Auditory Nerve

Question 1

How many sections of the 8th nerve are there? What are the sections? What is the mnemonic that helps remember which part section is above the other?

Answer 1

4 sections – superior vestibular nerve, inferior vestibular nerve, facial (VII) nerve, cochlear nerve

7up, Coke down

Question 2

What is the anatomical pathway that the auditory nerve fibers are attached to the hair cells?

Answer 2

Terminal buttons, unmyelinated fibers, habenula perforata, rosenthal’s canal and spiral ganglion

Question 3

What is a ganglion?

Answer 3

Any cluster of neurons located outside the central nervous system

Question 4

What is the auditory nerve innervation density as a function of frequency?

Answer 4

Base and apex: 400 fibers/mm ( 3-4 per IHC)

1-2 K Hz: 1400 fibers/mm ( 15 per IHC)

Question 5

How many auditory nerve fibers are there in a human?

Answer 5

30,000 in humans

Question 6

What percentage of AN fibers are type I?

Answer 6

Type 1 (Radial)- 90-95% of afferent fibers

Question 7

What type of connections do type I AN fibers have? Type II?

Answer 7

Type 1 fibers have many-to-one connection with IHCs- each only innervates about 1-2 IHCs

Type 2 fibers have a one-to-many connection (about 10 OHCs)

Question 8

What AN fibers surely encode sound?

Answer 8

Type 1

Question 9

Where do the afferent and efferent AN fibers connect to IHCs? OHCs?

Answer 9

IHCs are connected directly by afferent, and indirectly by efferent.

OHCs are connected directly by both afferent and efferent.

Question 10

Describe the tonotopic organization of the auditory nerve.

Answer 10

** note that the AN fibers are twisted when they leave the cochlea

Low frequencies at the core, mid freq in the middle, high frequencies on the outside

Question 11

What type of fibers do we know a lot about and why?

Answer 11

Type 1 It is hard to find and record from type 2.

Type I fibers have greater accessibility, size, and number
Type II fibers are difficult to locate in the AN core

Question 12

Most neurophysiological information about the auditory system is conducted in what kind of experiment? What is measured in these experiments?

Answer 12

Single cell recordings in physiology experiment, measuring series of action potentials.

Question 13

How is a PSTH generated?

Answer 13

“envelope of activity”
recorded over time from a single nerve that includes firing rates in response to an acoustic stimulus

The number of spikes, or times the neuron fires, is plotted depending on what time is fired for the duration of the stimulus. The x axis is ms and the y is the number of spikes.

Question 14

How is a period histogram generated?

Answer 14

Phase locking.

The spikes are plotted in time; as the x axis is time in ms, but the time resets after every cycle. The tone would be played and the number of spikes recorded would be plotted for each instantaneous amplitude.

The period histograms only show firing during half of the cycle, this is proof that phase does affect the way we encode acoustic signals. Trap doors on stereocilia are closed, so when it is closed there are no firings. You can see more firing as the amplitude is increased, and you can also see a saturation in the firing rate as the amplitude is increased past a certain level.

Question 15

How is an interspike interval histogram generated?

Answer 15

The interspike interval histogram plots spikes in time, but again the x axis is reset after every firing occurs, so this plot shows the response in spikes per second. The x axis is the duration of interval (ms) between firings and the y axis is the number of intervals. To create this plot, the length of the gap between firings would be recorded over time. Then the number of times the neuron fired at each interval would be added up and plotted. This plot gives information about how well a neuron is responding to different frequency stimuli. This can be recorded for a single neuron to see how well it is responding to different frequencies; this would indicate that a neuron with 1.6kHz CF responds best to 1.6kHz, but that the neuron still fires for a 400Hz stimuli.

Question 16

Describe the main sections of a primary-like PSTH? Why is it called primary-like?

Answer 16

This records the envelope of activity as you see the most firings at the onset of the response, less during the steady-state response period, and a very low number of spikes during the recovery period.

First thing that does the neural encoding (in the AN)

Question 17

How does a PSTH get converted to a tuning curve? Response area?

Answer 17

For TC you take a “horizontal” slice of the PSTH when firing is non-spontaneous

For RA you take a “vertical” slice of the PSTH when the nerve firing begins to saturate

Question 18

Describe the shape of low- and high-frequency tuning curves.

Answer 18

Low freq= symmetric. Low CF fibers have broad TC

High freq = asymmetric (steep HF sides and less steep LF sides). High CF fibers have sharp TC.

Question 19

What is the measure of sharpness of a tuning curve? How is it calculated? Is a higher number sharper or broader?

Answer 19

Measured by the Q10 value - The Q refers to the ratio of the CF to the bandwidth of the TC, and the 10 to the location within the TC where the ratio measurement is taken

Calculated by CF/BW
– you choose the BW at 10 dB up (hence the 10 in Q10)

Bigger Q10 means sharper tuning

Question 20

Do low-frequency AN fibers have absolutely large tuning? Relatively large?

Answer 20

Not absolutely large because if you measure in Hz it’s not a big #

Relatively large because when you normalize and do Q10 metric it’s broad compared to HFs

Question 21

At what spike rates do you subdivide thresholds for ANFs?

Answer 21

Low SR 18 spikes per second

Question 22

What percentage of ANFs are within 10 dB of absolute threshold?

Answer 22

70% within bottom 10 dB of range

Question 23

Describe the relationship between spontaneous rate and threshold for ANFs.

Answer 23

Negative correlation!

Low spon rate fibers have high thresholds (aka they need a lot of energy to respond)

High spon rate fibers have low thresholds (aka they don’t need much to respond)

Question 24

What is phase locking?

Answer 24

Phase-locking is related to the timing of the neural discharges in relation to the acoustic waveform.

The auditory nerve will tend to fire at a particular phase of a stimulating low-frequency tone

Question 25

What is the limit of phase locking for the auditory nerve?

Answer 25

Phase locking deteriorates above 1000 Hz

Volley principle allows up to 4000-5000 Hz

Question 26

What is the rate limit for firing for the auditory nerve?

Answer 26

800 times per second

Question 27

Why does the AN only fire in a preferred cycle/phase in response to a sine tone?

Answer 27

Phase locking
Related to stereocilia and trapped door–when stereocilia shift away, trap door opens=excitatory response=AN fires, when the trap door closes firing does not occur. That’s why in a period histogram you only see firing ½ cycle (phase locking)

Question 28

Why does the response of an ANF to a click have multiple peaks?

Answer 28

The ringing of the basilar membrane - the basilar membrane responds to the impulse but then takes a while to completely stop vibrating

Question 29

Why does the duration of the response of an ANF to a click get shorter with increasing CF?

Answer 29

Higher frequencies have larger bandwidths - you get less ringing with larger bandwidths

Question 30

Does the ANF respond at the rate of the input or at the intrinsic rate of the neuron?

Answer 30

The ANF responds at the frequency of input

Even though you have a characteristic frequency, lots of different frequency inputs can activate that neuron

Question 31

How is the characteristic frequency determined for an ANF?

Answer 31

ANF each respond best to a particular frequency – this frequency is usually determined by the frequency for which a given ANF’s threshold is the lowest - single cell physiology exp.

Question 32

What is the dynamic range problem for the AN?

Answer 32

Humans have a perceptual DR of 120-140 dB, but neurons have a dynamic range of only 20-50 dB

Question 33

How is the dynamic range problem solved?

Answer 33

We can encode our full DR by having multiple neurons firing at the same time.

Question 34

Since compression occurs, what might help us encode/perceive vowels?

Answer 34

Best synchronization at the peaks of the vowel formants.

Even when neurons are firing at maximum rate, synchronization of firing is used to discern those high levels to get the high part of the dynamic range

Question 35

What does this suggest about the range of useful formant frequencies?

Answer 35

If there were formants at 8k they would not be helpful because there is no phase-locking above 5k, so all useful formants are below 5k Hz.

Question 36

Describe the difference between suppression and inhibition.

Answer 36

Inhibition implies that the effect is at the level of the neuron, suppression implies interference that is caused from the basilar membrane

Question 37

What is one possible cause of two-tone suppression?

Answer 37

Non-linearity of the BM.

Question 38

How do you measure the suppressive sidebands of an ANF tuning curve?

Answer 38

Measure tuning curve, find CF, then go 20 dB above it (neuron will be well into it’s firing), then play a tone outside the tuning curve and nothing should happen for that fiber. But when you do, decrease in firing rate of the probe tone that was 20 dB above the threshold

Question 39

Describe the general shape of the response of an ANF nerve fiber to a narrowband noise as a function of bandwidth?

Answer 39

More and more energy as the bandwidth increases, happens to a certain point and then the curve turns around (due to suppression - going outside of the tuning curve of that neuron)

Peak drifts to the left with increasing intensity
At higher levels, more and more energy spills out to other areas in the BM

Question 40

Are all ANFs excitatory? Are any inhibitory?

Answer 40

Single AN fibers are always excitatory, never inhibitory.

Question 41

Briefly explain the volley principle of phase locking.

Answer 41

A single nerve fiber reaches its frequency limit (firing rate) and another nerve fiber is recruited into action. The two fibers fire alternately to increase the response rate, recruited more nerve fibers as each fiber reaches its limit.

Question 42

Briefly explain how lower than CF input frequencies might produce the largest firing rates of an ANF

Answer 42

1. Compression - on frequency gets compressed and off frequency is linear
2. Traveling wave grows towards the apex.

“Linearity and growing wave”