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Neural Circuits > L6 - Auditory System > Flashcards

L6 - Auditory System Flashcards

1
Q

What pressures did animals face as they moved out of water?

A

Evolutionary pressure to detect sounds travelling in air

Appearance of tympanic ear

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2
Q

The important of sensing sound developed in?

A

Early mammals - small and nocturnal

Evolved massive range of frequency and intensity sensitivity

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3
Q

How is sound faithfully relayed from HCs to the brain?

A

Highly specialised structures and mechanisms in cochlea

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4
Q

What do we use sound for?

A 
Communication
Topographic view of auditory world
Survival
Emotion 
Navigation
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5
Q

What 4 features of sound need encoding?

A

Frequency
Intensity
Latency
Duration

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6
Q

Sound frequency

A

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

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7
Q

Sound intensity

A

Loudness is measured in dB
A huge range of sound intensity has to be encoded (x1012)
Achieved by the firing rate of many ANFs

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8
Q

Sound latency

A

A rapid onset is important for localising different sounds and creating a topographic map

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9
Q

Sound duration

A

Ear has to remain sensitive to sounds for long periods without fatigue
The sensory cell synapses are specialised for sustained neurotransmission

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10
Q

Method of sound travel through the cochlea

A
  1. Sounds enter ear canal
  2. Causes vibration on the tympanic membrane
  3. Causes vibration of malleus, incus and stapes
    - Lever action of the bones amplifies the movement and pushes fluid in the cochlea
  4. Transmits vibrations of tympanic membrane to the round window
  5. Causes vibration of fluid inside the cochlea
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11
Q

Overview of the scala media

A

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

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12
Q

What lymph does the scala media contain?

A

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

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13
Q

What lymph does the scala vestibula contain?

A 
Top layer 
Perilymph 
Normal extracellular fluid 
- Low K – 5mM
- Normal Ca – 1.3mM
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14
Q

What lymph does the scala tympani contain?

A 
Bottom layer
Perilymph 
Normal extracellular fluid 
- Low K – 5mM
- Normal Ca – 1.3mM
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15
Q

What does the cochlear VIIII nerve innervate?

A

Innervates the organ of corti

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16
Q

What are spiral ganglions?

A

Found within the cochlea

Where cell bodies of all the neurons are found

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17
Q

What are the two types of sensory cells within the organ of corti?

A 
1 row of inner hair cells 
- 4,000
- Main sensory cells 
- Once damaged cannot be replaced 
3 rows of outer hair cells 
- 12,000
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18
Q

What are some supporter cells within the organ of corti?

A

Deiter cells - below OHC

Pillar cells – between IHC and OHC

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19
Q

What are the two types of neurones within the organ of corti?

A

Type 1 spinal ganglion neurons – innervate IHC
- Carry sound info from IHC to brain
Type 2 spinal ganglion neurons – innervate OHC

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20
Q

What are the two types of efferents within the organ of corti?

A

Allows some control of the sounds we concentrate on

  • Lateral effects – synapse with IHC
  • Medial efferents – synapse with OHC
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21
Q

How is the mammalian cochlea organised?

A

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

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22
Q

Human hearing frequency range

A

20Hz – 20kHz

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23
Q

Bat hearing frequency range

A

2kHz-120kHz

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24
Q

Mice hearing frequency range

A

900Hz – 79kHz

25
Q

Whale hearing frequency range

A

14Hz – 36Hz

Low frequency sounds travel better in water

26
Q

How is tonotopicity established?

A

By the basilar membrane travelling wave

27
Q

Establishing tonotopicity method

A
  1. Sound enters ear and initiates a wave along the basilar membrane, which is tuned to sound frequency
  2. Vibration of tympanic membrane
  3. Lever action causes pressure wave in fluid in cochlea
  4. 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
28
Q

Low frequency sounds cause maximum deflection of?

A

Travel further

Cause maximum deflection at apex of basilar membrane

29
Q

High frequency sounds cause maximum deflection of?

A

Cause maximum deflection at base of basilar membrane

Where its stiff and short

30
Q

Tonotopicity is preserved throughout the auditory pathway from?

A
  1. Cochlea where it established
  2. Auditory centres in brainstem
  3. Midbrain
  4. Auditory cortex
  5. Cerebral cortex
31
Q

General hair cell 5 key features

A 
Hair bundle 
Stereocilia 
Transducer channels 
K channels 
Ca channels
32
Q

Hair bundle facts

A

On top

Stimulated by movement of fluid in cochlea

33
Q

Transducer channel facts

A

On top of stereocilia
Mechanically gated
Attached to next to stereocilia via a tip link
Force in this tip-link opens the channels

34
Q

What is the role of K channels on the membranes of hair cells?

A

Outward movement of K to polarise cell

35
Q

What is the role of Ca channels on the membranes of hair cells?

A

Inwards movement of Ca to stimulate exocytosis of vesicle releasing neurotransmitter

36
Q

General hair cell at rest method

A
  1. Resting tension on tip links that opens some of transducer channels
  2. Allows some K ions to enter cell
  3. Depolarises cell to resting potential = -55mV
  4. This depolarisation activates some Ca channels producing resting activity in efferent fibre
37
Q

General hair cell excitation method

A
  1. Push hair bundle towards taller stereocilia
  2. Increases tension in tip-links
  3. Opens all channels in the hair bundle
  4. Maximum transducer current
    - Can be measured with a patch clamp
  5. Depolarises hair cells to -30 mV
  6. Opens Ca channels in cell
  7. Lots of neurotransmitter released
  8. Increased firing frequency of efferent fibre
38
Q

General hair cell inhibition method

A
  1. Sound stimulation pulls hair bundle back in inhibitory direction – away from taller stereocilia
  2. Closes all transducer channels in stereocilia
  3. Outflow of K through K channels
  4. Hyperpolarisation of cell to -65 mV
  5. Decreased firing frequency of efferent fibre
39
Q

Inner hair cells - hair bundle

A

Tall row and two small rows of stereocilia

Ions channels on small rows

40
Q

Inner hair cells - synaptic ribbons

A

Electron dense bodies in synapse region
Act as a store for synaptic vesicles
High rates of exocytosis for long periods of time

41
Q

Inner hair cells - K channels

A

3 types

  • Fast activating
  • Slow activating
  • Low voltage
42
Q

In response to sound inner hair cells?

A

Flicks between excitatory and inhibitory states, firing action potential in efferent fibre

43
Q

Inner hair cells - low and high frequency cells

A

Low frequency and high frequency cells respond to sound in different ways

44
Q

Outer hair cells - hair bundles

A

More W shaped than IHC bundles

45
Q

Outer hair cells - role

A

No role in transmitting sensory info

Involved in cochlea amplification

46
Q

Outer hair cells - innervation

A 
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
47
Q

What protein do outer hair cells have in their membrane?

A

Prestin
Motor protein that in response to Cl movement enables cell to contract or elongate
- Conformational change
Means OHCs Can respond faster than IHCs

48
Q

Outer hair cell - at rest

A

More depolarised than IHCs = -40mV

Allows cells to respond faster

49
Q

Outer hair cell - in response to sound

A

Cell contracts and elongates rapidly

  • Contraction – excitatory
  • Elongation – inhibitory
50
Q

What is the importance of outer hair cells being attached to the tectorial membrane?

A

The OHC receptor potential activates somatic motility that enhances the mechanical stimulation of IHCs

51
Q

Neurons in the cochlea - Type 1 afferents

A

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

52
Q

Neurons in the cochlea - Type 2 afferents

A

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

53
Q

Neurons in the cochlea - lateral efferent fibres

A

Innervate IHCs – type 1

Allow dynamic control of cochlear output

54
Q

Neurons in the cochlea - medial efferent fibres

A

Innervate OHCs – type 2

Allow dynamic control of cochlear output

55
Q

Auditory pathway - cochlear nucleus

A

All afferent fibres from the cochlea
Two main pathways:
- Red from ventral CN – sound localization
- Green from dorsal CN – sound recognition

56
Q

Auditory pathway - superior olivary complex

A

Sound localisation

57
Q

Auditory pathway - inferior and superior colliculus

A

Integration with non-auditory inputs

E.g. somatosensory and vision

58
Q

Auditory pathway - medial geniculate nucleus

A

Involved in learning and memory

59
Q

Auditory pathway - auditory cortex

A

Involved in cognition, attention, memory, decision making

Neural Circuits (15 decks)

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