L6 - Auditory System Flashcards
What pressures did animals face as they moved out of water?
Evolutionary pressure to detect sounds travelling in air
Appearance of tympanic ear
The important of sensing sound developed in?
Early mammals - small and nocturnal
Evolved massive range of frequency and intensity sensitivity
How is sound faithfully relayed from HCs to the brain?
Highly specialised structures and mechanisms in cochlea
What do we use sound for?
Communication Topographic view of auditory world Survival Emotion Navigation
What 4 features of sound need encoding?
Frequency
Intensity
Latency
Duration
Sound frequency
Pitch is measured in Hz
A wide range of sound frequency has to be covered (x103)
Achieved by cochlear mechanics and physiology of hair cells
Mainly encoded by the BM region stimulated
Sound intensity
Loudness is measured in dB
A huge range of sound intensity has to be encoded (x1012)
Achieved by the firing rate of many ANFs
Sound latency
A rapid onset is important for localising different sounds and creating a topographic map
Sound duration
Ear has to remain sensitive to sounds for long periods without fatigue
The sensory cell synapses are specialised for sustained neurotransmission
Method of sound travel through the cochlea
- Sounds enter ear canal
- Causes vibration on the tympanic membrane
- Causes vibration of malleus, incus and stapes
- Lever action of the bones amplifies the movement and pushes fluid in the cochlea - Transmits vibrations of tympanic membrane to the round window
- Causes vibration of fluid inside the cochlea
Overview of the scala media
Middle layer
Contains organ of corti and sensory cells
Separated from tympani by basilar membrane
Separated from vestibula by Reissners membrane
Specialised cells in the stria vascularis
What lymph does the scala media contain?
Endolymph
Normal intracellular fluid
- High K – 150 mM
- Brought about by cells in the stria vascularis
- Generates the endocochlear potential - +80mV
- Low Ca – 20 uM
What lymph does the scala vestibula contain?
Top layer Perilymph Normal extracellular fluid - Low K – 5mM - Normal Ca – 1.3mM
What lymph does the scala tympani contain?
Bottom layer Perilymph Normal extracellular fluid - Low K – 5mM - Normal Ca – 1.3mM
What does the cochlear VIIII nerve innervate?
Innervates the organ of corti
What are spiral ganglions?
Found within the cochlea
Where cell bodies of all the neurons are found
What are the two types of sensory cells within the organ of corti?
1 row of inner hair cells - 4,000 - Main sensory cells - Once damaged cannot be replaced 3 rows of outer hair cells - 12,000
What are some supporter cells within the organ of corti?
Deiter cells - below OHC
Pillar cells – between IHC and OHC
What are the two types of neurones within the organ of corti?
Type 1 spinal ganglion neurons – innervate IHC
- Carry sound info from IHC to brain
Type 2 spinal ganglion neurons – innervate OHC
What are the two types of efferents within the organ of corti?
Allows some control of the sounds we concentrate on
- Lateral effects – synapse with IHC
- Medial efferents – synapse with OHC
How is the mammalian cochlea organised?
Tonotopically
Cells at base – respond to high frequency sound
Cells at top – respond to low frequency sounds
Relay sounds to the cochlea nucleus in the brainstem
Human hearing frequency range
20Hz – 20kHz
Bat hearing frequency range
2kHz-120kHz
Mice hearing frequency range
900Hz – 79kHz
Whale hearing frequency range
14Hz – 36Hz
Low frequency sounds travel better in water
How is tonotopicity established?
By the basilar membrane travelling wave
Establishing tonotopicity method
- Sound enters ear and initiates a wave along the basilar membrane, which is tuned to sound frequency
- Vibration of tympanic membrane
- Lever action causes pressure wave in fluid in cochlea
- Pressure wave travels along cochlea and causes maximum stimulation of particular region on basilar membrane
- Reaches a peak at its best frequency
- Depends on stiffness of basilar membrane
Low frequency sounds cause maximum deflection of?
Travel further
Cause maximum deflection at apex of basilar membrane
High frequency sounds cause maximum deflection of?
Cause maximum deflection at base of basilar membrane
Where its stiff and short
Tonotopicity is preserved throughout the auditory pathway from?
- Cochlea where it established
- Auditory centres in brainstem
- Midbrain
- Auditory cortex
- Cerebral cortex
General hair cell 5 key features
Hair bundle Stereocilia Transducer channels K channels Ca channels
Hair bundle facts
On top
Stimulated by movement of fluid in cochlea
Transducer channel facts
On top of stereocilia
Mechanically gated
Attached to next to stereocilia via a tip link
Force in this tip-link opens the channels
What is the role of K channels on the membranes of hair cells?
Outward movement of K to polarise cell
What is the role of Ca channels on the membranes of hair cells?
Inwards movement of Ca to stimulate exocytosis of vesicle releasing neurotransmitter
General hair cell at rest method
- Resting tension on tip links that opens some of transducer channels
- Allows some K ions to enter cell
- Depolarises cell to resting potential = -55mV
- This depolarisation activates some Ca channels producing resting activity in efferent fibre
General hair cell excitation method
- Push hair bundle towards taller stereocilia
- Increases tension in tip-links
- Opens all channels in the hair bundle
- Maximum transducer current
- Can be measured with a patch clamp - Depolarises hair cells to -30 mV
- Opens Ca channels in cell
- Lots of neurotransmitter released
- Increased firing frequency of efferent fibre
General hair cell inhibition method
- Sound stimulation pulls hair bundle back in inhibitory direction – away from taller stereocilia
- Closes all transducer channels in stereocilia
- Outflow of K through K channels
- Hyperpolarisation of cell to -65 mV
- Decreased firing frequency of efferent fibre
Inner hair cells - hair bundle
Tall row and two small rows of stereocilia
Ions channels on small rows
Inner hair cells - synaptic ribbons
Electron dense bodies in synapse region
Act as a store for synaptic vesicles
High rates of exocytosis for long periods of time
Inner hair cells - K channels
3 types
- Fast activating
- Slow activating
- Low voltage
In response to sound inner hair cells?
Flicks between excitatory and inhibitory states, firing action potential in efferent fibre
Inner hair cells - low and high frequency cells
Low frequency and high frequency cells respond to sound in different ways
Outer hair cells - hair bundles
More W shaped than IHC bundles
Outer hair cells - role
No role in transmitting sensory info
Involved in cochlea amplification
Outer hair cells - innervation
Innervated heavily be efferent fibres - Release Ach which is linked to K channels - Inhibitory effect of cell Small amount of afferent fibre input - Not as much as IHCs
What protein do outer hair cells have in their membrane?
Prestin
Motor protein that in response to Cl movement enables cell to contract or elongate
- Conformational change
Means OHCs Can respond faster than IHCs
Outer hair cell - at rest
More depolarised than IHCs = -40mV
Allows cells to respond faster
Outer hair cell - in response to sound
Cell contracts and elongates rapidly
- Contraction – excitatory
- Elongation – inhibitory
What is the importance of outer hair cells being attached to the tectorial membrane?
The OHC receptor potential activates somatic motility that enhances the mechanical stimulation of IHCs
Neurons in the cochlea - Type 1 afferents
Innervate IHCs
95% of fibres
Carry sound information to the brain
Innervated by up to 30 type 1 afferent fibres
Each fibre has a limited rate at which it can fire action potentials
- Many fibres allows summation
Have both low threshold and high threshold fibres
- Low – quieter sounds
- High – louder sounds
Neurons in the cochlea - Type 2 afferents
Innervate OHCs
5% of fibres
Involved in nociception
Enters organ of corti and bends towards basal side
1 fibre innervated 10-30 OHCs
Only respond when all OHCs they innervate are activated
Neurons in the cochlea - lateral efferent fibres
Innervate IHCs – type 1
Allow dynamic control of cochlear output
Neurons in the cochlea - medial efferent fibres
Innervate OHCs – type 2
Allow dynamic control of cochlear output
Auditory pathway - cochlear nucleus
All afferent fibres from the cochlea
Two main pathways:
- Red from ventral CN – sound localization
- Green from dorsal CN – sound recognition
Auditory pathway - superior olivary complex
Sound localisation
Auditory pathway - inferior and superior colliculus
Integration with non-auditory inputs
E.g. somatosensory and vision
Auditory pathway - medial geniculate nucleus
Involved in learning and memory
Auditory pathway - auditory cortex
Involved in cognition, attention, memory, decision making
