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Systems: Neurology AB > Vestibular Function > Flashcards

Vestibular Function Flashcards

1
Q

What is the vestibular system?

A
  • A sensory system essential in the control of posture and balance
  • It is a series of fluid-filled membraneous tubes, (labyrinths), which are embedded in the temporal bone
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2
Q

Where is the vestibular system found?

A

Inner ear

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3
Q

What does the vestibular apparatus consist of?

A

The vestibular apparatus consists of 3 semi-circular canals, the utricle to which the semi-circular canals all connect and the saccule

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4
Q

How are the semi-circular canals arranged?

A

The 3 semi-circular canals are at right angles to each other (3 dimensions) and swellings at their bases, called ampulla, contain sensory hair cells.

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5
Q

What do the utricle and saccule both contain?

A

Sensory hair cells

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6
Q

What are the utricle and saccule collectively known as?

A

Otolith organs

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7
Q

What do the otolith organs do?

A

hey detect linear acceleration and encode information about the position of the head in space; back/front tilt is detected by the utricle, vertical movement is detected by the saccule.

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8
Q

What do the semi-circular canals detect?

A

The semi-circular canals (SCCs) detect rotational acceleration.

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9
Q

Where are the sensory cells of the semi-circular canals embedded?

A

The sensory cells of the semi-circular canals are embedded in swellings at the base of the bony canals called ampulla

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10
Q

What is inside the ampulla?

A

Inside those swellings are sensory receptors called cristae

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11
Q

What do the cristae consist of?

A

They consist of a flexible gelatinous structure called the cupula that stretches across the entire width of the ampulla and responds to movement of the endolymph fluid within the canals.

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12
Q

What is embedded within the gelatinous cupula?

A

Embedded within the gelatinous cupula are the cilia of hair cells which synapse directly with the sensory neurons of the vestibular nerve (CNVIII).

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13
Q

What do the hair cells contained in the gelatinous material detect?

A

Rotational acceleration

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14
Q

How do the hair cells detect rotational acceleration?

A

If the skull is rotated left or right from rest, the endolymph at first does not move because of its inertia. However the ampulla moves instantly because it is embedded in the skull.

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15
Q

What does the inertia of the endolymph produce?

A

The inertia of the endolymph produces drag which bends the cupula, and consequently the cilia embedded in it, in the opposite direction to movement. If rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds

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16
Q

How does dizziness occur?

A

Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness

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17
Q

What are the 2 types of cilia of the hair cells?

A
  • Single very large kinocilium

- Set of progressively smaller stereocilia

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18
Q

What does distortion of the cilia in the direction of the kinocilium cause?

A

Distortion of the cilia in the direction of the kinocilium causes depolarisation and increased discharge of APs in the vestibular nerve.

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19
Q

What does distortion of the cilia away from the kinocilium cause?

A

Distortion of the cilia away from the kinocilium leads to hyperpolarization and decreased discharge of APs in the vestibular nerve.

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20
Q

What does distortion of the kinocilium allow?

A

It allows the brain to determine movement in time and space. The orientation of the cupula are all slightly different so the brain can build a 3D image of body position using the pattern of firing and inhibition received.

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21
Q

Where does much of the integration of the sensory information take place?

A

Cerebellum

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22
Q

What is the sensory apparatus of the utricle and saccule are collectively known as?

A

Maculae

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23
Q

How is the macula in the utricle orientated?

A

Horizontal plane

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24
Q

How is the macula in the saccule orientated/

A

Vertical plane

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25
Q

What does the maculae consist of?

A

They have a set of cilia (one kinocilium and a series of stereocilium) which protrude into a gelatinous mass called the otolith membrane. Embedded in the otolith membrane are CaCO3 crystals called otoliths.

26
Q

Why is tilt of the head detected by the macula in the utricle affected more by gravity?

A

Tilt of the head is detected by the macula in the utricle (horizontal plane); otoliths have a density greater than endolymph

27
Q

What does tilting the head cause?

A

Tilting the head moves the otoliths and the otolith membrane in which they are embedded. This distorts the jelly, and moves the cilia.

28
Q

What do vertically orientated macula in the saccule respond to?

A

Vertically orientated macula in the saccule respond to vertical forces e.g. movement in a lift, and provide information on orientation of head when lying down.

29
Q

What does backward tilt of the head cause?

A

Backward tilt moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs. Opposite for forward tilt.

30
Q

How is the brain kept informed of its position in space and any direction of movement?

A

The distinct pattern of information being sent from the three cristae of the semi-circular canals, and the two maculae of the otolith organs, to the brain via the vestibular nerve keeps the brain informed of it’s position in space and any direction of movement.

31
Q

What does the fact that any movement of the head displaces the centre of gravity suggest?

A

Strong associations between the vestibular centres of the medulla, where many of the sensory afferents of the vestibular nerves terminate, and the cerebellar centres which co-ordinate the postural muscles required to maintain balance.

32
Q

What evidence is there that there are associations between the vestibular centres of the medulla and the cerebellar centres?

A
  • Projections from the vestibular nuclei on one side project ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular nuclei (extraocular muscles)). -Vestibular nuclei receive input from proprioceptors signalling limb and body position, also from neck and eye muscles
  • Vestibular nuclei project via the thalamus to the cerebral cortex leads to perception of movement and body position = kinaesthesia.
33
Q

What do the vestibular system reflexes involve?

A

Involve the vestibulocoritcal and vestibulospinal tracts

34
Q

What vestibular system reflexes are there?

A
  • Tonic labyrinthine reflexes
  • Dynamic righting reflexes
  • Vestibulo-ocular reflexes
35
Q

What are the tonic labyrinthine reflexes responsible for?

A

Keep the axis of the head in a constant relationship with the rest of the body. Use information from maculae and neck proprioceptors

36
Q

What are the dynamic righting reflexes responsible for?

A

Rapid postural adjustments that are made to stop you falling when you trip. Long reflexes, involving extension of all limbs. Most profound in cats

37
Q

What does the vestibule-ocular reflexes suggest a strong association between?

A
  • Vestibular apparatus
  • Visual apparatus
  • Postural control
38
Q

What is involved in the vestibule-ocular reflex?

A
  • Afferents from the semi-circular canals project and connect (within the vestibular nuclei) to afferent fibres travelling to the extraocular nuclei and thus have strong input to influencing eye movement.
  • The visual system also sends powerful descending projections which control posture. This is easily demonstrated by the increased difficulty experienced with balance when you close your eyes.
39
Q

What happens to the balance of people with destruction of the vestibular apparatus?

A

People with destruction of the vestibular apparatus can still maintain good balance if movement is relatively slow and eyes are open. Balance is lost immediately on closing eyes

40
Q

What are the different types of vestibule-ocular reflexes?

A
  • Static reflex

- Dynamic vestibular nystagmus

41
Q

What is the static reflex?

A

When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up.

42
Q

What dynamic vestibular nystagmus?

A

A series of saccadic eye movements that rotate the eye against the direction of rotation of the head and body so that the original direction of gaze is preserved despite head rotating. Think about being on a slowly rotating roundabout.

43
Q

What happens to the eye ball when it reaches the end of its range in nystagmus?

A

The extent of eye movement is restricted; when eyeball comes to the end of it’s range of movement, it rapidly flicks back to the zero position i.e. straight ahead.

44
Q

What does the direction of rotation tell us about nystagmus?

A

By convention, direction of nystagmus is the direction of the rapid flick back R rotation leads to R nystagmus

45
Q

How can nystagmus be used test vestibular function?

A
  • Post rotatory nystagmus

- Caloric stimulation

46
Q

How is post-rotatory nystagmus tested?

A

(Only in research units). Subjects are rotated in a Barany chair. If rotate to the left then during acceleration get a left nystagmus. At the end of rotation, for about 20 seconds, during deceleration get a right nystagmus.

47
Q

Why does post-rotatory nystagmus occur?

A

Due to endolymph catching up and now pushing the cupula in the opposite direction.

48
Q

What occurs in caloric stimulation?

A

Simple test for the horizontal SCC. When the outer ear is washed with either cold or warm fluid, the temperature difference from core 37C, gets through the thin bone and sets up convection currents which affect the endolymph.

49
Q

What precautions need to be taken when carrying out caloric stimulation?

A

When irrigating the outer ear it is important to use fluid of appropriate temperature. The stimulation of the vestibular system in the absence of movement can cause nausea and vomiting

50
Q

What eye sign may seen with lesions in the peripheral or central vestibular pathways?

A

Nystagmus can also be seen with lesions to peripheral or central vestibular pathways.

51
Q

How does temperature affect nystagmus?

A
  • Warm fluid (44C) causes nystagmus towards affected side, cold (30C) causes nystagmus away from affected side (COWS – Cold Opposite, Warm Same).
  • Warm into R ear gives R Nystagmus, Cold into R ear gives L nystagmus
52
Q

What can powerful maintained stimulation of the vestibular system give rise to?

A

Powerful maintained stimulation of the vestibular system can give rise to kinetosis = motion sickness.

53
Q

What are the symptoms of motion sickness?

A
  • Nausea and vomiting, decreased BP and dizziness, sweating and pallor.
  • All of which are autonomic NS symptoms
54
Q

When is motion sickness most likely to occur?

A

Motion sickness is most likely to occur if visual and vestibular system inputs to the cerebellum are in conflict e.g. if the vestibular system indicates rotation but the visual system does not

55
Q

What does the cerebellum do in motion sickness?

A

The cerebellum generates a “sickness signal” to the hypothalamus to bring about the ANS changes.

56
Q

What is labyrinthitis?

A

Acute interference with normal vestibular function as a result of infection (labyrinthitis) = all ANS symptoms + vertigo

57
Q

What is vertigo?

A

Vertigo is the perception (hallucination) of movement in the absence of movement. There may also be nystagmus. Gross impairment of posture and balance, very disabling

58
Q

What is Meniere’s disease?

A

Ménière’s disease: Vertigo, nausea, nystagmus and tinnitus = subjective noise. Associated with over production of endolymph causing  pressure. The cause is unknown.

59
Q

What is the inner ear sensitive to attack by?

A

Inner ear is sensitive to attack by drugs such as streptomycin

60
Q

How can chronic vestibular impairment be compensated?

A

If vestibular impairment is chronic, it can be quite well compensated by the visual system. Learning circuits are set up in the cerebellum but problems occur if subjects cannot use visual information.

61
Q

What can lead to nystagmus at rest?

A

Lesions of the brain stem lead to nystagmus at rest.

Systems: Neurology AB (51 decks)

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