L40 - Vestibular system

Question 1

3 major functions of the vestibular system?

Answer 1

1. Subjective sensation of motion, spatial orientation
2. Stabilization of eyes in space during head movement - vestibulo- ocular reflex
3. Adjustment of muscle activity and posture to prevent falling- vestibulo-spinal reflex

Question 2

Describe how hair cells in the vestibular apparatus operate and determine the polarization axis during head movement?

Answer 2

Hair cells: kinocilium, stereocilia: relative orientation determine the polarization axis

Head movement introduces shearing force:

• Bending of stereocilia towards kinocilium depolarizes receptor hair cell&raquo_space; increase in firing of the vestibular branch of CN VIII
• Bending of kinocilium towards stereocilia hyperpolarizes the hair cell&raquo_space; decrease firing of nerve

Question 3

Compare the function of the semicircular canal vs Otolith end organs?

Answer 3

Semicircular canal = Detect Angular acceleration, rotation of the head in 3-dimensional space

Otolith end organs = Detect Linear acceleration + Static head tilts + Gravity (hypergravity, hypogravity, microgravity)

Question 4

Describe the organization and function of the different otolith end organs?

Answer 4

Hair cells are organized in all directions

• Utricle on horizontal plane: detect multi-directional orientation
• Saccule in vertical plane: detect up-down orientation

Question 5

Describe the physiology of otolith end organs during head movement?

Answer 5

Translational head movement in any particular direction will displace hair bundles

2 concurrent processes:

• Increase the excitability of (depolarize) a subgroup of hair cells
• Decrease the excitability of (hyperpolarize) another subgroup on the same otolith organ

Question 6

Function of Otoconia crystals?

Answer 6

Otoconia crystals anchor otolithic membrane = gelatinous layer that embeds cilia together in position

Question 7

Describe the organization of semicircular canals?

Answer 7

Canals on 2 sides of the head work as complementary pairs with opposite actions:
i.e. increase in firing of vestibular nerve on one side is accompanied by decrease in firing on the other side

3 canals at each ear arranged perpendicular to each other:
 L / R horizontal pairs
 L anterior / R posterior pairs
 R anterior / L posterior pairs

Question 8

Describe the synchronous increase/ decrease in excitability of hairs cells in semicircular canals during the start of head rotation.

Answer 8

onset of head rotation (acceleration):

1) viscous endolymph within the canal (horizontal ducts) lags behind due to inertia
2) bends stereocilia of hair cells towards kinocillium
3) depolarize / increase in firing rate of the vestibular nerve

All hair cells have the same structural / functional polarization axis
» Hyperpolarization of the opposite semicircular canal hair cells (kinocillium bends to stereocillium)

Question 9

Describe the synchronous excitability of hairs cells in semicircular canals when head rotates at constant speed?

Answer 9

rotation continues but attains a constant speed:

1) cilia revert back to normal position
2) activity of vestibular nerve also returns to normal
3) subject could not detect any rotational signal without other sensory cues

Question 10

Describe the synchronous increase/ decrease in excitability of hairs cells in semicircular canals during the end of head rotation.

Answer 10

End of rotation = deceleration:

1) endolymph displaces in an opposite direction due to inertia (post-rotatory event)
2) decrease in firing rate of the vestibular nerve in one side of semicircular canals; increase firing in the opposite side

Question 11

List the 3 Vestibular pathways.

Answer 11

• Vestibulo-cortical pathway: Sensation to Cerebral cortex
• Autonomic nervous system (motion sickness)
• Sensori-motor interaction:
  i) Vestibulo-ocular reflex
  ii) Vestibulo-spinal reflex

Question 12

Function of Vestibulo-spinal reflex?

Answer 12

Balance of posture via actions on antigravity muscles

such as extensors of the neck, trunk and limbs

Question 13

2 subtypes of Vestibulo-ocular reflexes?

Answer 13

1) Otolith-related reflex: for lateral head tilt and linear acceleration
2) Canal-related reflex: for Rotation in the dark to induce nystagmus

Question 14

Role of cerebellum in vestibulo- ocular reflex pathway?

Answer 14

Canal deploarization to vestibulocerebellum (flocculus; flocculonodular lobe):

> > Controls open-loop reflex
Adjusts output of Purkinje cell
inhibits vestibular nucleus output to ocular motor nuclei via interneurons, motoneurons

Also modulates Vestibulo-spinal and Reticulospinal tract by inhibition of vestibular nucleus

Question 15

Describe the otolith-related eye movements.

Answer 15

1) Static head tilt:
- 50 degrees lateral tilt&raquo_space; 5 degrees ocular counter-rolling (doll’s eye reflex)
- Forward pitch (dip head)&raquo_space; eye elevation

2) Linear acceleration, Increase acceleration causes increase in eye elevation

Question 16

Function of canal-related vestibular nystagmus?

Answer 16

To maintain sharp visual acuity during head rotation

Question 17

Describe the eye movement phases in canal-related vestibular nystagmus at the start of head rotation?

Answer 17

1) Compensatory slow phase (2 s): eyes move Equal and opposite to head motion to focus on object
2) Fast phase (0.2 s): anticipatory eye movement to reset the eyeball to periphery

Question 18

If head to starting to rotate to the left, describe the neural control of the Left and right eye rectus muscles.

Answer 18

Compensatory slow phase:

Fluid move in ampulla:

1) Left horizontal canal depolarized&raquo_space; depolarize left vestibular nucleus&raquo_space; stimulate left CN III and left medial rectus + right CNVI and right lateral rectus
2) Right horizontal canal hyperpolarized&raquo_space; inhibit right CN III and suppress right medial rectus + inhibit left CN VI to suppress left lateral rectus

Question 19

Which cortical region processes vestibular signals? List sequence of vestibulo-cortical tract?

Answer 19

Vestibular nucleus&raquo_space; thalamus&raquo_space; entorhinal cortex&raquo_space; parietal cortex:

i) Superior sylvian gyrus
ii) Inferior intra-parietal sulcus

> > process Subjective Vestibular Sensation

Question 20

What sensory information is integrated for spatial awareness?

Answer 20

Somatic/somatosensory + Proprioceptive + Labyrinth = Subjective vestibular sensation / perception (conscious awareness of body orientation)

Question 21

How to identify the source of dizziness?

Answer 21

Vestibular:

• Spinning sensation
• Episodic
• Nausea and vomiting

Non-vestibular:

• Lightheaded, floating sensation
• Constant
• Perspiration, pallor, palpitation

Question 22

Define Benign Paroxysmal Positional Vertigo?

Answer 22

short lasting, but severe, room-spinning vertigo

• Benign: not very serious or progressive
• Paroxysmal: sudden and unpredictable onset
• Positional: change in head position
• Vertigo: causing a sense of dizziness

Question 23

List 3 types of vertigo with increasing duration?

Answer 23

1) Benign positional vertigo (sec)
2) Meniere’s syndrome (min)
3) neuritis, trauma, infarction induced vertigo (days)

Question 24

Pathogenesis of Meniere’s syndrome?

Answer 24

Fluctuating pressure of the endolymph, caused by:

i) blockade of the drainage system, too much fluid secreted by Stria Vascularis
ii) Constantly distort cupula (jelly-like mould capping hair cells)

Labyrinth membranes become dilated – “hydrops”

Question 25

Pathogenesis of head trauma causing constant vertigo?

Answer 25

Head trauma cause cupulolithiasis:

Displaced (dislodged) otoconia (on surface of urticle) enter posterior semicircular canal

> > block normal endolymph movement
usually relieved by Changing head position to dislodge otoconia crystals

Question 26

What causes physiologic dizziness?

Answer 26

visual-vestibular conflict