Auditory Nerve - Anatomy & Function
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- How many fibres are in each of the two portions of the VIIIth Nerve?
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Auditory portion: 30,000 - 35,000 fibres
Vestibular portion: 20,000 fibres
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What form the spiral ganglion?
Are they bipolar? -
Cell bodies of the first-order neurons
Yes. - Where does the VIIIth nerve enter the brainstem?
- At the Cerebello-Pontine angle, where the cerebellum, medulla oblongota and pons meet.
- What are the two types of afferent fibers and where do they come from?
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1. Type I Radical: one IHC
2. Type II Outer Spiral: ten OHCs - What is the difference between the effect fibres of the IHC and the OHC.
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IHC: synapses on afferent - no direct connection with the IHC
OHC: direct contact - 85-95% of afferent neurons are what type?
- Type I Radical
- How many Type I radicals per IHC?
- 8-10
- Which type of afferent neurons are thicker/have better myelin and are faster?
- Type I Radical
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Efferent neurons can be from the ipsi side, via the _________.
Or the contra side via the ________. -
Uncrossed olivocochlear bundle
Crossed olivocochlear bundle -
Efferent fibres originating more medially synapse with...
Efferent fibres originating more laterally synapse with... -
OHCs
Type I afferents leaving the IHCs - Type I fibres serving the apex are in...
- the centre of the bundle that makes up the cochlear nerve
- Does depolarization cause inhibition or excitation?
- Excitation
- What is spontaneous nerve firing?
- Nerve firing ("spikes") in absence of a stimulus.
- What is the neural threshold?
- The minimum stimulus level causing a specified increase in discharge rate (e.g. an increase of 10% or an increase of 5 spikes/sec.)
- A single neuron can only fire so fast due to...
- its refractory period
- Spike rate is proportional to...
- Basilar membrane velocity (CM
- Will discharge rate continue to increase with stimulus level no matter how high it goes?
- No, discharge rate will continue to increase with stimulus level, but only up to a point (saturation)!
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What does dynamic range mean?
And what is the dynamic range of a typical single-fibre rate-level function?
Explain the graph with the star on page 4. -
The range from threshold to saturation.
30-40dB - What is the characteristic frequency?
- The frequency at which the lowest stimulus level is required to achieve neural threshold.
- Explain the graph with the heart on page 4
- do it :)
- If you were to draw a graph of the auditory neuron frequency tuning curve, showing the bandwidth differences between a passive and an active response of basilar membrane...how would you draw it?
- See page 5A
- When we have two graphs of frequency tuning curves for different auditory nerve fibres...one being a linear scale and one being log...what are the differences between how the two graphs look?
- It looks like the tuning is getting worse with frequency on the linear scale...but actually if you look at the log scale, you can see narrower peaks which means better tuning. Page 6A.
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What is the tip-to-tail difference for auditory neurons?
They are NOT very responsive for (pick one: high/low) frequency stimuli.
At which frequency level do you start seeing a tail? -
45-50 dB
low
2000 Hz