Audition Test 3 - Auditory Nerve

What is the difference in myelination of type I and type II SGN?

type 1 is myelinated

type 2 is not myelinated

What happens at the Ranvier node for the type 1?

At the node of Ranvier, the action potentials are 'jumping' i.e., passing on to each one so it saves energy and increases speed.

Which SGN has a larger diameter and what does this lead to?

type I therefore this means that there are larger AP and higher AP conduction speed

Which can be recorded easier? Type 1 or typer 2 SGN?

type 1

What is the total approximate number of SGN in humans?

30,000

Are type 1 SGN myelinated? What about type 2?

type 1 myelinated

type 2 not

What is the function of type 1 SGN?

1) speed up signal conduction

2) reduce interference from each other of ANFs

What is the function of type 2 SGN?

little is known about function

Do type 1 SGN have small or large diameter? What about type 2?

type 1: large diameter

type 2: small diameter

Are type 1 SGN recordable? What about type 2?

type 1: easily recordable

type 2: not easily recordable

Type 1 SGN make up what percentage of the total SGN

85-95%

Type 2 SGN make up what percentage of the total SGN

5-15%

Type 1 SGN innervate

IHC

Type 2 SGN innervate

OHC

What is the ratio of HC to type 1SGN?

10:1

each IHC receives innervation from more than 10 type 1 SGNs

What is the ratio of HC to type 2 SGN?

1:16

each fibre innervates up to 16 OHCs

Which SGN is convergent? Which is divergent?

type 1: convergent

type 2: divergent

What direction do type 1 ANFs go?

radial direction: radial fibers

What direction do type 2 ANFs go?

cross the tunnel of Corti at the bottom, go to OHC region and then turned over: outer spiral fibers

Are type 1 SGN's bipolar or monopoly? What about type 2?

type 1: bipolar

type 2: monopolar

What are ribbon synapses?

Specific to the auditory system; They facilitate the release of neurotransmitters by holding the vesicles close to the active zone. (Pre-synaptic regions have these 'ribbons')

What are the two main roles of ribbon synapse relating to transmission?

Quickening the response and ensuring long lasting response.

Which role of the ribbon synapse is unique to the cochlea? Which is shared with the visual system?

- ensuring a long lasting response is shared with visual system

- ensuring a quick response is specific to the cochlea

What is the neurotransmitter of the ribbon synapse in the inner ear?

Glutamate

What are 6 pieces of evidence for the neurotransmitter of the ribbon synapse in the inner ear?

1) Exists in the synapse region

2) the synthesis system (where it is made) exists

3) releases in response to a stimuli

4) causes action potentials in ANF

5) there is a glutamate receptor (GluR)

6) when blocking the GluR the AP is cancelled

What direction is efferent vs afferent fibers?

The efferent is from the brain to the hair cells; Afferent is from the hair cells up to the brain

Where do efferent fibers originate from?

The neurons come from the superior olivary complex in the brainstem.

Where do peripheral fibers pass through?

olivary cochlear bundle

What percentages of the MOCB is crossed? uncrossed?

crossed - 26%

uncrossed - 11%

What percentages of the LOCB is crossed? uncrossed?

uncrossed 48%

crossed 15%

Is the MOCB thick or thin? What about LOCB?

MOCB - think

LOCB - thin

What does MOCB innervate?

OHC bodies

What does LOCB innervate?

innervates IHCs (not directly but on the afferent terminal below it)

Why do we understand MOCB fibers better?

- MOCB fibers cross the middle line at the surface of the brainstem facing the forth ventricle

- we can put the electrode here, as it is a spot for easy access

- this spot has mainly MOC fibers, so that is why we know so much

Are there more MOC fibers or LOC fibers?

LOC, but we still know more about MOC

What is the bottom of the OHC surface mainly covered by?

efferent synapses

How important is the action potential waveform in encoding? Why or why not?

- for information encoding, AP waveform is not important since there isn't a difference (its either generated or not)

- APs are all or none

What is the function of type 1 afferent?

primary auditory neuron that carries information from cochlea to brainstem

How is information encoded by a single neuron?

Single neuron can change the time rate and firing (info is encoded in specific time sequence)

How it encoded by a group of neurons?

Information is encoded in a group of neurons as an array of firing (frequency is encoded through temporal and spatial patterns)

What is the refractory time of nerve fibers?

Nerve fibers have a refractory time of ~2 ms after each action potential

What is the maximal firing rate of actin potentials?

The maximal firing rate is ~500 action potentials/second

How does the refractory time relate to Rutherford's frequency theory?

Rutherford thought 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 does increasing the intensity lead to? What kind of coding is this? Is this possible by single or groups of neurons?

- the rate of action potential is increase significantly in a manner related to sound (intensity)

- rate code

- single neurons and groups can do it

What is a spontaneous action potential?

Spontaneous is an action potential generated without an external stimulus; random in timing.

How do you know if something is spontaneous or caused by an external stimulus?

- If it is random then the sound level is below the threshold of the neuron and it doesn't seem to be affected by the sound which is why upon repetition, and comparing how it is with and without sound

- if the AP rate in the window with sound is significantly higher than the quiet window, we can judge that one is driven by sound

How are fibers grouped?

Fibers are grouped based on spontaneous rate (SR) --> 3 groups (high, medium, low)

How is threshold related to spontaneous rate?

higher SR = lower threshold

low/median SR = higher threshold

What is the threshold distribution like for low SR compared to medial/high?

low SR has a wide distribution

high SR has a narrower distribution

What is the functional importance of spontaneous rate? What is a possible reason for this?

- SR is inversely related with behavioural threshold

- they may work for sound coding in different sound levels

What does it mean to have a high vs low threshold?

Low thresholds respond quicker

What is a behavioral threshold?

Our audiology hearing tests (pure tone testing, standard hearing test) - play the beep and patient responds, trying to get the lowest sound they can hear, but this is based on the behavior of the patient, it is only based on their response

For the IHC, which side of the fiber is likely to have a low SR? Which side is likely to have a high SR?

modular side is likely to have low SR

pillar (OHC) side is likely to have high SR

*Important

Low SR ANF are _____ synapsed around IHC

medially

High SR ANF are _____ synapsed around IHC

laterally

What is the ribbon size on IHC synapse for low SR ANF?

large

What is the ribbon size on IHC synapse for high SR ANF?

small

What is the terminal size on IHC synapse for low SR ANF?

small

What is the terminal size on IHC synapse for high SR ANF?

large

What is the dynamic range of low SR ANF?

large

What is the dynamic range of high SR ANF?

narrow

What is the approx dB of the dynamic range for low SR ANF?

no value because no saturation

What is the approx dB of the of the dynamic range for high SR ANF?

30-40 dB

What is the sensitivity to noise damage for low SR ANF?

more sensitive

What is the sensitivity to noise damage for high SR ANF?

less sensitive

How important are low SR ANFs to signal coding?

more important

How important are high SR ANFs to signal coding?

less important

Are low SR ANF saturated at a high level?

not saturated at high level

Are high SR ANF saturated at a high level?

saturated

What is the threshold spread compared to absolute threshold for low SR ANF?

within 60-80 dB above

What is the threshold spread compared to absolute threshold for high SR ANF?

within 20-40 dB above

Which ANF has greater efferent innervation, low or high SR?

high SR ANF

What portion of low SR ANF make up the total ANFs?

less than 10% of total ANFs

What portion of high SR ANF make up the total ANFs?

most ANFs

What is the rate threshold?

Rate threshold: the sound level at which the firing rate is just about the spontaneous rate (when it is assumed the firing is due to stimulus)

What is a dynamic range?

Dynamic range: intensity range in which increasing intensity increases the spike rate

What does the rate level function show?

how the spike changes with the intensity

What is the difference to indicate a dynamic response in rate level function?

- If SPL changes cause spike rate change, this is a dynamic response

- if SPL change does not cause spike rate change, this is NOT dynamic

- when the curve is saturated, it is no longer dynamic

Describe the graph shape of the high vs low SR for rate level function?

High SR saturate around 40 dB

Low SR does not saturate, more of a linear shape graph (which also means more dynamic range)

What is contradictory in hearing function evaluation to what we think we know about the low SR unit?

Low SR are the ones coding at high sound levels but there is just a small number so the accuracy should be poorer but this is not the case in hearing tests! The mystery at this point is that this indicates that there MUST BE ANOTHER MECHANISM.

What are low-SR units more capable of doing since they are not saturated at high sound level?

more capable of coding in background noise where there high-SR fibers are saturated

What are the two main ways that frequency is coded?

Frequency information is coded mainly by two mechanism:

Place coding

Temporal coding

What is place coding?

- the ANF innervation the IHC at different places along the cochlea and showing different frequency features

What is temporal coding?

- the ANF changing the firing pattern based on the temporal information related to frequency (timing of the signal)

What kind of curve shows the lowest sound level that drives the ANF to fire?

Threshold tuning curves: showing the lowest sound level (y axis) that can drive the ANF to fire

Why is it called a threshold tuning curve?

Think of tuning as fine tuning so the threshold is changing with the frequency and getting tuned or more focussed

What is CF?

- the CF is the characteristic frequency

- this is the frequency where the ANF has the lowest threshold (happens at the tip with tuning)

Where should the neuron be recorded (which side) to see the cochlea's high frequency CF vs low?

At the basal end = very high CF

At the apical end = very low CF

Which side has a more symmetrical tuning curve?

the low frequency curves

How is tonotopic organization shown?

Frequency selectivity and place coding since it goes along that order on the cochlea

What is the dynamic range at CF for the CF rate level function?

narrow dynamic range

What is the dynamic range at low/high frequency for the CF rate level function?

more linear at low/high F

Which one has a lower maximum (for CF rate level function)?

lower maximum at high F (it starts to also just stop here)

Does a spontaneous AP mean there is no stimulus?

- not always

- an external signal may be applied, but if the neuron does not hear the sound or the sound is below the threshold of the neuron then it would still have spontaneous activity even though a signal is present

What is the main takeaway for the different SR levels of neurons?

- the 3 groups of neurons show different thresholds, this makes the neurons work in different sound level range

- this means the task of intensity coding is divided into different neurons

What does a threshold tuning curve tell us? What do the tips tell us?

- how individual neurons respond to sound

- tips tell us the lowest threshold of neurons

When at the CF, what is the threshold like?

At CF, the ANF has lowest threshold: most sensitive

When at the CF, what happens to the spike rate?

spike rate increases quickly with sound level

When at the CF, when does it get saturated?

gets saturated quickly at relatively low sound level (60 dB)

What is another way to describe dynamic range?

nonlinearity (compression)