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Neurology > Auditory & Vestibular Systems > Flashcards

Auditory & Vestibular Systems Flashcards

1
Q

Describe the basic structure of all hair cells

A

Hair bundle composed of actin - stiff and rigid sits on top of hair cell
Hair cell synapses onto an auditory/sensory nerve fibre and projects to the brain

Overlying extracellular matrix (gelatinous substance) in all hair cells

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2
Q

What is the role of the hair cells?

A

Hair cells convert motion of the cilia hair bundles into release of neurotransmitter → electrical activity to the brain

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3
Q

What are the different types of extracellular matrix in different hair cells?

A
  • Tectorial membrane (in auditory organs),
  • Otoconial membrane (in the maculae of vestibular
    system) for linear motion
  • Cupula (in cristae of vestibular system) for rotational
    movement
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4
Q

How are stereocilia arranged in the ear?

A

Stereocilia (hairs) are arranged in ‘bundles’ (e.g. 30-300 stereocilia in each bundle in the ear)

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5
Q

How are stereocilia linked?

A

Within the bundle stereocilia can be connected via a number of links:
Lateral-link connectors: top connectors, shaft connectors and ankle links.

Tip links: Found at the top of the cilia

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6
Q

What are lateral-link connectors?

A

Lateral-link connectors between the shafts of stereocilia hold
the bundle together to allow it to move as a unit

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7
Q

How o tip links connect hair cells to allow motion?

A

Tension in the ‘Tip-links’ distorts the tip of the stereocilia mechanically.
This distortion allows channels to open and close with cilia movement. Current flows in proportionately.

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8
Q

Outline the tip link transduction mechanism

A
  1. ‘Tip-links’ open ion-channels.
  2. Endolymph high in K+.
  3. Potassium ion (K+) influx depolarises the cell.
  4. Voltage gated Ca2+ channels open.
  5. Ca2+ triggers neurotransmitter release at the synapse
  6. Postsynaptic potential in nerve fibre triggers action
    potential
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9
Q

How do hair cells aid motion and balance?

A

Displacement of the cilia causes a change in membrane potential

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10
Q

What are lateral line systems?

A

Hair cells that act as water motion detectors in amphibians

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11
Q

What is the inner ear composed of?

A
  • Semicircular canals (vestibular system)
  • Cochlea (auditory system)
  • Vestibulocochlear Nerve
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12
Q

What is the vestibulocochlear nerve?

A

The vestibulocochlear (VIII Cranial Nerve) is the afferent nerve formed from the vestibular and cochlear nerves

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13
Q

What is the role of the vestibular system?

A

The vestibular system is responsible for balance and motion in mammals

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14
Q

What are the 6 motions of balance and motion in mammals regulated by the vestibular system?

A

Orientation due to 6 motions

3 Linear motions: up/down, left/right, backwards/forwards

3 Rotations: roll, pitch, raw

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15
Q

What is the role of the semicircular canals of the inner ear?

A

Semicircular canals detect rotation

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16
Q

How are semicircular canals able to sense rotation?

A

Rotation causes fluid motion in the semicircular canals.

Hair cells at different canals entrances register different directions

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17
Q

Which rotations are detected by the semicircular canals?

A
  • Posterior semicircular canal : rotation around x axis (roll)
  • Anterior semicircular canal: rotation around y axis (pitch)
  • Horizontal semicircular canal : rotation around z axis
    (yaw)
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18
Q

What is the endolymph?

A

Endolymph is the fluid contained in the membranous labyrinth of the inner ear

The major cation in endolymph is potassium

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19
Q

Explain how semicircular canals detect rotation

A

As you move your head, the endolymph moves in the opposite direction to your head motion
At the entrance of the semicircular canals there is an ampulla - opening with sensory receptors (hair cells)

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20
Q

Outline how the structure of the ampulla allows rotation sensing

A

Cilia are connected to the gelatinous cupula.
Under motion, fluid in the canals lags due to inertia, pulling the cupula in the opposite direction to the rotation of the head.

Cilia are displaced, depolarising hair cells.

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21
Q

Which part of the inner ear detect linear motion?

A

In the otolith organs (Maculae) they are sensitive to linear acceleration.

Gravity is also acceleration

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22
Q

What are the 2 maculae components of the inner ear?

A
  • Utricular macula

- Saccular macula

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23
Q

What does the utricular macula sense?

A

The utricular macula hair cells are arranged in a curving lateral plane detecting when we move sideways

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24
Q

What is the role of the saccular macula?

A

The saccular macula can detect up/down motion (as hair cells are arranged in up/down motions) and also forwards/backwards motion (as hair cells are arranged in opposing directions forwards/backwards as well)

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25
Q

How do otolith hair cells sense motion?

A

Otolith hair cells don’t detect fluid motion, but instead detect motion of crystals sitting on top of the extracellular matrix

Hair cells are topped by a rigid layer of otoconia crystals.

Under acceleration the crystal layer is displaced, deflecting the cilia.

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26
Q

Outline the structure of otolith hair cells

A

Otolithic membrane is a gelatinous layer that sits on top of the stereocilia

On top of the stereocilia is a layer of otoconia crystals

27
Q

How does movement of otoconia crystals cause motion detection?

A

The otoconia crystals are heavier and denser compared to the gelatinous membrane. When we move, the crystals also move and bend, causing the stereocilia to leave opening up ion channels.

28
Q

Where in the body is the auditory system located?

A

Starts from the ear → cochlear → multiple brain regions

29
Q

Which brain regions are associated with the auditory system?

A
  • cochlear nucleus
  • olivary complex
  • lateral lemniscus
  • inferior colliculus
  • medial geniculate body
  • auditory cortex
30
Q

What are the roles of the different auditory brain regions?

A

Each different nuclei serves a different purpose within the auditory system
e.g.
Olivary complex is responsible for computing location

Inferior colliculus → auditory cortex separates auditory objects relative to one another

31
Q

What is sound?

A

The rapid variation of air pressure of molecules in time and space
- Longitudinal pressure waves in the atmosphere.

32
Q

How is the wavelength and frequency of sound detected?

A

The rate at which the compression and rarefaction of a wave occur determine the distance between two peaks in the wave (wavelength) and the rate at which the pressure cycles between compression and rarefaction (frequency)

33
Q

What is the relationship between frequency and wavelength of sound?

A 
Frequency and wavelength are inversely related. 
	λ = c/f
c = speed of sound (344m/s)
f = frequency
λ = wavelength

higher frequency = shorter wavelength

34
Q

What sound level can humans detect and hear?

A

The difference in amplitudes between the quietest sounds we can hear is massive

Normal air pressure: 100k Pascals.
We can hear a .000000001% change in pressure

35
Q

What is the Pinna of the ear?

A

The outer ear is called the pinna and is made of ridged cartilage covered by skin.

36
Q

How big is the pinna in humans?

A

Size and shape varies from person to person.

37
Q

What is the role of the pinna?

A

Sound funnels through the pinna into the external auditory canal, a short tube that ends at the eardrum (tympanic membrane)

38
Q

How does the pinna filter sounds?

A

The outer ear filters sound, influencing the frequency response.
Pinna features influence the entering sound differently, and can amplify sounds

e.g. The filanga adds high frequency amplification (1-2kHz)
The meatus amplifies low frequency (~3kHz)

39
Q

What is microtia?

A

Congenital deformity where the pinna (external ear) is underdeveloped

Development of the pinna can alter the way you hear

40
Q

What are the 4 Grades of microtia?

A

Grade I: A less than complete development of the external ear with identifiable structures and a small but present external ear canal

Grade II: A partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss.

Grade III: Absence of the external ear with a small peanut-like vestige structure and an absence of the external ear canal and ear drum. Grade III microtia is the most common form of microtia.

Grade IV: Absence of the total ear or anotia.

41
Q

What is the significance of the middle ear?

A

Critical to transduction mechanism and adds amplification

42
Q

Describe the structures of the middle ear

A

The ‘ear-drum’ vibrates in response to sound.
Middle ear bones (ossicles) are visible through the membrane.

Manubrium of malleus is the point at which the tympanic membrane connects to bone (ossicles)

43
Q

What are the ossicles?

A

Smallest bones in the human body.

Connects the tympanic membrane to the oval window of the cochlea

44
Q

Which 3 bones make up the ossicles?

A

The ossicles are comprised of 3 bones:

Malleus connects to the eardrum

Incus acts as a lever - as malleus pushes down on it, incus
levers → turns a small motion into a larger motion

Stapes connects to incus and oval window of cochlea

45
Q

Describe the transduction mechanism through the middle ear

A

Vibrations through the eardrum are transmitted through the ossicles and amplified at the point of the incus, leading to a pushing motion onto the oval window of the cochlea

46
Q

What is glue ear (otitis media)

A

Middle ear fills with fluid which impedes motion of the ossicles.

Reduces middle ear gain, raises hearing thresholds.

Very common in small children (<5 yrs) - can lead to development problems.

47
Q

What is the cochlea?

A

The cochlea: fluid filled spiral canal divided by a flexible membrane.

48
Q

Which 3 chambers form the cochlea?

A

3 chambers of the cochlea:

  • Scala Vestibuli (upper)
  • Scala Media
  • Scala tympani
49
Q

How are the chambers separated within the cochlea?

A

Scala vestibuli and scala media are separated by a membrane

Scala media and scala tympani separated via basilar membrane

50
Q

How does the transduction mechanism of the middle ear create sound?

A

Stapes pushing on oval window causes compression of cochlear fluid causing basilar membrane movement

Basilar membrane filters sound
according to frequency.

51
Q

How does basilar membrane filter sound?

A

The basilar membrane is more rigid at the stapes end compared to the other apex end where its wider

Close to the oval window, the basilar membrane oscillates at higher frequencies

At the opposite end, the membrane moves slower and responds to lower frequencies

52
Q

How does the travelling wave of endolymph produce sound?

A

Travelling wave of compression moves through cochlear fluid causing basilar membrane movement

53
Q

How are higher and lower frequency sounds produced by the basilar membrane?

A

Membrane moves more compliantly at apex end ⇒ larger movements (~2kHz) at apex end
Where as a higher frequency (~6kHz) creates larger movements closer to the base

Wave rises gradually, peaks, then decays rapidly.
Peak location depends on stimulus frequency.

54
Q

What is the organ of corti?

A

The sensitive element in the inner ear - body’s microphone

55
Q

Where is the organ of corti located?

A

It is situated on top of the basilar membrane in the scala media (one of the three compartments of the Cochlea)

56
Q

Describe the structure of the organ of corti

A

It contains four rows of hair cells which protrude from its surface.
Inner and outer hair cells are mounted on it.

Extracellular matrix on top of the organ of corti hair cells is the tectorial membrane, which can move up and down (lever)

57
Q

How is the organ of corti activated?

A

Motion of the organ of corti on the basilar membrane causes displacement of the stereocilia.

Outer hair cells contact the tectorial membrane. Inner hair cells do not.

As tectorial membrane moves, it pushes the hair cells sideways to open up ion channels causing firing in the afferent nerves

58
Q

How do the functions of inner and outer hair cells differ?

A

Inner hair cells are involved in the transduction mechanisms and hearing via brain signalling

Outer hair cells act as amplifiers

59
Q

What causes outer hair cell activation?

A

Outer hair cells are motile.

Influx of positive ions makes the outer hair cells contract

60
Q

Outline the mechanism of action of the cochlear amplifier

A
  1. Influx of +ve ions
  2. Prestin in short conformation state
  3. Outer hair cell contracts
  4. This pulls the basilar membrane toward the tectorial
    membrane.
    → larger influx of +ve ions
61
Q

What is the purpose of the cochlear amplifier?

A

Amplifies by as much as 50dB!!
Quiet sounds are amplified.
Loud sounds are not amplified
– helps us deal with 120dB of dynamic range.

Tuning is sharper than the passive vibration of the basilar membrane.

62
Q

What is the ‘battery’ driving the cochlear hair cells?

A

The high potassium concentration of the endolymph in the scala media creates a 2x amplification. (+80mV)

63
Q

What would be the consequence of a low [K+] endolymph?

A

If it were not potassium rich then inner hair cell output (of the cochlear nerve) would be halved, making sound perceptually quieter.

And the cochlea amplification would be much smaller, again making sounds perceptually quieter.

Neurology (20 decks)

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