Auditory Nerve l Flashcards
What are the three characteristics that are encoded by ANF?
– Intensity
– Frequency
– Temporal pattern
How are the three characteristics encoded by the ANF? (3)
– Rate change (rate code)
– Place code
– Temporal code (phase locking
How important is the action potential waveform encoding?
AP waveform not important since there isn’t a difference (its either generated or not)
How is information encoded by an AP of a single neuron?
Single neuron can change the time rate and firing (rate coding)
How is the information encoded by AP of groups of neurons?
By produces an array of firing, spatial issues, frequency and timing
What is the refractory time of nerve fibers?
2ms after each AP
What is the maximal firing rate of actin potentials?
500 AP per second
How does the firing rate of neurons relate to Rutherfords frequency theory
Believed that the firing rate variation is what encodes BUT not true since they all have the refractory after 2ms so even increasing the speed that doesn’t match; can’t go beyond that number
What are spontaneous APs?
Action potentials that are generated without an external signal (no stimulus)
What are characteristics of spontaneous APs? (3)
- The spontaneous APs are random in timing.
- External signal may be applied.
- However, if APs occur randomly, the neuron does not “hear” the sound. Or the sound level is below the threshold of the neuron
What does increasing the intensity lead to?
Increasing the intensity can see the action potential where firing rate increases
How can we verify a neuron’s response to sound? (3)
– Compare the spike rates between the two time-windows of equal length
– One window cover the period of external signal and the other contains no sound.
– If the AP rate in the sounded-window is significantly higher than the quiet window (by certain criterion), the we judge that the neuron is driven by the sound.
What kind of coding is this?
Rate coding (single and group neurons can do it)
How are fibers grouped in relation to the spontaneous rate of firing?
High Spontaneous Rate, Medium SR and Low SR (lower the SR, higher the threshold)
What is the functional importance of fibers with high SR in the graph? (circles)
It is inversely related to the behavioral threshold and may work for sound coding in different sound levels
What does it mean to have a high vs low threshold in terms of response speed?
Lower threshold = Quicker Response
Higher Threshold = Slowed Response
What is an example of a behavioral threshold?
Our audiology hearing test
What is the relationship between each of the SR groups to the threshold?
higher the SR, lower the threshold
Explain the location and SR likely hood of SGN synapses around each IHC. (Fiber A vs. Fiber B)
SGNs synapse around
each IHC.
Fiber A: at modiolar side:
likely to have low SR,
Fiber B: at pillar (OHC)
side likely to have high-
SR
For the IHC, which side of the fiber is likely to have a low SR? Which side is likely to have a high SR?
Modiolor have low SR;
Pillar (OHC side) have a higher SR
What are the ribbons and terminals of the synapse like on each of the modiolor and piller sides for the auditory nerve fiber?
The medial low SR have larger ribbons and smaller terminals
ANFs synapsing around IHC
show different SRs, Ribbons and terminal sizes from medial to lateral synapses: (2)
Medially synapsed: low SR ，
large ribbons, small terminals
Laterally synapsed: high SR,
small ribbons, large terminals
Fill in the chart:
What is the rate threshold?
The sound level at which the firing rate is just above the spontaneous rate and when we can assume the firing is due to this stimulus
What is a dynamic range?
The intensity range in which increasing the intensity increases the spike rate.
What does the rate level function show?
How the spike rate changes with the intensity
What is the change to indicate a dynamic response?
If the SPL change causes a spike rate change then there is a dynamic response
Describe the graph shape of the high vs low SR:
High SR gets saturated and the low SR is more linear (which also means more dynamic range)
What is the dynamic range of High -SR ANFs?
30-40 dB: if the threshold is 20 dB, thefiring rate increases until 60 dB, the saturated
Give 3 characteristics of the behavior for high-SR ANFs. (3)
Low threshold,
Narrower dynamic range
Plateau at moderate sound level
Give 3 characteristics of the behavior for Low-SR ANFs. (3)
- Wider than high-SR units
- Not really saturated at high sound level
- Therefore more capable for coding in background noise where the high-SR fibers are saturated
What is contradictory in hearing function evaluation to what we think we know about the low SR unit?
SR are the ones coding at high sound levels but there is just a small number
There must be another mechanism
What are the two main ways that frequency is coded?
Place coding and temporal coding
What is place coding?
Place coding is the ANF innervation of the IHC at different places along the cochlea and showing different frequency features
What is temporal coding?
Temporal coding is the ANF changing the firing pattern based on the temporal information related to frequency
What kind of curve shows the lowest sound level that drives the ANF to fire?
Threshold tuning curve
Why is threshold tuning curve called by this name?
The threshold is changing with tunning/change of the frequency
What is the characteristic frequency?
Frequency that the ANF has the lowest threshold (With this tuning, the tip shows the the frequency that spikes the ANF the most)
Where should the neuron be recorded (which side) to see the cochlea’s high frequency CF vs low?
At the basilar turn its high frequency vs apical
Which side has a more symmetrical tuning curve?
The Low frequency’s curves
Which side has a more symmetrical tuning curve?
The Low frequency’s curves
How is tonotopic organization shown what is the order like? (2)
Frequency selectivity and place coding since it goes along that order on the cochlea
For the CF rate level function, describe the dynamic range following situations:
At CF?
Low/high frequency?
Which one has a lower maximum?
At CF- narrow dynamic range
Low/high frequency – more linear
Which one has a lower maximum?- the higher frequency (it starts to also just stop there)
Based on the graph, how many ANF are represented?
This is one ANF but it responds for ones that are not its characteristic frequency
When at the CF, what is the threshold like?
Lowest threshold, most sensitive
When at the CF, what happens to the spike rate?
Quick increase with sound level
When at the CF, around what point does it get quickly saturated?
Saturated quickly at a low sound level
What is another way to describe the dynamic range?
Narrow so compression and non-linear
What is the cause of the different rate level functions across the CF?
The OHC
“The different RLFs across CF are due to the OHC active mechanism, which is level and frequency dependent—it acts at narrow frequency region and at low-moderate sound - level dependent amplification causing non-linearity in RLF”
How can the temporal firing pattern be shown experimentally?
Using a Post stimuli’s histogram
Why are time bins used here?
What does prevalence mean in this context?
-PSTH count the number of spikes in each time bin after the onset
-Time bin- have equal time durations (the width varies with accuracy)
-The number of spikes in each bin represents the prevalence/occurrence of neural firing in timing with respect to the stimulus
-To show the prevalence in PSTH, we need to record responses to many repeated stimuli and add them together. This is because the response has randomness.
What are the five stages that a tone burst goes through?
Onset peak, fast then slow adaptation, offset depression, and recovery
At what frequency does phase locking occur?
Low frequency
What happens in the other frequencies than low frequencies in Phase Locking? (3)
Sometimes the AP are missed
(1) APs occur in certain phase of sound;
(2) the certain phase does not necessary have to be at the peak of sound;
(3) APs may not exist in every cycle of sound; higher the frequency, more cycles will see no APs.
Explain the location and SR likely hood of SGN synapses around each IHC.
SGNs synapse around
each IHC.
Fiber A: at modiolar side:
likely to have low SR,
Fiber B: at pillar (OHC)
side likely to have high-
SR
Explain the location and SR likely hood of SGN synapses around each IHC.
SGNs synapse around
each IHC.
Fiber A: at modiolar side:
likely to have low SR,
Fiber B: at pillar (OHC)
side likely to have high-
SR
What are the three characteristics that are encoded by ANF?
– Intensity
– Frequency
– Temporal pattern
