Auditory Nerve

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

Type 1 is myelinated, and Type 2 is not

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 type of SGN can be recorded easier?

Type I

What is the total (approximate) number of SGN in humans?

30,000

What is the function of type 1 SGN? Type 2?

Type 1 function is speed up signal conduction; type 2 function is to reduce interference from each other of ANFs

Diameter of type 1 SGNs? Type 2?

1 - thicker fibers, 2 - thinner fibers

Recordability of type 1 SGNs? Type 2?

Type 1 are easily recorded, whereas type 2 are not

Action potentials and type 1 SGNs

larger APs, higher conduction speed

Action potentials and type 2 SGNs

smaller APs, slower conduction speed

Total percentage of type 1 SGNs

85-95%

Total percentage of type 2 SGNs

5-15%

Which hair cells innervate type 1 SGNs? Type 2?

Type 1 - inner hair cells, type 2 - outer hair cells

Ratio of hair cells to type 1 SGN

10:1 (SGN:IHC)

Ratio of hair cells to type 2 SGN

1:16 (SGN:OHC)

Is innervation of type 1 SGNs convergent or divergent? What about type 2?

1 - convergent, 2 - divergent

Direction of type 1 ANFs

radial direction (radial fibers)

Direction of type 2 fibers

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

Are type 1 SGNs bipolar or monopolar? What about type 2?

Type 1 - bipolar cells, Type 2 - pseudo-monopolar

_____ SGNs synapse with one IHC at bottom around _____ - at ______, and ______ - at ______

10-30, Fiber A - at modiolar side, and Fiber B - at pillar (OHC) side

Each type 1 SGN forms ____ synapse with ____ IHC

one, one

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.

Where are ribbon synapses?

in pre-synaptic regions

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

Glutamate

What are 6 pieces of evidence for this?

Exists in the synapse region, the synthesis system (where it is made) exists, releases in response to a stimuli, causes action potentials in ANF, there is a glutamate receptor (GluR), when blocking the GluR the AP is cancelled

What are the functional features of ribbon synapses?

(1) ribbons ensure long-lasting responses to continuous stimuli (shared by retina sensorial cells) and (2) ribbons ensure quick responses and therefore high temporal resolution (not critical in eyes)

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 (SOC) in the brainstem (or peri-nuceli around SOC).

Where do peripheral efferent fibers pass through?

Olivary cochlear bundle (OCB)

What two groups are efferent fibers divided into?

medial olivary complex bundles (MOCB) and lateral olivary complex bundles (LOCB)

Which efferent fiber bundle (MOC or LOC) is medial and which is lateral?

MOC - Medial, LOC - Lateral

Are MOC bundles crossed or uncrossed (percentages)?

MOC - mainly crossed (26%), 11% uncrossed, LOC - mainly uncrossed (48%), 15% crossed

Describe diameter and myelination of MOC

thick and myelinated

Describe diameter and myelination of LOC

thin and unmyelinated

Which hair cells does MOC innervate? Which part?

innervate OHC bodies

Which hair cells does LOC innervate? Which part?

innervate IHCs, not on the IHC directly but on the afferent terminals below it

Do we know more about MOC or LOC?

Know more about MOC, function of LOC is not clear

Which efferent fiber group is more likely to be recorded and why?

MOC fibers, because they produce larger APs

Why is MOC function more understood?

MOC is more accessible, MOC fibers cross middle line in brainstem at the surface facing 4th ventricle - where cutting and recording can be done and MOC fibers are thicker and myelinated (so larger AP amplitude)

Which spot of the brain allows for easy access to mostly MOC fibers?

above the 4th ventricle (lift cerebellum to expose surface of brainstem)

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

AP waveform not important since there isn't a difference (it's either generated or not)

How is information encoded by APs?

BY single neuron - can change the time rate and firing, and by group of neurons - so products 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 action potentials?

around 500 AP per second

How does firing rate relate to Rutherfords 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?

increasing the intensity can see the action potential where firing rate increases

What kind of coding is it when increasing stimulus intensity?

This is shown by even single neurons and is an example of a rate code

Is it possible to increase intensity (rate code) by single or groups of neurons?

Single neurons and group neurons can do it.

APs are simplified as _______

spikes

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 stimuli?

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 (sounded window) and without sound (quiet window), have to see that the sound window has more of rate (significantly higher) than the 'no sound'

How are fibers grouped?

High spontaneous rate (SR), and low spontaneous rate (SR) and medium (lower SR higher the threshold)

What is the functional importance of the spontaneous rate?

It is inversely related to the behavioral threshold, so the lower the SR, the higher the threshold

What is a possible reason for the threshold/spontaneous rate differences among auditory nerve fibers (ANFs)?

They might work to code sounds in different levels (efferent innervation, LOC)

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

Low thresholds respond quicker

What is a behavioral threshold?

audiology hearing tests

What is the relationship between each of the SR groups to the threshold?

Higher the SR, lower the threshold

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

Low SR has a wide distribution

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

Which spontaneous rate group (low, high, medium) of auditory nerve fibers are medially synapsed around IHCs?

Low spontaneous rate auditory nerve fiber

Which spontaneous rate group (low, high, medium) of auditory nerve fibers are laterally synapsed around IHC?

High spontaneous rate auditory nerve fibers

Ribbon size on IHC synapses of low spontaneous rate auditory nerve fibers

large

Ribbon size on IHC synapses of high spontaneous rate auditory nerve fibers

small

Terminal size on IHC synapses of low spontaneous rate auditory nerve fibers

small

Terminal size on IHC synapses of high spontaneous rate auditory nerve fibers

large

What is the threshold for low spontaneous rate auditory nerve fibers?

High

What is the threshold for high spontaneous rate auditory nerve fibers?

Low

What is the threshold distribution for low spontaneous rate auditory nerve fibers?

large distribution

What is the threshold distribution of high spontaneous rate auditory nerve fibers?

narrower distribution

What is the dynamic range of low spontaneous rate auditory nerve fibers?

large dynamic range

What is the dynamic range of high spontaneous rate auditory nerve fibers?

narrower dynamic range

Approximate dB of dynamic range of high spontaneous rate auditory nerve fibers?

30-40dBs

What is the sensitivity to noise damage for high and low spontaneous rate auditory nerve fibers?

low SR ANFs are morel ikely to be damaged by noise

Describe the importance for signal coding in noise for high and low spontaneous rate auditory nerve fibers.

Signal coding in noise is more important for low SR ANFs

Describe the saturation at high levels for high and low spontaneous rate auditory nerve fibers.

Low - no saturation at high levels, high - saturation at high level

What is the threshold spread compared to absolute threshold for low auditory nerve fibers?

60-80 dBs above absolute threshold

What is the threshold spread compared to absolute threshold for high auditory nerve fibers?

20-40dBs above the absolute threshold

Which spontaneous rate group (low or high) of auditory nerve fibers has greater efferent innervation?

high spontaneous rate auditory nerve fibers

What is the portion of total ANFs for low SR auditory nerve fibers (above the absolute threshold)?

less than 10%

What is the portion of total ANFs for high SR auditory nerve fibers (above the absolute threshold)?

A majority, most high SR ANFs are above the threshold

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 (RLF) show?

How the spike rate changes with the intensity (or SPL)

What is the difference to indicate a dynamic response?

If that SPL changes cause a spike rate change

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 explains why the high SR ANFs cannot code sound at high levels?

the saturation

What are the two main ways that frequency is coded?

Place coding and temporal coding.

Describe place coding.

Place coding is the ANF innervation the IHC at different places along the cochlea and showing different frequency features

Describe 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 the threshold tuning curve called that?

Tuning where think of it as fine tuning so the threshold is changing with the frequency and getting tuned or more focussed

What is the characteristic frequency (CF)?

With this tuning the tip shows the frequency that the ANF has the lowest threshold; This frequency is called the characteristic frequency

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

At the basilar turn it's high frequency vs the apical turn for the low frequencies.

Which side has a more symmetrical tuning curve? T

The low frequency's curves

How is tonotopic organization shown?

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

For the characteristic frequency (CF) rate level function, describe the dynamic range following situations - At CF, low/high frequency, and which has a lower maximum?

Narrow dynamic range at CF, more linear at low/high frequency, and the lower maximum at high frequency

When at the characteristic frequency, what is the threshold like?

Lowest threshold, most sensitive

what happens to the spike rate at characteristic frequency?

Quick increase with sound level

What is another way to describe the dynamic range at characteristic frequency?

Narrow, so compression and non-linear