Vestibular function

Question 1

What is the vestibular system?

Answer 1

A sensory system essential in the control of posture and balance.

Found in the inner ear, it is a series of fluid-filled membranous tubes, (labyrinths), which are embedded in the temporal bone (petrous part)

Question 2

Describe the overall structure of the vestibular system?

Answer 2

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

Question 3

Describe the structure of the semi-circular canals found in the vestibular apparatus

Answer 3

3 semi-circular canals are:

• at right angles to each other (3 dimensions)
• have swellings at their bases, called ampulla
• contain sensory hair cells

The utricle and saccule also contain sensory hair cells

Question 4

Do the thing

Answer 4

Question 5

Cristae are structures found within the ampullae of SC canals

What do they do?

Answer 5

Sensory receptors for rotational acceleration

There are also receptors called Thermaculae for linear acceleration and head position

Question 6

What parts of the vestibular apparatus make up the otolith organs?

What are their functions?

Answer 6

Utricle & saccule

Utricle:

• back/front tilt and horizontal acceleration

Saccule:

• Vertical acceleration

Question 7

Describe the structure of the ampullae of the semi circular canals

Answer 7

Inside the Ampulla (swelling) at the bottom of canal:

• A flexible gelatinous structure called the cupula stretches across the entire width of the ampulla
• Hair cells are embedded into this cupulla
• Endolymph fills the space
• Nerves at bottom (vestibular nerve)

Cupula is distorted by endolymph movement within the canals (during rotational acceleration)

Question 8

Identify the parts of the ampulla

Answer 8

Question 9

What are the types of cilia (hair cells) and their differences

Answer 9

a single very large kinocilium and a set of progressively smaller stereocilia

Distortion of the cilia:

• in the direction of the kinocilium causes depolarisation and increased discharge of APs in the vestibular nerve.
• away from the kinocilium leads to hyperpolarization and decreased discharge of APs in the vestibular nerve

This allows the brain to determine movement in time and space

Question 10

Maculae is the term for the sensory apparatus of the saccule and utricle, so the maculae sense linear movements

What is the difference between the maculae in the saccule and in the utricle?

Answer 10

Utricle:

• Orientated in horizontal plane
• Detects tilt of head

Saccule:

• Vertical orientation
  *

Question 11

Describe the structure of Maculae of the otolith organs

Answer 11

The maculae:

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

Question 12

Label the parts of the Maculae

Answer 12

Question 13

What feature of otolith crystals means they are useful in detecting movements such as tilt

Answer 13

Otoliths have a density greater than endolymph and thus are affected more by gravity.

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

Question 14

Describe what happens on the macula when the head is tilted backwards

Answer 14

Backward tilt moves the otoliths in the direction of the kinocilium causing depolarisation and increased discharge of APs.

(Opposite for forward tilt)

Question 15

Describe how the vestibular information travels in the brain

Answer 15

Numerous neural pathways connect 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

Question 16

What are the tonic labyrinthine reflexes?

Answer 16

Keep the axis of the head in a constant relationship with the rest of the body.

Use information from maculae and neck proprioceptors

Question 17

What are the dynamic righting reflexes?

Answer 17

Rapid postural adjustments that are made to stop you falling when you trip.

Long reflexes, involving extension of all limbs.

Most profound in cats

Question 18

What are the vestibulo-occular reflexes?

Answer 18

2 reflexes (static & dynamic) that work to maintain a clear focus on an object/image during movement of the head

Static:

• Lateral tilting ==> Intorsion/extorsion
• When you tilt your head, the eyes intort/extort to compensate, so that over a certain range, the image stays the right way up.

Dynamic:

• Rotation of head ==> Rotation of eye against
• direction of nystagmus is the direction of the rapid flick back. So R rotation = R nystagmus

Question 19

How can induced Nystagmus be used to test vestibular function?

Answer 19

Caloric Stimulation:

• Test of Horizontal SCC
• Outer ear is washed with either cold or warm fluid causing convection currents in endolymph
• Warm fluid (44°C) causes nystagmus towards affected side and Cold (30°C), away
• COWS (cold = opposite, warm = same)

Post rotatory nystagmus:

• Spun in a Barany chair
• Not important

Question 20

Powerful maintained stimulation of the Vestibular system can give rise to what sickness?

Answer 20

kinetosis = motion sickness

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

cerebellum generates a “sickness signal” to the hypothalamus to bring about the ANS changes – nausea, vomiting, lowered BP and dizziness, sweating and pallor

Question 21

Identfy the labels of the ampulla

Answer 21