HUF 2-47 Vestibular system and postural control

Question 1

Why do we need vestibular system?

Answer 1

1. “Which way is up?”
2. “Where am I going?”
3. Head-eye coordination
4. Postural eqm.

Question 2

Semi-circular canals

Answer 2

• Sense head rotation (hair cells in ampulla of semicircular adjoining utricle)
• Ampulla:
  hairs of hair cells embedded w/i gelatinous diaphragm called ampulla
• Hair cells transduce mechanical stimuli into neural signals
• 3 planes perpendicular to each other => each canal maximally sensitive to rotations in its plane

Question 3

Otolith organs

Answer 3

• Sense linear accelerations (incl. head orientation relative to gravity)

Question 4

Mechanisms behind sensing head rotation

Answer 4

• Cupula displaced by movement of endolymph fluid inside semicircular canal (∵ inertia)
  => Deflects hairs of hair cells
  => Neural signals related to head rotation
• Head rotating counterclockwise
  => relative clockwise movement of endolymph fluid

Question 5

Anatomy of hair cells in utricle

Answer 5

• Hairs of hair cells project ototolithic membrane (covered by CaC03 particles - otoconia)
• Movement of otoconia relative to epithelium of utricle
  => bend hairs of hair cells
  => Translational acceleration detected

Question 6

Vestibulo-ocular reflex (VOR)

Answer 6

• Head-eye coordination
• Head turning to L.
  => ↑ firing in L. + ↓ firing in R.
  => Eye movement directed to R. (away from side with ↑ firing)
  => Steady vision
• Excitatory + inhibitory connections
• Gain of VOR is adaptable (glasses for shortsightedness / farsightedness)
• Modulated by vestibuocerebellum (-ve. from Purkinje cell)

Question 7

Excitatory connections of vestibulo-ocular reflex

Answer 7

(Head turns L.)
Ipsilateral:
- Vestibular nu.
=> Ascending tract of Deiters
=> Oculomotor nu. (+ abducens nu.)
=> CN3
=> L. medial rectus

Contralateral:
- Vestibular nu.
=> Abducens nu.
=> CN6
=> R. lateral rectus

Question 8

Inhibitory connections of vestibulo-ocular reflex

Answer 8

Vestibular nu.
=> Abducens nu.
=>
Ipsilateral: L. lateral rectus
Contralateral: Oculomotor nu. => R. medial rectur

Question 9

VOR during persistent head rotation => Nystagmus

Answer 9

1. Slow Phase: direction of eye movement for steady vision
2. Eyes reach limit of orbital range
3. Quick Phase: Re-setting eye movement in direction of head movement

e.g. Head turns to L.
=> Eyes move to R. (slow)
=> Eyes move to L. (quick)
∴ Left-beating nystagmus

Question 10

Pathological nystagmus

Answer 10

Quick phase moves towards healthy side (side with more firing from vestibular n.)
e.g. L. vestibular neuritis

Question 11

What is maintaining postural eqm.?

Answer 11

Active resistance to ext. forces acting on body (for locomotion / other tasks)

e. g. gravity
- unsteady surface (on boat)
- small base of support (on string)

Question 12

Strategies of postural responses

Answer 12

• Complex ms. activation patterns based on strategy and biomechanical context

1. Bring centre of mass back over base of support
   - Hip flexors activated when standing on narrow beam
2. Widen base of support
   - Ankle extensors activated when standing on platform

Question 13

Insufficiency of spinal cord to generate motor responses for balance

Answer 13

• Ankle flexors normally elicited by backward perturbation
• Spinal cord transection
  => No evoked activity in flexor ms.

Question 14

Maintenance of balance and postural regulations from brainstem

Answer 14

• Vestibular nu.; reticular formation
• Expression of other skilled / voluntary movements requiring distal ms.
• Work tgt with motor cortical areas during voluntary movement

1. Medial vestibulospinal tract
   - Bilateral terminations
   => Activate neck ms. in response to head rotation signals
   => Head position
2. Lateral vestibulospinal tract
   - Ipsilateral
   => Proximal ms. (limb extensors for resisting gravity)

Question 15

Role of reticular formation in postural maintenance

Answer 15

• Feedforward, anticipatory motor commands
• Under control of higher motor areas (motor cortical areas; hypothalamus)

Functions of RF:

• Cardiovascular and respiratory control
• Eye movement coordination
• Sleep and wakefulness
• Postural maintenance

Question 16

Example demonstrating role of reticular formation in postural maintenance

Answer 16

1. Subject instructed to pull on a handle in response to an auditory tone
2. Pulling
   => Biceps brachii contract
   => Pull whole body forward
3. Gastrocnemius activated before biceps
   => Counteracts forward motion from biceps activation
   => Stabilise posture

• Effect of biceps contraction anticipated (feedforward control) by motor system

Question 17

Feedforward and feedback control in postural response

Answer 17

Refer to diagram

Question 18

Two routes from motor cortical areas to spinal cord for movement / posture

Answer 18

• Direct: Motor cortex => Spinal cord (distal ms.)
• Indirect: Motor cortex => RF => Spinal cord (proximal ms.)
• Corticospinal tract important for distal ms. and fine movement
• Direct route severed (transect corticospinal tract)
  => Monkeys could still climb and jump
  NOT reach and grasp objects with hands