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Neuroscience Y2 > Auditory System > Flashcards

Auditory System Flashcards

1
Q

Describe impedance matching

A
  1. Fluid has greater inertia so requires more energy to move
  2. Lever action from stapes
  3. Eardrum to oval window SA ratio is 20:1
  4. These two things cause a 26X pressure amplification
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2
Q

Describe role of middle ear

A
  1. Sound in air medium
  2. Vibrates ear drum
  3. Vibration causes Malleus to move the Incus to move the Stapes
  4. Stapes moves oval window
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3
Q

Describe the attenuation Reflex

A

When exposed to loud sound (>80dB)

  1. Stapes muscle and Tensor Tympani muscle stiffen the lever action
  2. Reduces energy transferred to inner ear
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4
Q

Describe the Cochlear

A
  1. 35mm spiral
  2. Top section is Scala vestibuli (perilymph)
  3. Middle is scala media (endolymph)
  4. Bottom is Scala tympani (perilymph)
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5
Q

Describe the organ of corti

A
  1. Inner (1 row) and outer hair cells (3 rows) (innervated by spiral ganglion to form auditory nerve)
  2. Sit in the flexible basilar membrane
  3. Tectorial membrane sits on top of bait cells
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6
Q

Describe the structure of hair cells

A

Inner:

  • straight row of sterocilia (hair bundle)
  • largely afferent innervation

Outer:

  • V shaped rows of sterocilia
  • largely efferent innervation
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7
Q

What are sterocilia formed off

A
  1. Actin
  2. Myosin
  3. Frimbrins
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8
Q

What are tip links

A
  1. Fine filaments between sterocillia

2. Made of cadherins (CDH23 and PCDH15)

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9
Q

How do tip links function

A
  1. Linked to transduction ion channels on sterocilia
  2. Non-specific cation channels (TMC1/2)
  3. Mutations in these cause deafness
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10
Q

Describe the cochlear fluids

A

Perilymph:

  • resembles CSF
  • bathes cell bodies in organ of corti

K+ 7mM
Na 140mM

Endolymph

  • resembles extracellular fluid
  • bathes sterocilia

K+ 145mM
Na+ 1mM

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11
Q

Describe the propagation of sound

A
  1. Movement of oval window displaces fluid
  2. Bends basilar membrane at base
  3. Traveling wave moves along basilar membrane
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12
Q

Describe the Basilar membrane

A
  1. Structure of basilar membrane varies along the length
  2. Position of maximal displacement depends on frequency of sound
  3. Base: narrow and stiff (higher frequency)
  4. Apex: wide and floppy (lower frequency)
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13
Q

What happens when the basilar membrane moves?

A

It takes the organ of corti with it- causing the tectorial membrane to move with it, pushing the inner and outer hair cells to bend

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14
Q

Describe how bundle stimulation opens ion channels (inner hair cells)

A
  1. When sterocilia move towards tallest, causes tension between the tip links
  2. Tip links attached to transducer channels and therefore open
  3. Causes K+ influx into the cell
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15
Q

Describe inner hair cell activation in vivo

A
  1. Sterocilia bathed in K+ rich endolymph (+80mV)
  2. -60mV in hair cell, so -140mV driving force)
  3. Depolarisation causes Ve-gated Ca+ ion channels to open
  4. Synaptic transmission to afferent neurones
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16
Q

What happens (in terms of receptor potential) when sterocilia are pushed in positive/ negative direction

A

Neutral: transducer current slightly +Ve
Negative: Ve- stimulus
Positive: Ve+ stimulus

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17
Q

Describe the relationship between nerve firing and receptor potential

A

Receptor potential responds to the wave pattern of said frequency

This creates a burst firing pattern in the nerve

18
Q

Describe a step by step from sound to neuronal firing

A
  1. Sounds stimulus
  2. Bundle movement
  3. Changes in transducer current
  4. Graded changes in membrane potential
  5. Changes in transmitter release
  6. ## Changes in neuronal firing‘Phased locked’ - sound phases represented in neuronal firing
19
Q

Describe the volley principle

A
  1. Neurones have max firing frequency of 1kHz
  2. Groups of neurones can respond to multiple cycles as one output
  3. Increases range of phase locking to 4kHz
20
Q

How is frequency of up to 20kHz detected

A
  1. The structural variations in basilar membrane
  2. Neurones have a characteristic frequency by responding with the greatest frequency
  3. Maintains a tonotopic map over a range of frequencies
21
Q

Describe isofrequency bands In the brain

A

Different frequency’s represented in tonotopic map in auditory cortex

22
Q

Describe the role of the cochlea amplifier

A
  1. If cochlea was a passive structure we would see a viscous dampening of basilar membrane (would lose energy)
  2. Causes an application of inner hair cell stimulation
23
Q

Describe the role of outer hair cells in the cochlear amplifier

A
  1. Inner hair cells depolarise
  2. Prestin (motor proteins) shorten up to 100nm
  3. This amplifies basilar membrane movement
  4. Modulated by efferent inputs
24
Q

What are otoacoustic emissions

A
  1. Noise produced by the ear

2. Caused by the active mechanisms of the ear

25
Q

Describe the afferent innervation of hair cells

A

Inner:

  1. Type 1 afferent
  2. 1:1 neurone to IHC
  3. Thick/ myelinated

Outer

  1. Type 2 afferent
  2. 1:50 neurones to OHC
  3. Thin and unmyelinated
26
Q

How is sound amplitude encoded

A
  1. Populations of afferent nerves
  2. Fibres excited at low, medium and high levels of sound intensity
  3. Summation of these populations
27
Q

From cochlear to brain

A
  1. Spiral ganglion
  2. Ventral cochlear nucleus
  3. Superior olive
  4. Inferior follicles
  5. Medial gelatinous nucleus
  6. Auditory cortex
28
Q

What is conductive hearing loss

A
  1. Damage to outer/middle ear
  2. Defects in ossicles
  3. Damage to tympanic membrane
29
Q

What is sensortineural hearing loss

A
  1. Damage or loss of sensory hair cells

2. Damage or loss of neurones

30
Q

Two types of hair cell defects

A
  1. Loss of OHC
    - increased threshold
    - poor frequency discrimination
    - hearing aid helps with intensity only
  2. Loss of IHC and OHC
    - complete deafness
    - cochlear implant only current treatment
31
Q

Describe hair cell defect- noise trauma

A
  1. 80-90 dB
  2. Disruption of hair bundle and breakage of tip links
  3. May be repairable
32
Q

Describe severe noise trauma

A
  1. > 130dB
  2. Complete loss of hair cells due to build up for free radicals
  3. ## Scar formation fills voidAlso excitotoxicisty from excessive glutamate release can permanently damage synapse
33
Q

From Cochelar to brain

A
  1. Spiral ganglion
  2. Ventral cochlear nucleus
  3. Superior olive
  4. Inferior colliculus
  5. MGN
  6. Auditory cortex
34
Q

How is representation of both ears on each side of the brim achieved

A

Binaural neurones that bicussate allowing trigagulation of source of noise

35
Q

How is intensity encoded

A

Populations of afferent nerves

- highest rate of firing at highest intensity

36
Q

What are the two types of deafness

A

Conductive:

  • outer/ middle ear
  • defect of ossicles/ tympanic membrane

Sensorineural hearing loss

  • damage to or loss of sensory cells
  • damage to neurones
37
Q

Result of loss of hair cells

A

OHCs:

  • Increased threshold (50dB)
  • poor frequency discrimination

OHCs and IHCs:

  • complete deafness
  • Cochlear implant
38
Q

How to test OHC loss

A

Speech audiogram

- ability to discriminate words

39
Q

Describe mild noise trauma

A

80-90dB

  • disruption of hair bundle and breakage of tip-links
  • may be repairable
40
Q

Describe severe noise trauma

A
  • complete loss of area of hair cells due to free radicals

- scar tissue fills void

Neuroscience Y2 (12 decks)

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