Auditory Nerve

Question 1

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

Answer 1

There are four sections: 7th nerve, 8th nerve, superior vestibular nucleus, and inferior vestibular nucleus. The mnemonic is 7 up and coke down.

Question 2

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

Answer 2

The auditory nerve travels from the hair cells through the habenula perforate in the osseous spiral lamina to the IAM.

Question 3

What is a ganglion?

Answer 3

A ganglion is a group of neuronal cell bodies 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 dibers/mm (3-4 IHC)

1-2 kHz: 1400 fibers/mm (15/IHC)

Question 5

How many auditory nerve fibers are there in a human?

Answer 5

There are 30,000 auditory nerve fibers in a human.

Question 6

What percentage of AN fibers are type I?

Answer 6

90-95% of auditory nerve fibers are Type I.

Question 7

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

Answer 7

Multiple Type I fibers innervate each inner hair cell and have a many-to-one connection. Unlike Type I fibers, a Type II fiber innervates multiple outer hair cells.

Question 8

Which AN fibers surely encode sound?

Answer 8

Type I fibers surely encode sound.

Question 9

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

Answer 9

Inner hair cells are contacted directly by afferents and indirectly by efferents. Outer hair cells are contacted directly by both afferents and efferents.

Question 10

Describe the tonotopic organization of the auditory nerve.

Answer 10

Auditory nerve fibers are ordered low-to-high in radial fashion. High frequency fibers are on the outside, middle frequency fibers are on the next ring, and low frequency fibers are directly in the middle.`

Question 11

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

Answer 11

Most knowledge comes from type I fibers because type II fibers are difficult to find and record.

Question 12

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

Answer 12

Most neurophysiological information about the auditory system is conducted with single cell neurophysiology experiments. The experiments occur when a microelectrode is placed into a single neuron. Stimuli is presented to the animal’s ears and the frequency of the stimulus is played over a large range of levels. If the stimulus is the frequency and sensitivity that the neuron is tuned to, the action potential from the neuron can be measured.

Question 13

How is a PSTH generated?

Answer 13

In a physiology experiment, a stimulus is presented and the action potentials from a single neuron is recorded. To generate a PSTH, the clock is restarted at the onset of each stimulus. The firing over the entire stimulus is observed.

Question 14

How is a period histogram generated?

Answer 14

A stimulus is presented and the action potentials from a single neuron is recorded. To generate a period histogram, the clock is restarted for each period. This demonstrates the phase locking and how firing only occurs in the rarefaction phase.

Question 15

How is an interspike interval histogram generated?

Answer 15

A stimulus is presented and the action potentials from a single neuron is recorded. To generate an interspike interval histogram, the clock is restarted after each spike. This helps show that spikes occur at integer multiples of the period, and observe phase locking.

Question 16

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

Answer 16

The main sections of a primary-like PSTH are the resting potential, the onset, the steady-state response, and the recovery period. During the resting potential, the neuron is not responding to a specific stimulus, but there is some spontaneous firing. During the onset, the neuron has been depolarized and the action potential is occurring due to the presentation of a stimulus. Then the neuron has a steady state response to that stimulus. Lastly, the neuron enters the recovery stage, which is when the neuron is hyperpolarized and is trying to return to its resting potential.

It is called primary-like because it is the first neural encoding of the sound, and there is no real transformation of the auditory stimulation.

Question 17

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

Answer 17

To convert a PSTH to a tuning curve, a section is examined from the Y axis. For example, look at the response of three different neurons at 20 spikes/sec. To convert a PSTH to a response area, a section is examined from the X axis. For example, look at the response of the three different neurons at 50 dB SPL.

Question 18

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

Answer 18

Low frequency tuning curves are symmetric and are broadly curves. High frequency tuning curves are asymmetric and are sharply tuned.

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

Sharpness of tuning curve is measured by Q10 value. The Q10 value is calculated by dividing the center frequency by the bandwidth. A higher number indicates a sharper tuning curve and a lower number indicates a broader tuning curve.

Question 20

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

Answer 20

The auditory nerve fibers measured in Hertz has a small absolute bandwidth.

The low frequency fibers have broad tuning and the high frequency fibers have sharp tuning when considering the center frequency.

They have absolutely small and relatively large for low frequency fibers.
Relatively speaking, neurons are more narrowly tuned when talking about proportions.
Auditory bandwidths increase with center frequency. Also correct to say high frequencies have relatively sharply tuning.

Question 21

At what spike rates do you subdivide thresholds for ANFs?

Answer 21

Low: 0-5 spikes/second
Medium: 5-18 spikes/second
High: >18 spikes/second

Question 22

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

Answer 22

70%

Question 23

Describe the relationship between spontaneous rate and threshold for ANFs.

Answer 23

High spontaneous rates have low thresholds and low spontaneous rates have high thresholds.

Question 24

What is phase locking?

Answer 24

Phase locking is a clear and fixed relationship between some aspect of the neural response and the phase (or time) of some aspect of the stimulus.

Question 25

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

Answer 25

The limit is between 4 and 5,000 Hz.

Question 26

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

Answer 26

Most auditory nerve fibers can only fire at about 800 Hz.

Question 27

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

Answer 27

It only fires to the preferred phase because it only fires during rarefactory periods.

Question 28

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

Answer 28

The multiple peaks are due to temporal smearing, or ringing, as a result of passing the acoustic stimuli through a filter.

Question 29

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

Answer 29

As the center frequency increases, the period will get smaller, thus resulting in a shorter duration of the response.

Question 30

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

Answer 30

The ANF will respond at the intrinsic rate of the neuron.

Question 31

How is the characteristic frequency determined for an ANF?

Answer 31

The characteristic frequency is determined by the frequency for which a given AN fiber’s threshold is the lowest and exhibits compressive nonlinearities.

Question 32

What limits the phase locking of an ANF?

Answer 32

Phase locking of an ANF is limited by high frequency inputs.

Question 33

What is the dynamic range problem for the AN?

Answer 33

The dynamic range problem is the difference between the dynamic range of humans and the dynamic range of the neurons. Humans have a perceptual dynamic range of 120 to 140 dB, but the auditory nerve neurons have a dynamic range of 20 to 50 dB.

Question 34

How is the dynamic range problem solved?

Answer 34

The dynamic range problem is solved because multiple neurons encode the intensity of the incoming auditory stimuli.

Question 35

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

Answer 35

Synchronization of the neurons still occurs even though there is compression. The synchronization provides the best phase-locking to the stimulus.

Question 36

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

Answer 36

The range of useful formant frequencies is up to 5000 Hz.

Question 37

Describe the difference between suppression and inhibition?

Answer 37

Suppression occurs in an excitatory neuron and causes it to fire at a lower rate. Inhibition is the stopping of neural firing because it’s the fibers innate nature.

Question 38

What is one possible cause of two-tone suppression?

Answer 38

One possible cause of two-tone suppression is the non-linear properties of the BM.

Question 39

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

Answer 39

Trace the sidebands of the tuning curve when it reaches 20%

Question 40

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

Answer 40

When the bandwidth is increased, the neuron is firing more and more until it reaches the region of the tuning curve. Once the firing rate hits that region, suppression occurs, which is evident in the shape of the response of the ANF nerve fiber.

Question 41

Are all ANFs excitatory? Are any inhibitory?

Answer 41

All auditory nerve fibers are excitatory and none are inhibitory. Although, suppression can occur to an excitatory auditory nerve fiber.

Question 42

What is the “typical” DR of an ANF?

Answer 42

The typical dynamic range of an auditory nerve fiber is 20 to 50 dB.

Question 43

Briefly explain the volley principle of phase locking.

Answer 43

Neurons share the load of an incoming signal for high frequencies. Since an AN fiber may only fire on every second, third, or fourth cycle of high frequencies, other fibers are recruited to lock into the cycle that the original fiber is not firing at.

Question 44

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

Answer 44

On frequency, there is compression. Off frequency is a linear response. Traveling wave is asymmetrical. So, a low frequency tone is going to have a large envelope. The traveling wave grows as it goes down the basilar membrane.