Vestibular System

Question 1

What is the function of the vestibular system?

Answer 1

• Mediates posture & movement of body and eyes relative to the external environment

Question 2

What is it made up of?
What are the sacs and ducts present?
note that the sacs have a combined name- what is this?

Answer 2

• Makes up the membranous labyrinth (filled with endolymph)
• Two sacs: the UTRICLE and SACULE (otolithic organs) in the VESTIBULE
• Four ducts: 3 Semicircular ducts in canals [+cochlear duct -> hearing]
o Note ducts = membranous labyrinth, canals = bony
o The SEMICIRCULAR DUCTS ALL drain into the UTRICLE

o The COCHLEAR duct drains into the SACCULE

The semicircular ducts & otolithic organs are suspended within the bony labyrinth rather than being stretched across it like the cochlear duct. Pressure changes caused by movement of the stapes are therefore equally dist

Question 3

Where are the SENSORY CELLS located? What are they CALLED? And what do they do?

Answer 3

• Functionally, sensory receptors for balance are organized into unique structures that are located in each of the components of the vestibular apparatus.
• Located in the MACULA (of utricle & saccule) & CRISTA AMPULARIS (within the ampulla of the semicircular ducts)

Question 4

What do the MACULAR do?

Answer 4

Maculae
• Detect linear motion of the head with respect to gravity - Changes in head position; They depend on “hair cells” as well

•Kinocilium & stereocilia on sensory cells: Kinocilium have microtubules

Question 5

What is the membrane that overlies the kinocilium and sterocilia on sensory cells called?

Answer 5

Otolithic membrane – gelatinous material containing calcium carbonate crystals, denser then endolymph

• > This is a gelatinous layer that overlies the hairs.
• called OTOCONIA
• > Above the topmost layer are calcium carbonate crystals (denser)

Question 6

What is the AUDITORY membrane that overlies the hair cells in the cochlear called?

Answer 6

Tectorial Membrane

Question 7

Describe what hapens when the hairs deflect

Answer 7

• Tilt in head → deflection of hairs
• Saccule & utricle give you information regarding position of the head

Opens potassium channels; K+ ions flow into the cell → depolarize cell membrane
Opens voltage gated calcium channels → calcium ions enter the cell
Trigger release of transmitter (aspartate or glutamate)
Afferent fibers discharge

Deflection towards longest hairs (kinocilium) depolarizes them.
Towards the shortest cell – hyperpolarizes them

Question 8

Which organs hairs would be stimulated tilting head forward/back

Answer 8

• Tilting head forward/backwards maximally stimulates hair cells of the utricle
o This is due to the positioning of the hair cells

Question 9

Which organs hairs would be stimulated head side/side

Answer 9

Tilting head side ways maximally stimulates hair cells of the SACCULLE

Question 10

Where is the crista ampularis located? Crista Ampullaris

Answer 10

• Located in ampulla of semicircular duct (dilation at one end of the duct)
o There are 3 semicircular ducts, more or less 90 degrees to each other
• Detect rotational movement of the head
• Rotation of head in any plane can be detected due to spatial orientation of semicircular canals
• Horizontal (lateral), anterior (superior), posterior semicircular ducts

Question 11

What are the hairs of the Crista Ampularis embedded in?

Answer 11

CUPULA
The hair cells of the crista ampullaris are embedded in the cupula (spans thickness of ampulla) – a glatinous mass, the same density as endolymph
All the hair cells of a given crista are aligned with their kinocilia facing in the same direction, so deflection of the cupula in one direction causes the afferents that innervate that crista to increase their firing rate, and deflection in the opposite direction causes them to decrease their firing rate

• When rotation is initiated:
- >Endolymph lags because of inertia -> Distorts the cupula, hairs are deflected- > As rotation continues & terminates, cupula deflects in the opposite direction

Question 12

Describe the relative orientation of semicircular ducts:

Answer 12

• Horizontal canal – tilted backward 30 degrees
• Anterior canal on one side is parallel to posterior canal on contralateral side
o Movement of endolymph in one – you will see corresponding movement in the opposite one

Question 13

Given the example of the head rotating in a horizontal plane to the LEFT, describe the deflection

Answer 13

• Left turn → receptor depolarization & afferent excitation in horizontal semicircular duct on the left side; afferents on right side are inhibited
o Contralateral side deflects hairs to shorter one
o Left side exited, right side inhibited

Question 14

What is the Vestibular Ganglion?

What is another name for it?

Answer 14

• Contains bipolar afferent neurons
• Peripheral processes innervate hair cells
• Central processes form vestibular component of VIII

Central processes enter the brainstem & synapse with vestibular nuclei ->Vestibular component of VIII terminates in the vestibular nuclei

Question 15

where is the vestibular nuclei located?

Answer 15

• There are 7 vestibular nuclei. Together they extend through quite a large section of the midbrain
• from the rostral medulla almost to caudal pons

Question 16

What are the other inputs to the vestibular nuclei?

Answer 16

Other input into vestibular nuclei, besides vestibular component of CN VIII
• Reciprocal vestibular projections
• Cerebellum (flocculonodular lobe)
• Spinal cord (some fibers from posterior spinocerebellar tract)

Question 17

What are the OUTPUTS from the Vestibular nuclei?

hint- descending and ascending

Answer 17

Lateral vestibulospinal tract (IPSILATERAL) – postural adjustments with change in environment
Eg spinning around → dizzy, staggering, exaggerated activity of LVST

Medial vestibulospinal tract (bilateral) – compensatory neck movements

• Innervates cervical motor neurons
• You need both these tracts to work so that you can adjust your posture
• both of these travel in the MEDIAL LONGITUDINAL FASCICULUS

Cerebellum (flocculonodular lobe) - info about head movement to influence eye movement
Occulomotor, abducens, trochlear nuclei (control of eye movements – changes in position of the head needs to communicate with these nuclei)
Reticular formation (brainstem – neuronal control of emesis)
Thalamus

Question 18

What does vomiting require?

Answer 18

Vesibular nuclei;; vagal afferents (from GI tracts); Area postrema (toxins, prengncies), Cognitivie (memory, anxiety)
—->
Emetic Centre -> LMNs- < oesophagus mm, valve muscles, abdo muscles

Question 19

Vestibular info goes to the thalamus and then to –> ?

Answer 19

projections to pre & post central gyrus

Question 20

Explain the Vestibuloocular reflex

Answer 20

• Generates eye movements to compensate for head movement. It is modifiable, may be suppressed or adjusted by flocculus of cerebellum. Conscious control can come in

Semicircular duct – if you rotate your head to the left, you get deflection of the cupula against longest hairs on the LHS → excitation. Output from the vestibular nuclei → contralateral abducens nucleus (links up with lateral rectus). At the same time, neurons project out from the MLF and terminate on contralateral oculomotor nucleus (medial rectus). So when you move your head left, the VOR acts to abduct the right eye (abducens) and adduct the left eye (medial rectus) to maintain gaze on the subject.
• This reflex is modifiable – it can be suppressed or adjusted by input from the cerebellum

Question 21

Explain the Oculocephalic Reflex (doll’s eye movement)

Answer 21

A form of VOR - involves the abducens & occulomotor nucleus & MLF
In a comatose patient, the absence of this reflex suggests damage to the brainstem

Question 22

what is nystagmus

Answer 22

RAPID INVOLUNTARY MOVEMENT OF THE EYE
• Very rapid eye movement in the opposite direction to that in VOR when rotation is large
o It is a normal response
• Named for the direction of fast component
o Eg left-beating nystagmus
• Fast eye movement involves signals from the reticular formation
• Nystagmus & VOR can be used to assess level of brainstem damage in patients

Question 23

Optokinetic nystagumus- what is this

Answer 23

induced by moving visual stimuli

Question 24

What is caloric nystagmus

Answer 24

Endolymphatic convection current induced by cool water - - - Excitation of vestibular organs is caused by movement of the endolymph,
- Instead of using movement of head to create endolymph movement, you can induce this by a change in temperature
• Head is tilted back 60 degrees
• Cool water in right ear → effect is similar to rotation of head to the left → very fast movement of the eye to the left –> Nystagmus in left eye

Mneumonic – COWS: cool, opposite; warm, same

Question 25

What would a tumour involving the vestibular nerve show up as

Answer 25

Diziness, nausea, spatial disorientation