Lecture 26: Vestibular Physiology

Question 1

What is balance?

Answer 1

• Perception of balance (maintaining steady position, posture) and spatial orientation (position in environment).

Maintaining a steady position, posture, position in the environment

• Coordination of sensory and motor systems

Question 2

What are the forms of balance?

Answer 2

Two forms

• Static postural equilibrium
• Dynamic balance

= Prevents falling when standing or moving

Question 3

How is a sense of balance achieved?

Answer 3

Highly integrated motor and sensory functions

Involves integration of:
- Vision, vestibular, proprioceptive inputs to generate motor outputs

Question 4

Why do we have balance?

Answer 4

Needs ability to sense surroundings and position of body in environment

Postural equilibrium “centre of mass is controlled over the base of support”

Question 5

How is balance maintained?

Answer 5

Controlled by reflexive (i.e neck muscles and vestibular senses prevents falls) and conscious actions

Static and dynamic adjustments to muscle to maintain stable position

Question 6

Describe the role of the vestibular system in balance:

Answer 6

Vestibular = head movements (and in essence rest of body)

• Senses dynamic and static head position
• Detect linear and angular head acceleration
• Conscious awareness of head position and reflex control of eye movements

Question 7

What is the vestibular occular reflex?

Answer 7

Maintenance of eye gaze with motion

Question 8

What is oscillopsia?

Answer 8

Sensation that surrounding environment constantly in motion; usually symptom of conditions that affect eye movement of ability to stabilise images, especially during movement

Question 9

How sensitive is the vestibular system?

Answer 9

Vestibular system is exquisitely sensitive and finely balanced.

Question 10

What can an acute loss of vestibular function lead to?

Answer 10

An acute loss of vestibular function can have catastrophic effects on balance, causing VERTIGO (sense of losing balance, movement) disorientation and nausea.

Question 11

What is vertigo?

Answer 11

• Perception of motion or environment (the room is spinning) when there is none
• Accompanied by autonomic symptoms i.e pallor, sweating, nausea, and vomiting

Question 12

What is motion sickness?

Answer 12

Conflict between vestibular, visual, proprioceptive inputs compared with an expected internal model

Question 13

What is a longer term sense of motion i.e after boat or long plane ride called?

Answer 13

Mal de debarquement

Question 14

Can loss of vestibular function be compensated for?

Answer 14

Chronic and gradual loss of vestibular function may have limited symptoms because of compensation in central nuclei.

Question 15

What are the vestibular structures of the inner ear?

Answer 15

Three membranous semicircular canals, in three planes

1. Super/anterior (vertical)
2. Lateral (Horizontal)
3. Posterior (vertical)

Two otolithic organs

1. Utricule
2. Saccule

Question 16

Describe the ampullae of semicircular canals:

Answer 16

Contain the cristae ampullaris that detects angular acceleration

Question 17

Describe the utricle and saccule:

Answer 17

Contain otolithic organs, macula sacculus and utriculus that detect static head position and linear acceleration

Question 18

Describe the structure of the vestibular nerve:

Answer 18

Superior and inferior vestibular nerve with cell bodies in scarpas ganglion part of the vistibulocochlear nerve in internal auditory meatus.

Question 19

What does the superior division of the vestibular nerve innervate?

Answer 19

Utricle, anterior part of saccule, and lateral and anterior semicircular canals

Question 20

What does the inferior division of the vestibular nerve innervate?

Answer 20

Posterior part of saccule and posterior semicircular canal

Question 21

Describe the pairings of the canals:

Answer 21

Lateral (horizontal) canals of each ear are paired

Anterior canal is paired with posterior canal of opposite ear.

Question 22

What is an important point regarding the function of the vestibular organs?

Answer 22

Vestibular sensory organs operate bilaterally as pairs

Question 23

Whats the function of the cristae ampullularis?

Answer 23

Each semicircular canal has one. It contains the capula (gelatinous mass) which covers the type 1 and 2 hair cells.

The canals allow fluid (endolymph) to move with angular acceleration and distort the capula and thus hair cells. Linear acceleration results in equal force applied to the capula thus nothing is sensed.

Question 24

Describe the sensory cells of the vestibular organs:

Answer 24

Similar to those in cochlear except have KINOCILIUM which polarise the hair cells. All kinocilium are on the same side of each hair cell.

Thus aligned in the same direction, movement towards kinocilium = depolarisation. Movement away = hyperpolarisation and decreased nerve activity.

Question 25

Describe the structure of the otolithic organs:

Answer 25

Hair cells in otolithic organs arranged in button like (Macula) sheets. Stereocilia prokect into otolithic membrane. Otoconia (CaCO4 crystals) (give mass and thus sense gravity).

Question 26

Whats another very important point when it comes to the vestibular system?

Answer 26

Hair cells of the vestibular organs are polarised and arranged so that they are aligned in the same direction.

Question 27

What is benign paroxysmal postural vertigo?

Answer 27

Displaced otoconia travel to crista ampullaris of posterior canal and turn it into a gravity receptor.

Question 28

What is the sensory transduction of vestibular hair cells?

Answer 28

Same as seen in cochlear

Question 29

Describe the stimulation of hair cells and nerve fibres:

Answer 29

• Constant low-level current through hair cells causes resting discharge in vestibular nerve
• Stimulation towards kinocilium -> Cell depolarisation and increased nerve activity
• Stimulation away from kinocilium -> hyperpolarisation and decreased nerve activity

Question 30

Describe neural firing when the ampulla senses acceleration:

Answer 30

Crista detects acceleration with increased firing. Inertia overcome in steady state and firing returns to rest.

I.e constant motion = no firing

Question 31

Describe the neural firing from the otolithic organs:

Answer 31

Otolithic organs detect head placement; Firing constant for change in position

i.e firing returns to basal discharge only when head tilt finishes.

Question 32

Whats the implication of the hair cells being polarised in the vestibular organs?

Answer 32

Because of hair cell polarisation rotation of head leads to stimulation on one side and decreased activity on other side

Question 33

Differing rates of neural firing from both sides allow what?

Answer 33

Comparison of bilateral vestibular nerve discharge pattern and rate provides information on direction and acceleration of head movement.

Question 34

Describe vertigo from peripheral nerve damage:

Answer 34

• At rest, theres no difference between left and right side standing discharge rate = brain interprets as no movement
• Following peripheral nerve damage no discharge from on side is compared with standing discharge from other side which is interpreted as movement = vertigo

Question 35

How is vertigo from peripheral nerve damage eventually compensated?

Answer 35

Eventually compensated by increased standing discharge from damaged side nucleus to match undamaged side.

Central Compensation

Question 36

What is the vestibuloccular reflex? How does it work?

Answer 36

Maintain equilibrium and gaze with movement

Vestibular information is integrated with motor information to determine head orientation. The CNS sounds motor outputs to the eye muscles to ensure gaze is maintained relative to the head orientation.