AN Flashcards
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?
- 4
- Anterior/superior: CN VII
- Anterior/inferior: cochlear nerve
- Posterior/superior: vestibular nerve
- Posterior/inferior: vestibular nerve
- 7up/coke down
What is the anatomical pathway that the auditory nerve fibers are attached to the hair cells?
- IAM via the habenula perforata
- Reaches through the bony/perforated modiolus to the spiral lamina
What is a ganglion?
- Cluster of neuron-like cells outside the CNS
- Spiral ganglion: neuron-like cochlear cells
What is the auditory nerve innervation density as a function of frequency?
- High frequency: high density ANFs
- Low frequency: low density ANFs
How many auditory nerve fibers are there in a human?
~30,000
What percentage of AN fibers are type I?
-90-95%
What type of connections do type I AN fibers have? Type II?
- Type I: many-to-one connections (1-2 IHCs per ANF)
- Type II: one-to-many connections (1 ANF to ~10 OHCs)
What AN fibers surely encode sound?
-Type 1
Where do the afferent and efferent AN fibers connect to IHCs? OHCs?
- IHCs: direct connection to afferent fibers, indirect connection to efferent fibers
- OHCs: direct connection to both afferent and efferent fibers
Describe the tonotopic organization of the auditory nerve.
- High frequencies are encoded around the periphery/lateral edges
- Low frequencies encoded at the core
What type of fibers do we know a lot about and why?
- Type I
- Easier to find/record
Most neurophysiological information about the auditory system is conducted in what kind of
experiment? What is measured in these experiments?
- Single cell physiology experiments
- Record single ANF firing to a presented stimulus over time
How is a PSTH generated?
- Record a single ANF firing/spike rates over a specific time period (i.e. 1 ms) over multiple recording
- Average and bin measurements to create a continuous histogram
How is a period histogram generated?
- To obtain a period histogram, a single-unit electrode should be used to record how often a single auditory nerve fiber spikes over the course of one cycle of the stimulus.
- The number of spikes recorded can then be compared to the phase waveform of the stimulus
How is an interspike interval histogram generated?
- To obtain an interspike interval histogram, one single auditory nerve fiber’s spikes should be recorded
- Average number of intervals between spikes can then be calculated and plotted as a function of the interval duration.
Describe the main sections of a primary-like PSTH? Why is it called primary-like?
- Onset, steady state (rate adaptation), recovery
- Simplest neural firing (no modifications/transformations performed)
How does a PSTH get converted to a tuning curve? Response area?
- Take a single point in the PSTH to generate a rate-level function
- TC: horizontal slice of the rate-level function
- RA: vertical slice of the rate-level function
Describe the shape of low- and high-frequency tuning curves.
- Low frequency: broad, symmetric
- High frequency: sharp, asymmetric
What is the measure of sharpness of a tuning curve? How is it calculated? Is a higher number
sharper or broader?
- Q10
- CF/BW measured at 10 dB above the peak
- Higher Q10 = sharper TC
Do low-frequency AN fibers have absolutely large tuning? Relatively large?
- Absolute: small
- Relative: large
At what spike rates do you subdivide thresholds for ANFs?
- Low: <0.5 spikes/s
- Mid: 0.5-18 spikes/s
- High: >18 spikes/s
What percentage of ANFs are within 10 dB of absolute threshold?
-70%
Describe the relationship between spontaneous rate and threshold for ANFs.
- High SR: low threshold
- Low SR: high threshold
What is phase locking?
- Time locking neural discharge to the stimulus waveform
- Firing at a preferred phase of the stimulus waveform (rarefactions, not condensations)
What is the limit of phase locking for the auditory nerve?
-4-5 kHz
What is the rate limit for firing for the auditory nerve?
-800 Hz
Why does the AN only fire in a preferred cycle/phase in response to a sine tone?
-Only fires during rarefactions, when BM is lifted upward and ion channels are sheared lateraly
Why does the response of an ANF to a click have multiple peaks?
-Ringing (temporal smearing)
Why does the duration of the response of an ANF to a click get shorter with increasing CF?
-As CF increases, period decreases
Does the ANF respond at the rate of the input or at the intrinsic rate of the neuron?
-Rate of the input
How is the characteristic frequency determined for an ANF?
- Firing at the lowest neural threshold
- Compressive nonlinearities
What limits the phase locking of an ANF?
-Refractory periods following ANF firing
What is the dynamic range problem for the AN?
- ANF has a smaller DR than HCs
- ANF DR: 20-50 dB
- Human perception DR: 120-140 dB
How is the dynamic range problem solved?
-Firing of multiple ANFs of various spontaneous rates/DRs to encode full perceptual DR
Since compression occurs, what might help us encode/perceive vowels?
-Neural firing can be saturated but synchronization will occur at higher intensities
What does this suggest about the range of useful formant frequencies?
-Can only go up to 4-5 kHz
Describe the difference between suppression and inhibition?
- Inhibition is chemical
- Suppression occurs because of something non-chemical that reduces ANF firing rate
What is one possible cause of two-tone suppression?
-Nonlinear mechanics of the BM and HCs
How do you measure the suppressive sidebands of an ANF tuning curve?
- Present a single ANF with tones until you find CF
- Present probe tone at CF at 10-20 dB SL
- Introduce second tone of varied frequency/intensity
- Find where ANF firing decreases by at least 20% following introduction of second tone
Describe the general shape of the response of an ANF nerve fiber to a narrowband noise as a function of bandwidth?
-Increasing BW adds more energy, so neuron fires more until the point at which we are outside the tuning curve → start seeing suppression from neighboring areas
Are all ANFs excitatory? Are any inhibitory?
-All are excitatory
What is the “typical” DR of an ANF?
-20-50 dB
Briefly explain the volley principle of phase locking.
- Most ANFs can only fire at about 800 Hz because after firing, the nerve has to recover before it can fire again (aka the refractory period)
- Therefore, multiple ANFs are needed to encode temporal patterns of high frequency tones (record multiple ANFs, sum response to encode total response)
Briefly explain how lower than CF input frequencies might produce the largest firing rates of
an ANF.
- On compression → compression, off frequency → linear response
- Traveling wave is asymmetrical (so lower than CF tone→ large envelope)
- TW grows as you go down the BM
