4d. Central Auditory System Flashcards
Afferent Innervation of the Cochlea
- IHC Neurotransmitter
IHCs release glutamate that acts on AMPA receptors on type I ganglion cells.
Afferent Innervation of the Cochlea
- IHC Neurones
Synapse with 90% of auditory nerve fibres, which are type I ganglion cells
Each type I ganglion cell connects to only 1 IHC and each IHC can be innervated by up to 20 type I ganglion cells
Afferent Innervation of the Cochlea
- OHC Neurones
Synapse with 10% of auditory nerve, which are type II ganglion cells, which are un-myelinated
Each type II ganglion cell innervates many OHCs
Afferent Innervation of the Cochlea
- Afferent Ganglion Cells
Bipolar spiral ganglion cells
Form the cochlear part of cranial nerve VIII (vestibulocochlear).
Travel to cochlear nuclei in the brainstem
Temporal Adaptation
- Location
Likely takes place at the hair cell - afferent fibre synapse
Temporal Adaptation
- Description
Action potential frequency decreases over time with a steady stimulus
Auditory Nerve Spontaneous Firing Rate
60% of auditory nerve fibres have spontaneous firing
Frequency Coding
- 3 Mechanisms
- Labelled line coding
- Phase locking
- Lateral inhibition
Labelled Line Coding
There are >10,000 overlapping bandpass filters which allow the tonotopicity of the basilar membrane to be preserved from cochlea to cortex
Bandpass filters are narrower at higher frequencies
Labelled Line Coding
- Characteristic Frequency
The most sensitive point of the bandpass filter
Labelled Line Coding
- Q10dB
Q10dB = CF/Bandwidth 1dB above CF Threshold
Phase Locking
- Reason
Low frequency afferents have large bandpass filters so frequency encoding is requires to encode frequencies >3kHz
Phase Locking
- Description
Temporal coding that ensures action potentials can only fire during the rising phase of the vibrational stimulus sinusoid, as this is where hair cells are depolarised and neurotransmitter is released.
Phase Locking
- Limitation
Action potentials do not fire at every vibrational sinusoid
The upper limit of phase locking is higher than the upper limit of action potential frequency (1,000Hz), which is limited by the refractor period.
Instead fibre recruitment at the upper end of frequencies is required
Lateral Inhibition
- Description
Tightens the frequency response by the OHCs
Amplitude Coding
- 2 Methods
- Action potential frequency, while maintaining phase locking
- Fibre recruitment
Amplitude Coding
- Action potential Frequency
Louder sound increases rate of action potential firing while maintaining phase locking
Amplitude Coding
- Fibre Recruitment
Louder sound increases fibre recruitment
Spontaneous Discharge Rate
Low threshold fibres:
- High spontaneous firing rate
- Narrow dynamic range
- Synapse on the lateral side of hair cells
High threshold fibres:
- Low spontaneous firing rate
- Wide dynamic range
- Vulnerable to acoustic overstimulation
- Synapse on medial side of hair cells
Parallel Processing in the Cochlear Nucleus
- Neurones that preserve temporal information
- Neurones that preserve amplitude
- Neurones that show strong lateral inhibition
Parallel Processing in the Cochlear Nucleus
- Temporal Information
These neurones have end-bulb of held synapses, which are among the largest synapses in the brain
Parallel Processing in the Cochlear Nucleus
- Amplitude Information
These neurones have a wide dynamic range and sum information from many auditory nerve fibres
Parallel Processing in the Cochlear Nucleus
- Tonotopic Information
Tonotopic information is preserved into the cortex
Central Pathways
- Scheme
- Cochlear nucleus in brainstem
- Superior olivary complex in the brainstem
- Colliculi in the midbrain
- Medial geniculate nucleus of the thalamus in the midbrain
- Primary auditory cortex
