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Motor Systems 21332 > Lectures 17 & 18 (Nick Glossop) > Flashcards

Lectures 17 & 18 (Nick Glossop) Flashcards

Vestibular system

1
Q

What two basic questions is the vestibular system design to answer and how does it answer them?

Week 9 - Vestibular Systems

A
  1. Which is up/down
  2. which direction am i Moving in

Answers based on outputs of five organs that independently measure linear & angular acceleration

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2
Q

How many vestibular systems/labyrinths do we have?

Week 9 - Vestibular Systems

A

Two, they are mirror-symmetric one in each inner ear

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3
Q

What are the 5 receptor organs in the vestibular labyrinth

Week 9 - Vestibular Systems

A

Utricle
Saccule
Anterior (superior) Semi-Circular canal
Posterior (inferior) Semi circular canal
Horizontal (lateral) Semi Circular canal

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4
Q

What is attatched to the semi circular canals

Week 9 - Vestibular Systems

A

Endolymphatic sac

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5
Q

What do the utricle and saccule provide in the vesitbular system?

Week 9 - Vestibular Systems

A

They transduce linear accelerations of the head

(i.e acceleration forwards, lift going down)

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6
Q

What do the semi-circular canals provide in the vestibular system

Week 9 - Vestibular Systems

A

They transduce angular accelerations of the head

(e.g moving head from shoulder to shoulder)

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7
Q

What does the combined info of vestibular organs provide a measure of?

Week 9 - Vestibular Systems

A

A measure of angular accelerations about any axis & linear acceleration along any axis

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8
Q

What is the vestibular labyrinth and its structure?

Week 9 - Vestibular Systems

A

The structure in the inner ear housing vestibular organs.Has fairly simple organisation - each organ is lined w/ a continuous sheet of epithelial cells.

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9
Q

What do the epithelial cells that line the vesitbular organs produce and how do they do this?

Week 9 - Vestibular Systems

A

Produce endolymph, do so by action of ion pumps

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10
Q

What is endolymph?

definition and ionic composition

Week 9 - Vestibular Systems

A
  • An extracellular fluid that washes over atypical cellular surfaces
  • is rich in K+, poor in Na + and Ca2+
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11
Q

What is the structure of hair cells and where are they found in the vesitbular labyrinth

Week 9 - Vestibular Systems

A
  • flask-shaped, have an array of stereocilia at their apical ends & a kinocilium at end of the array
  • found in five clusters -one for each organ
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12
Q

What are tip links

Week 9 - Vestibular Systems

A

A filamentous process that physically attatches an ion channel at one end of a stereocillium to the longest adjacent stereocillium

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13
Q

In reference to tip links

What can swaying of stereocilia cause?

Week 9 - Vestibular Systems

A
  • The opening/closing of the ion channel attatched to the tip link
  • this allows for the influx of potassium and calcium (cations)
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14
Q

What happens when stereocilia are deflected towards the kinocilium?

Week 9 - Vestibular Systems

A

Ion channels open - depolarisation - enhanced transmitter release

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15
Q

What happens when the stereocilia are deflected away from the kinocilium

Week 9 - Vestibular Systems

A

Ion channels close - cell hyperpolarised - reduced transmitter release

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16
Q

Which cranial nerve seves as an output to the vestibular system. What is it comprised of

Week 9 - Vestibular Systems

A

Vestibulocochlear nerve - Cranial nerve VIII - made of around 20,000 myelinated afferent axons

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17
Q

How do hair cells fire?

Week 9 - Vestibular Systems

A
  • Tonically & phasically
  • some adapt to continious stimulation

(tonic firing = sustained/continious, phasic = in phases)

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18
Q

What do hair cells code for?

Week 9 - Vestibular Systems

A

info about abrupt & sustained accelerations & translations of the head

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19
Q

What contains the hair cells of the utricle and saccule

Week 9 - Vestibular Systems

A

The macula of each organ

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20
Q

How many hair cells do the utricle and saccule have respectively

A

Utricle = 30k
saccule = 16k

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21
Q

What is the otolithic membrane

Week 9 - Vestibular Systems

A

A gelatinous sheet in the endolymphatic space the stereocilia at the apex of each hair cell attatch to.

It covers the entire macula

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22
Q

What is the otoconia

Week 9 - Vestibular Systems

A

Fine dense particles of calcium carbonate embedded within & lie on the otolithic membrane

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23
Q

What are otoliths/otoconia?

Week 9 - Vestibular Systems

A

Individual calcium carbonate particles that make up the otolithic membrane.

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24
Q

What happens to the otolithic membrane during linear acceleration of the head

Week 9 - Vestibular Systems

A
  • membranous labyrinth moves
  • otolithic membrane is free to move within saccule & utricle & lags behind the movement of the head because of its inertia
  • otolithic membrane therefore shifts relative to the underlying epithelium
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25
Q

How are linear accelerations transduced?

(4 points)

Week 9 - Vestibular Systems

A
  • movement deflects the hair bundles, eliciting an electrical response in the hair cells.
  • The macula of each utricle is oriented to lie in the horizontal plane.
  • Any substantial acceleration within the horizontal plane will deflect at least some of the utricle hair cells.
  • The hair cells are oriented within the macula so that their axes of greatest mechanosensitivity lie in all possible directions.
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26
Q

What does any horizontal acceleration in any given direction cause in relation to utricle hair cells?

A
  • There will be maximal acceleration of one group of cells while another group of oppositely oriented hair cells will be suppressed.
  • cells w/ intermeidate orientations are not affected
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27
Q

Why is there redundacy in the utricle

Week 9 - Vestibular Systems

A

one of the bundles of stereocilia alone can detect where the head is moving → you can ablate a large poriton of the utricle and you’ll still be able to detect linear motion

+ + have a utricle in each ear, and they do the same thing

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28
Q

What is the orientation of the maculas in the saccules

Week 9 - Vestibular Systems

A

Vertical

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29
Q

What is the orientation of hair cells in the saccule?

Week 9 - Vestibular Systems

A

similarly to utricle - hair cells have their axes of greatest mechanosensitivity lying in all possible directions within macula

there are also some saccular hair cells attuned to accelrations in the horizontal plane, particulrly in the anterior posterior axis

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30
Q

What are the saccules sensitive to?

Week 9 - Vestibular Systems

A

Verical accelerations (bungee jumping roller coasters etc)

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31
Q

How does the brainstem project to the vestibular system

Week 9 - Vestibular Systems

A

Brainstem sends inputs to hair cells of vestibular systems - these can have dramatic effects on the sensitivity of hair cells

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32
Q

How do the semi-circular canals compliment the activty of the utricle

Week 9 - Vestibular Systems

A

The utricle is activated by either linear acceleration or head tilt, so the signal on its own is ambigious as to which is going on.

Semi-circular canals detect angular acceleration so provide signal during head tilt or have absent signal during linear acceleration

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33
Q

When does angular acceleration occur

Week 9 - Vestibular Systems

A
  • Whenever an object alters its rate of rotation about an axis
  • head undergoes this frequently (turning, tilting, dancing etc)
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34
Q

What detects and reports angular accelerations

Week 9 - Vestibular Systems

A

The three semi-circular canals in each vistibular labyrinth

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35
Q

What are the semi circular canals comprised of

Week 9 - Vestibular Systems

A

closed tubues filled w/ endolymph that extend from the saccule & utricle

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36
Q

how do semi-circular canals detect accelerations

Week 9 - Vestibular Systems

A

Using the motion of the endolymphatic fluid - specifically the mass of the endolymph

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37
Q

What is the cupula

Week 9 - Vestibular Systems

A
  • A gelantinious diagphram spread across the canals widest region
  • it firmly attatches to the epithelium except at the ampullary crista (where it is penetrated by the stereocilia of around 7k hair cells)
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38
Q

What physically happens when the endolymphatic fluid moves due to acceleration in the semi-circular canals?

A

It presses against the cupula, causing it to bow.

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39
Q

How do hair cells in a single canal respond to angular acceleration?

A

Angular acceleration in one direction excites the hair cells, while an angular acceleration in the opposite direction suppresses them

40
Q

Do the semi-circular canals act alone or in pairs?

Week 9 - Vestibular Systems

A

They act in pairs

Horizontal left SC canal paired with horizontal right SC canal

if not simple horizontal rotation, the pairs of semicircular canal activation can be compliated

41
Q

What happens to the semi circular canals if the head is turned left

(only on the horizontal plane)

Week 9 - Vestibular Systems

A

Left horizontal canal hairs are activated, right horizontal canal hairs are inhibited

42
Q

What kind of patterns of activation do actual human movements elicit from vestibular organs?

Week 9 - Vestibular Systems

A

Actual human movements elicit complex redundant patterns of activity from all receptor organs on both sides of the body.

43
Q

Why do infants require many months of training to learn how to support themselves when they start walking?

Week 9 - Vestibular Systems

A

because even simple movements like getting out of a car evoke extremely complex spatiotemporal patterns of activation within the vestibular system.

44
Q

Why do pilots need extended periods of training to adjust to flying?

Week 9 - Vestibular Systems

A

to get used to the new patterns of vestibular stimulation associated with flying.

45
Q

What does damage to either of the two vestibular labyrinths cause and why?

Week 9 - Vestibular Systems

A

Disorientation & vertigo because of conflicting infomation from the two inner ears

(one set of correct info > two sets of conflicting info)

46
Q

How does the central nervous system (CNS) respond to vestibular activity

Week 9 - Vestibular Systems

A

learns to associate a specific pattern of vestibular activity for each motor behavior, and any unusual vestibular responses usually evoke a protective reflex.

47
Q

What may be the only option in severe cases of vestibular dysfunction

Week 9 - Vestibular Systems

A

surgical removal of the labyrinth

48
Q

What is menieres disease?

(description, effects, associations, cause,treatment)

Week 9 - Vestibular Systems

A
  • described by sporadic relapsing vertigo lasting from 10 mins to 10 hours
  • effects receptor cells of vestibular labyrinth
  • associated w/ tetanus (white noise in ears) and distorted hearing, suggesting cochleae involved
  • cause unknown, but suggested to be poor drainage of endolymph
  • Surgically implanted shunts that bypass this drainage port
    sometimes work
49
Q

Poor drainage of the endolymph has been suggeted as a possible cause for menieres disease. How is the endolymph normally drained

Week 9 - Vestibular Systems

A

Endolymph normally exits the vestibular labyrinth via
the endolymphatic duct before being reabsorbed
into the spinal fluid in the endolymphatic sac.

50
Q

What are vestibulo-ocular reflexes (VORs)?

A

A system by which the vestibular system tells the oculomotor system how fast and in which direction the head is rotating.

This allows the oculomotor system to move the eyes to compensate for head movements and stabilise vision.

51
Q

What is the difference between visual and vestibular processing and what happens if you lose VORs?

A

Visual processing is much slower and less efficient than vestibular processing for image stabilisation.

If you lose the reflexes, you’ll have a hard time reading text - especially when locomoting.

You’ll also have problems recognising friends while walking - you’ll need to stop to do so.

52
Q

What are the three forms of VOR?

A

The rotational vestibulo-ocular reflex

The translational vestibulo-ocular reflex

The ocular counter-rolling response

53
Q

What is the role of the rotational vestibulo-ocular reflex?

A

Compensates for head rotation.

(Shaking head side to side)

54
Q

What is the role of the translational vestibulo-ocular reflex?

A

Compensates for linear head movements.

(How you focus when looking out a car window).

55
Q

What is the role of the ocular counter-rolling response?

A

Compensates for head tilt in the vertical plane.

(How you track a target whilst nodding your head).

56
Q

How does the rotational VOR work?

A

It causes the eyes to slowly rotate in the opposite direction to any rotational head movement detected by the vestibular system.

Eventually, the eyes reach the edge of their orbit and will make a rapid resetting movement back across the centre of gaze.

57
Q

What is vestibular nystagmus?

A

Nystagmus is involuntary eye movement can be part of the vestibulo-ocular reflex.

The eyes moving first in the direction of the lesioned side followed by a quick correction to the opposite side or away from the lesioned side.

(Or when you are doing a rotational VOR and you reach edge of orbit, you snap back via nystagmus).

The direction of nystagmus is defined by the direction of its quick phase.

58
Q

Which systems drive which part of nystagmus?

A

The vestibular signal drive the slow phase of nystagmus.

Brain stem circuits generate the quick phase (when the eyes are near the edge of orbit)

59
Q

What are the symptoms of clinical vestibular nystagmus?

A

If the vestibular is damaged, it sends erroneous signals to the oculomotor system.

Patients will report feeling dizzy, double vision, constantly flickering eyes side to side.

60
Q

How does the translational VOR work?

A

They compensate for linear head movements by taking into account the distance of the object being viewed.

It’s a complex process because of this factor of depth.

(Looking out a train window and things move by at different speed depending on their distance to you)

61
Q

How does the ocular counter-rolling response work?

A

Gravity exerts a constant acceleration on the head so the vestibular system can sense the orientation of the head relative to gravity.

Therefore, the vestibular system is used to estimate the tilting of the head in the vertical plane and initiates the counter-rolling response of the eyes to compensate.

(Think intorsion/extorsion).

62
Q

What are the two main problems with the VOR?

A

It habituates.

The vestibular system does not respond well to very slow head movements.

63
Q

What is/How does the optokinetic reflex work?

A

It uses the fact that as we move our eyes in space, the world moves in the opposite direction.

When large parts of a scene move, the optokinetic reflex drives our eyes in the direction of the observed image motion.

If this response is perfect, it should stabilise the image on the retina.

64
Q

What are the two complimentary properties of the optokinetic reflex to the VOR?

A

It takes time to develop:
- It provides a motion signal that takes over as the vestibular signal decays.

It is activated by very slow visual image motion:
- Vestibular system isn’t good at slow motion.

65
Q

Give an example of circular vection and how it works?

A

Sitting in an enclosed cylinder with black and white stripes on the walls.

As it slowly rotates around you, you perceive it to slow down until it ‘stops’ moving ~30 secs in.

Then you will feel like you’re moving.

This is due to the optokinetic reflex slowly kicking in.

66
Q

What does the circular vection study tell us about the optokinetic reflex?

A

As the vestibular system should tell you that you are not moving (cause you’re not) it means that the visual system MUST be able to stimulate the vestibular nuclei.

This is what allows this illusion to give the false impression that the vestibular system is working when it is not.

67
Q

Why can the perception of self-motion be induced and why?

A

When large parts of a scene move.

This happens because vision-related neurons project to the vestibular nuclei and stimulate it.

(So visual neurons can stimulate the same neurons as vestibular system - inducing the feeling of motion)

68
Q

Describe the findings of the monkey circular vection study.

A

Recordings from the vestibular nuclei of head fixed primates presented with circular vection showed activity even though there was no vestibular activation.

Which accounts for the sensations experienced by human observers and provides evidence that the vestibular system can be activated by visual inputs.

69
Q

What is the relationship of integrating optokinetic and vestibular inputs?

A

The vestibular nuclei respond identically to movement of an image on the retina and head movements.

However, there is a difference in latency and duration of these.

The vestibular input fires quickly (low latency) but decays exponentially to zero with continued rotation (due to endolymph rotating at the same speed as you)

At the same time, the optokinetic pathway generates an exponentially rising signal into the vestibular brain stem nuclei.

In normal circumstances, the summed output of these two signals results in a constant response.

70
Q

What causes the sensation of being dizzy after you stop spinning?

A

Once you are spinning at a constant rate, the endolymph in the semi-circular canals will spin at the same rate you are.

Once you stop spinning, the fluid continues to spin and trigger the hair cells which tells you that you are still moving when you are not.

This = dizzy.

71
Q

What are the two ways we know we are falling?

A

Visual

Vestibular

72
Q

How does the vestibular system tell us we’re falling?

A

As we fall, our head moves from its normal position relative to the force of gravity.

The vestibular system then responds MUCH fast than the visual system, signalling to the vestibulospinal reflexes to allow for preventative measures.

73
Q

What are vestibular spinal reflexes?

A

An automatic reflex that helps maintain balance, posture, and head stabilisation.

74
Q

What is the vestibular complex?

A

It is a group of four major nuclei which lies on the floor of the fourth ventricle.

Forming the front part of the medulla and spanning a bit into the pons.

75
Q

What are the 4 vestibular nuclei named?

A

Medial, Lateral, Superior and Descending.

76
Q

What are the sources of inputs combined in the vestibular nuclei?

A

They combine information from the vestibular labyrinths with those from the visual system, the spinal cord and the cerebellum

77
Q

What are the outputs of the vestibular nuclei? (5)

A

They send outputs to the reticular and spinal centres concerned with skeletal movements, the vestibular regions of the cerebellum, the thalamus, and the oculomotor nuclei.

78
Q

What are the inputs and outputs of the superior and medial vestibular nuclei?

A

Mainly get inputs from the semi-circular canals.

They send outputs to the spinal cord and oculomotor centres.

79
Q

What are the transmission and effect of the medial and superior vestibular nuclei?

What do they help generate?

A

Medial:
- Mostly excitatory.
- Glutamatergic.

Superior:
- Mostly inhibitory.
- GABAergic.

They help generate gaze reflexes - dealing with rotational velocity.

80
Q

What is gaze?

A

Gaze is the coordination of head and eye movements to direct the fovea towards a point of interest.

81
Q

What is the relationship between the head and eye in small gaze movements?

A

The head is much heavier than the eye.

So, during small gaze movements the eye reaches its target well before the head begins to move.

Small gaze movement translates into a saccade, followed by a head movement with VOR.

82
Q

What is the relationship between the head and eye in large gaze movements?

A

During larger gaze movements the eyes and the head move at the same time.

This is because the VOR is suppressed so that they move together (head snaps to target fast so eyes don’t need to saccade).

83
Q

Outline the inputs, outputs and role of the lateral vestibular nucleus.

A

Both the semi-circular canals and the otolith organs send inputs to the lateral nucleus.

The lateral nucleus send outputs mainly to the lateral vestibulospinal tract.

It is involved with postural reflexes.

84
Q

Outline the inputs, outputs and role of the descending vestibular nucleus.

A

Inputs from the otolith organs.

It sends outputs to the spinal cord, the cerebellum and reticular formation, as well as to the contralateral vestibular nuclei.

It seems to be mainly interested in integrating the central motor signals with vestibular signals.

85
Q

Define posture.

A

The relative position of the various parts of the body with respect to one another and the environment.

86
Q

What are the two types of postural equilibrium?

A

Static equilibrium:
- Refers to a balance of forces acting on the body at rest.

Dynamic equilibrium:
- Refers to a balance of forces when a body is in motion.

87
Q

What are the differences between smaller and larger quadrupeds in postural control?

A

To minimise energy needed to support their weight, elephants lock their knee joints in line with gravity.

Smaller quadrupeds tense the muscles around their flexed hindlimb joints, allowing them to flee at the first site of danger.

88
Q

What are preparatory movements and give an example of one.

A

They are movements that occur prior to postural readjustment to maintain balance.

For example:
- Lifting one leg sideways while trying to remain balanced on the other leg.
- To maintain balance, we shift our weight to the standing leg before lifting the other.

89
Q

Why is head position important is postural equilibrium?

A

Knowing the position of the head relative to the environment is important in stabilising vision.

Knowing its position relative to the rest of the body is important for maintaining erect posture.

90
Q

What types of control do anticipatory postural responses depend on?

A

Primarily feedforward control.

However, feedback control is needed to generate postural adjustments to unexpected events.

91
Q

Outline the findings of the cat postural control study.

A

Cats can lose the support of one of their legs and remain standing.

They can be trained to expect the loss of support from one paw as they learn to shift their centre off balance to be supported by the three remaining limbs.

92
Q

What is the result of cerebellar lesions on adaptive changes in postural control?

A

Patients are unable to make these adaptive changes.

93
Q

What did the Horak and Diener (1994) study show in patients with anterior cerebellar regions in postural control?

A

They failed to SCALE their response. (Problems modulating the magnitude of response).

They tensed far more than they needed as their baseline rate and did not change it much.

Controls show a graded differentiation of contraction dependent on the situation they were in.

94
Q

Give and example that shows how postural control is learned during locomotion.

A

Walking on a circular rotating treadmill requires precise neuromuscular control of horizontal sway of the body relative to foot placement on the treadmill.

After exposure and learning, healthy controls learnt to walk in a circle blindfolded.

People in wheelchairs will always fall off…

95
Q

Outline the vestibular cat relearning study results.

A

Normal cats can learn to walk along a rotating beam without falling.

Animal with bilateral labyrinthectomy never recovers this ability.

Animal with a unilateral labyrinthectomy can relearn this depending on when they are ALLOWED to.

Animals that are restrained for a period of a week during this retraining period (up to 20 days post op) NEVER fully regain the ability.

Motor Systems 21332 (7 decks)

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