Overview of the auditory system Flashcards
Describe the development of sensing sound over evoultion
- First amphibians had some sensory cells - sense the movement of water around the body
- As move out of water onto land - evolutionary pressure to sense sounds carried by the air
- Evolved massive range of frequency and intensity sensitivity
- Lead to the formation of the tympanic ear
What is important about the evolution of the tympanic ear?
Appeared 3 times over evolution, in:
- Mammals
- Amphibians
- Birds
Shows that this structure is very important
What is sound frequency?
What is it measured in?
- Amount of waves per second
- The pitch of sound
Measured in Hz
How are wide ranges of sound frequency detected?
By the cochlear mechanics and physiology of the hair cells
What is sound intensity?
What is it measured in?
The volume of the sound
Measures in dB
How are wide ranges of sound intensity detected?
By the firing rate of many afferent nerve fibres that innervate the hair cells of the cochlea
What is a rapid onset of sound important for?
Localising different sounds
and
Creating a topographic map of the auditory sound in the brain
How are the sensory cell synapses of the auditory system different to conventional synapses?
Why?
They are specialised for sustained neurotransmitter release
So that the ear remains sensitive to sounds constantly
Describe the pathway for sound from the outer ear to the cochlear
- Auricle funnels the sounds into the ear canal
- Vibrates the tympanic membrane
- Vibrates the ossicles, which transmit the vibration of the tympanic membrane to the round window
- Causing a vibration of fluid in the cochlear
What is the auricle of the ear?
What does it do?
The external, visible part of the ear
Shapes the sounds and funnels them into the ear canal
What is another name for the tympanic membrane?
The ear drum
What are the ossicles of the ear?
Why are they necessary?
Malleus, Incus and Stapes
- Transmit the sound induced vibration of the tympanic membrane to the fluid movement in the cochlear
- To AMPLIFY the movement of air
What would happen if the ossicles were not present
Air would bounce off the round window
What is contained in the middle ear?
Tympanic membrane
Ossicles
What is contained in the inner ear?
Cochlear
Vestibular organs
What are vestibular organs important in?
Balance
What are the vestibular organs?
- Semi-circular canals
- Utricle and saccule
What are the chambers of the cochlear? (base to apex)
Scala Tympani
Scala Media
Scale Vestibuli
What is the scala media also called?
The cochlear duct
What is the cochlear innervated by?
Where does it enter the cochlear?
What does it innervate?
The cochlear nerve (VIII)
Enters the cochlear through the core (modiolus)
Innervates the organ of corti
Where are the cell bodies of the neurons in the ear?
In the spiral ganglion
Where are the sensory cells are the ear?
In the organ of corti
Which chamber/s of the ear contain endolymph?
Scala media
Which chamber/s of the ear contain perilymph?
Scala tympani
Scala vestibuli
Describe endolymph
Like intracellular solution:
- High K
- Low Ca2+
- Positively charged (80mV)
Describe perilymph
Like extracellular solution:
- Low K+
- Normal levels of Ca2+
- 0mV
What causes the high levels of K+ in the scala media/endolymph?
What does this produce?
Stria vascularis - actively transports K+ into the scala media
Produces an endocohclear potential of +80mV
What is around the outside of the scala media?
Stria vascularis
What separates the scala tympani from the scala media?
Basilar membrane
What separtates the scala media from the scala vestibuli?
Reissner’s membrane
In which chamber of the cochlear is the organ of corti present?
In the scala media
What does the organ of corti constist of?
- Tectorial membrane
- 1 row of IHCs
- 3 rows of OHCs
- Supporting cells
What happens to the hair cells in the ear once they are damaged?
They cannot be replaced
How many IHCs are there?
How many hair cells in total?
4,000 IHCs
16,000 total hair cells
What are the support cells in the organ of corti and where are they present?
- Deiter cells (underneath the OHCs)
- Papillar cells (between the OHCs and the IHCs)
What do the type I spiral ganglion neurons do?
How is this different tot he type II SGNs?
Innervate IHCs
Type II innervate the OHCs
What is the function of type I SGNs?
Carry sound information from the IHCs to the brain
What is the function of type II SGNs?
Believe they are involved in nociception (dangerously loud stimulation)
What is the function of efferent feedback from the brain to the organ of corti?
Allow control over the sound we concentrate on
Prevents the cells from becoming over stimulated
What do lateral efferents synapse with?
Type I SGNs
What do medial efferents synapse with?
Directly with OHCs
Which cells in cochlea respond to high frequency?
Low frequency?
Base - high frequency
Apex - low frequency
Where is the base of the cochlea?
Apex?
Base - near the ossicles
Apex - point of the cochlear (middle of the spiral)
Where does the cochlear relay sound information to?
The cochlear nucleus in the BRAIN STEM
What is the organisation of the cochlear reflected in?
The organisation in the cochlear nucleus and further along in the auditory pathway
What is the hearing range of humans?
20Hz - 20kHz
Which animals have high frequency hearing?
Low frequencies?
High frequencies:
- Bats
- Dolphins
- Mice
Low frequencies:
- Whales
Why is an advantage of mice to have high frequency communication?
Birds (predators) hear low frequencies
What does the pressure wave in the cochlear cause?
What is this determined by?
Maximum deflection of the basilar membrane, depending on stiff the membrane is - dependant on how far the wave travels
How far the wave travels is determined by the FREQUENCY of the sound
What is the structure of the basilar membrane at the base?
What causes maximum deflection here?
Stiff and narrow
Maximum deflection by HIGH frequency sounds
What is the structure of the basilar membrane at the apex?
What causes maximum deflection here?
Wide
Maximum deflection by LOW frequency sounds
What does deflection of the basilar membrane by the pressure wave cause?
Stimulation of the hair cells in that region of the cochlear
What is tonotopicity?
How is it established and preserved in the auditory pathway?
The spatial arrangement of where sounds of different frequency are processed in the brain
Established in the cochlear by the traveling pressure wave
Persevered ALL the way along the auditory pathway (brainstem, midbrain, auditory cortex, cerebral cortex) - Kept seperate
What are the 4 major components of the hair cell?
1) Hair bundle
2) Stereocilia
3) K+ current
4) Voltage gated Ca2+ channels
What channels do the sterocilia have?
How are these channels opened?
Mechanically gated K+ channels
Channels opened by generating force in the tip links
How are sterocillia linked to each other?
Through tip links
What occurs in the hair cells at REST?
- Resting tension on the tip links
- SOME K+ channels open
- SOME Ca2+ channels are activated
- RESTING activity in the afferent fibre
What occurs to the hairs cells when ACTIVATED by sound?
Hair bundle moves backwards and forwards, causing PULES of transmitter release and activity in the afferent fibre
What happens when the hair cells are pushed in the direction of the TALLEST stereocillia?
Excitation:
- Increase tension in tip links
- K+ channels open
- Maximum inward current
- Depolarises the hair cell to -30mV
- Opens majority of Ca2+ channels
- More neurotransmitter released - increasing the firing of the afferent fibre
What is the resting potential of the hair cell?
-60mV
What happens when the hair cells are pushed in the direction of the SMALLEST stereocillia?
Inhibition:
- Slack on the tip links
- Closes K+ channels AND the small resting current
- Hyperpolarisation - outward movement of K+ through a different channel (not transducer channel)
- Potential to -65mV
- Ca2+ channels closed
- Reduced firing of action potentials
Describe the sterocilia in IHCs
Tall sterocilia and 2 shorter rows
Where are the ion channels in the sterocilia present?
In the shorter rows
What are synaptic ribbons of the IHCs?
Specialised electron dense bodies in the synaptic region of IHCs that STORE synaptic vesicles
What is the function of synaptic ribbons?
Store synaptic vesicles to maintain high levels of EXOCYTOSIS over long periods of time
Where are synaptic ribbons not found?
In CONVENTIONAL synapses
What channels are present in IHCs?
Transducer channel
3 types of K+ channel
- Fast acting
- Slow acting
- Active at -ve voltages
How do high frequency and low frequency IHCs respond to sound?
In DIFFERENT ways
What can the cell membrane of the IHC do?
Filter out the phasic component (when the hair cells move backwards and forwards, leading to activation and inhibition)
And respond with a SUSTAINED POTENTIAL for the duration of the stimulus
What do OHCs involved in?
How?
Cochlear amplification
When they contract - causes IHCs to contract more
How are the hair bundles of the OHCs arranged?
In a V shape
What are OHCs innervated by?
What can these neurons do?
Medial efferent fibres from the brain
These neurons can release Ach, to activate Ach receptors that are linked to the K+ channels of the OHCs
- Inhibiting the OHC
What is prestin? What does it allow?
Wheres is it present?
Motor protein that enables the cell to contract or elongate in response to Cl- movement
Present in the cell membrane of the OHCs
What happens prestin in the OHCs when they are depolarised?
What does this cause?
Conformational change in prestin
Shortening the length of the OHC
What happens prestin in the OHCs when they are hyperpolarised?
What does this cause?
Conformational change
OHC elongates
What is the resting potential of OHCs?
Why?
-40mV (higher than IHCs)
Allows the OHCs to respond faster than the IHCs
Which hair cells are attached to the tectorial membrane?
OHCs
How many type I afferent fibres innervate IHCs?
Why is this needed?
MANY (10-15)
Allows SUMMATION to occur
How do the IHCs respond to volume of sound?
Each afferent type I fibre that innervate one IHC has a different threshold for sound
When do type II afferent neurons respond?
When all OHCs that they innervate are active
How do the efferent fibres from the MEDIAL centres of the brain act?
- INHIBITORY affect on the OHCs
- Turn off the action of the amplifier when active
How do the efferent fibres from the LATERAL centres of the brain act?
- INHIBITORY
- Protect the afferent fibres by switching them off to prevent overactivity
What are the stages of the auditory pathway?
1) Cochlear nucleus
2) Superior olivary complex (SOC)
3) Inferior (IC) and superior colliculus (SC)
4) Medial geniculate nucleus (MGN)
5) Auditory cortex (AC)
Where are the cochlear nucleus and SOC?
In the brain stem
Where do ALL fibres from the cochlear go to?
The cochlear nucleus
What are the branched from the cochlear nucleus?
What are these branches involved with?
DCN (dorsal) - sound RECOGNITION
VCN (ventral) - sound LOCALISATION
What is the SOC important for?
Sound LOCALISATION
What is the binaural first site of auditory pathway?
Superior olivary complex
What happens in the inferior and superior colliculus?
Integration of auditory and non-auditory inputs
ALL sensory information
What is the medial geniculate nucleus involved in?
Learning and memory
What is the auditory cortex involved in?
Cognition, attention, memory, decision making
