What is the Central Auditory Nervous system responsible for?
What is its mode of operation?
- Information Processing
State the five functions an auditory system has to do.
For each of them, state which part of the central auditory system is responsible.
- Detect sounds (sensitivity) --> Auditory Nerve
Discriminate between sounds (selectivity) --> Auditory Nerve
- Determine source position --> Midbrain (brainstem)
Group & segregate sounds --> Cortex
Interpret sounds --> Cortex
What does selectivity (discrimination between sounds) allow you to do?
- Distinguish & focus on one sound in a noisy environment
(e.g. listen to one person)
Briefly outline the central auditory pathway.
- Cochlear Nuclei
- Auditory Nerve
- Superior Olivary Complex
- Lateral Lemniscus
- Inferior Colliculus
- Medial Geniculate Body
- Auditory Cortex
How many Inner Hair Cells (IHCs) does a Type I Fibre contact with?
What do each Inner Hair Cells (IHC) synapse onto?
- Many (around 10) Auditory Nerve Fibres (ANFs)
(1 to many)
What are the two types of hair cells?
- Inner Hair Cell
- Outer Hair Cell
What do OHCs synapse onto?
Lots of OHCs synapse onto one auditory nerve fibre
What happens when a sound of a particular frequency is fed into the ear?
Movement of the basilar membrane takes place at a particular point
Hair cells respond to this sound
What type of frequencies do hair cells close to the base (outside bit) respond to?
What type of frequencies do hair cells close to the apex (centre) respond to?
What is tonotopy?
Map of frequency
Adjacent cells will respond to similar frequencies
Other than in the basilar membrane, what other place is tonotopy seen?
- Auditory Nerve Fibres (preserved)
What type of Auditory Nerve Fibre do Outer Hair Cells (OHCs) synapse onto?
- Type 2 ANFs
What type of Auditory Nerve Fibre do Inner Hair Cells (IHCs) synapse onto?
- Type 1 ANFs
What type of cell is an ANF?
Where does its cell body reside?
- Bipolar Cell
- Cell Body --> in the Spiral Ganglion
What percentage of ANFs synapse with IHCs?
(of the roughly 30,000 ANFs)
Roughly how any Type I fibres synapse onto a single IHC?
- Average is 10
- Up to 20
How many IHCs does a single Type I Fibre synapse with?
What do Type I Fibres do?
- Afferent Fibres
- Transmit information from the cochlea to the CNS
Briefly, what is the role of an IHC & OHC?
IHC --> responsible for hearing sound
OHC --> responsible for amplifying sound
Collectively, what are ANFs called when they leave the cochlea?
- ANFs Congregate
- Join the Vestibular Apparatus
- Make up the Vestibulo-Cochlear Nerve
Where do efferent fibres to the cochlea come from?
Superior Olives in the Brainstem
What does an increase in 10 dB mean in real terms?
- 10x increase in power
Why do you need 30,000 ANF (large amount)?
- Cover a wide range of different stimuli intensities
Roughly what is the human range for hearing?
- Between 10-110 decibels (dB)
Generally, how is amplitude coded for (2 things)?
Different neurones respond to different intensities of sound
Frequency of Action Potentials
What three things can Auditory Nerve Fibres vary in?
(i.e. differnet types of SPR properties)
- Spontaneous Rate
Coding Range (saturation point)
What three types of Single Auditory Nerve Fibres (SPRs) exist?
- High SPR
- Medium SPR
- Low SPR
When does an SPR respond?
- During Coding Range
- When Intensity is Above Threshold & Below Saturation Point
What is the ANF spontaneous rate?
- The basal level of firing
- Some SPRs will fire even if there is low/no sound
What is threshold for SPR?
ANF begins to respond to the increase in intensity
(i.e. reached coding range)
What is the coding range?
- Middle section
- It is the dynamic range where there is a change in response in Action Potentials as intensity changes
Which SPR is sensitive to quiet sounds?
- High SPR
(think of SPR as sensitivity)
Which SPR esponds to medium sounds?
- Medium SPR
Which SPR responds to loud sounds?
- Low SPR
(does not respond at all in the absence of sound or low sound)
Why does it help having different types of fibres (SPRs)?
Extends Dynamic Range of the Auditory Nerve
- Encodes for sounds over a 120-140 dB range
What is the distribution like between high & low SPRs?
- Many more high SPRs compared to low SPRs
(even in hearing problems people tend to have problems with hearing in loud areas not in quieter places that is why they show up as fine in audiologist test)
What happens in terms of ANF action potential responses for low frequencies?
- Whenever there is a peak in sound there is a peak in the APs (firing rate)
- Phase locked
(hair cell membrane potential can track rapid fluctuations at low frequencies but not high frequencies)
Why are ANFs (Auditory Nerve Fibres) specific?
- Phase locked
- Only respond to a particular point on a wave on the basilar membrane (i.e. partiuclar frequency)
How do ANFs respond to high frequency sounds?
- Respond to Sound
- However, they will not modulate its firing rate (unlike low frequency) in response to peak & troughs of sound
Failure of Phase locking (no temporal structure)
(There are no distinct peaks in frequency)
How does phase-locking work?
- Vibration at a point on basement membrane
- This displacement occurs in a particular direction --> causing depolarisation of hair cell
- If in other direction --> causes hyperpolarisation (fewer APs)
Thus can track the difference --> Phase-locking