather more chance appears at certain interval values
- Integer relationship among peaks
- Multiple presentations show trend
- If neuron is going too fast, it might not be able to keep up (works better at low frequencies)
Phase Locking is a method of ____
First peak is responding to ____
- fre coding
- First peak is responding to one period of sound
Explain the importance of phase locking at low frequencies
At low frequencies, there is more of a chance that phases locking will occur in every cycle (so the interspike interval will be one period)
Do not see this at the higher frequencies
Temporal pattern III: PRH
- what is the name
- what are you measuring
- what is it showing
- PRH—period histogram
- Time axis divided into equal time bins
- Only shows several periods or cycles.
- Spikes counted in each time bin for many presentations of stimulus
- Shows periodicity or phase locking of firing pattern
Use a two tone combination to see how a neuron codes a ____ signal
Complex
Phase locking to complex - two tones of low frequencies
Phase locking occur to
- First tone
- Second tone
- Both
Phase locking to complex - two tones of high frequencies (small diff)
Phase locking to beat/fundamental frequency
Phase locking to complex - complex signal
Phase locking occurs to temporal envelope
What is the response to complex tonal stimuli?
Phase locking to envelope
Explain phase locking to the envelope
- A pure tone is amplitude modulated (AM), resulting in amplitude fluctuation
- PSTH of the neurons follows the envelope of the sound.
What is the importance of the envelope?
- Envelope has slow temporal cues
- Important information carrier
- Encoding in cochlear implants
What does temporal coding require?
Requires neurons to quickly respond to the signal over time
First limiting site is synapse between ____ and ____
IHC, SGN
Phase locking and temporal pattern provide the bases for ____
Temporal coding
Phase locking follows the amplitude ____ of sound
Envelope
There is no place code in cochlea for ____ information
Temporal
Speech coding
- Peaks mostly at low levels, at high levels, they get lost
- When speech is presented at a low sound level (28 dB), the response against the AN shows 2 peaks
- Increasing sound level the 2 peaks disappear
Phase locking can improve ____ analysis across sound levels
Frequency
If high frequency signal is temporarily modulated it can also be encoded by ____
Phase locking
Place coding works for ____
Whole frequency range
Phase locking works for ____
Low frequency range
Frequency coding at high intensity and high fre region is not ____
Fully understood
What happens to the 8th nerve at high frequency? What doesn’t go down and why)
- Frequency selectivity of 8th N fiber poorer
- However, psychological fre. discrimination does not go down. Why? Phase locking may improve fre dis. at high level
Phase locking at low and high frequencies
At low f region, phase locking contributes (better at low freq)
At high f, phase locking doesn’t contribute (so we don’t know the mechanism for signal freq at high freq)
How dynamic range of hearing is established by AN?
- The dynamic range of human hearing is something around 120 dB
- The dynamic range of an auditory nerve fiber is typically 20-40 dB, few up to 60 dB.
- How is the gap filled? Different coding methods are used
What are the 7 methods that contribute to how the dynamic range is established by AN?
- Greater DR at onset
- Increase dynamic range by threshold distribution
- Sloping saturation
- Spread of excitation
- Two-tone suppression shifts rate-level function
- Effect of background noise
- Phase locking
- Greater DR at onset
- As intensity increases, the spike rate fully saturates
- With a smaller time window, there isn’t as much saturation for high sound levels (DR is larger)
- The onset response (as soon as sound happens) may be weighted more in our brain
Behaviour dynamic range is related to ____ (you can sense the loudness change)
intensity change
- Increase Dynamic Range by Threshold Distribution
- Some low SR (SR <18/sec) neurons have higher thresholds.
- They increase the distribution of thresholds
- However, the number of neurons in this group is small, how can they handle the job at high intensity
Distribution of idealized rate-intensity functions
This is showing that the DR makes the ANs have different working thresholds (not all starting at the same place, they are staggered which is showing a wider distribution of LSR)
Shows the covering of 0-120
- Sloping saturation
- Might not saturate as aggressively as we think
- Maybe the saturation is more gradual than we think, therefore, increasing DR (which is how the rate is changing with intensity)
- Spread of excitation
- BM vibration spreads to high fre areas at high intensities
- More 8th N fibers will be accumulated to code intensity
- Problems with this model:
- Can’t explain broad band noise
- Failed in exp using band rejected noise (if spreading works, response in notch noise should saturate quicker) - Even though the sound is saturated at a higher sound level, there is more neurons
When intensity of sound increases, the frequency range of which the neurons are excited is ____
larger (large sound excites more neurons)
Increasing sound level, more neurons are ____
recruited
- Two-tone suppression shifts rate-level function
- When we present second tone, the first tone is depressed if the second tone is at an appropriate level
- Pushes the RLF to the right
- There is suppression when B is played along with A
Suppression but not inhibition
- Adding a second tone may cause a decrease in the discharge rate of the first tone; as if tone b inhibited neural activity for tone a
- The discharge rate is reduced only if tone b is the right freq and intensity (inhibitory area)
- Occurs quickly—via mechanical interaction between two tones, no inhibitory circuit involves.
The reduction of the spike rate is due to the ____ interaction between the 2 tones
Mechanical (no latency)
If it was inhibition we would see a ____
Latency
Frequency range for two-tone suppression (response area)
- what is the response of two tone suppression
- Two tone suppression TC
- When you play the two tones, there will be a response area, but it will be smaller
- Result of 2 tone suppression is that it shifts RLF to the right = covers a wider DR
- Effect of background noise
- what does background noise do?
- what is it due to?
- how does this relate to LSR
- Background noise shifts the working range of ANFs (push it up)
- When background noise is present, RLF shifts to the right (higher level)
- The reason why it shifts to the right is due to the efferent system (takes longer to see the effect of background noise because it has to activate the efferent system)
- This result is an indication that low SR units take responsibility for the coding of signal at high levels against background noise
- 2 tone suppression does not involve anything from the brain, but the effect of background noise does so it takes longer to happen
Impact of background noise (low and high SR)
- Low SR ANFs: highly resistant to noise (in term of their response to signal)
- Low SR units are critical for signal coding in noise
- Low SR units are more sensitive to noise damage
- High SR units, saturated by background noise
How background noise change ANF function?
- By mechanical interaction like two-tone suppression
- By efferent control
- Phase locking
- increased phase locking with ____
- How we have increased phase locking with intensity
- Increasing sound level the strength of phase locking increases
Phase locking improves ____
threshold
Threshold of synchronization is lower than ____
Rate threshold
Phase locking threshold is lower than ____
rate threshold
Rate threshold vs. phase locking threshold
- Phase locking is way stronger with an increase in intensity
- Phase locking threshold is lower the the rate threshold
- See phase locking before rate increase
AN use all 7 mechanisms together to make the ____
120 dB DR
