Lecture 32: Vestibular physiology

Question 1

Role of vestibular system and balance in cognition?

Answer 1

Self-motion perception

Bodily self-consciousness

Spatial navigation and learning

Spatial memory and object recognition memory

Question 2

Vestibular system function?

Answer 2

Sense of dynamic and static position of the head, detecting linear and angular acceleration of the head (but not velocity)

Conscious awareness of head position and reflex control of eye movement (where is my head)

Question 3

Vertigo symptoms?

Answer 3

• Perception of motion or environment (the room is spinning) where ther is none.
• NOT lightheadedness or diziness
• Accompaied by autonomic symptoms (eg. pallor, sweating, N + V)

Question 4

Motion sickness is?

Answer 4

• A conflict between vestibular, visual, proprioception inputs with an expected internal model.
• Mismatch of inputs promotes te symptoms associated with vertigo and motion sickness

Question 5

Vestibular system inputs and outputs?

Answer 5

• A sensory organ of the inner ear that projects via the VIII cranial nerve to the vestibular nuclei in the brainstem
• Outputs from the vesibular nuclei

Question 6

Vestibular system structure?

Answer 6

Within the bony labyrinth of the temporal bone is the cochlea as well as the vestibule and three semicircular canals of the vestibular system.

It contains perilymph, a fluid similar to ECF being high in Na and low in K

Superior (anterior), Posterior and Lateral (horizontal) canals all opening into vestibule. Utricle and Saccule also found between cochlea and semicircular canals.

Ampullae of the canals contain the cristae ampullaris and these detect angular accelleration.

Utricle and Saccule contain the otolithic organs that have crystals of calcium carbonate on them giving them mass, the macula sacculus and utriculus detect static head position and linear acceleration.

Question 7

Main nerves associated with the vesibular organs/

Answer 7

Superior and inferior vesibular nerve with cell bodies in Scarpa’s ganglion exist as part of the Vesibulocochlear (VIII) nerve in the internal auditory meatus

Question 8

Innervation of superior and inferior division?

Answer 8

S = Utricle, ant part of saccule, and lateral and anterior semicircular canals

I = Posterior part of saccule and posterior semicircular canal

Question 9

Pairing of canals?

Answer 9

Lateral (horizontal) cannals of each ear are paired and lie at 30˚ to horizontal sloped backwards

Anterior is paired with posterior canal of the other ear.

NB: therefore, these organs operate as bilateral pairs

Question 10

Difference between sensory cells in cochlear and vesibular organs?

Answer 10

The main difference is that the vestibular organ cells have a KINOCILIUM that sits at one end and is a very tall proper cilia and it polarises the cell.

Question 11

How do the semicircular canals work?

Answer 11

• The semicircular canal system is sensitive to angular acceleration of the head.
• As the head rotates the endolymph remains stationary for a moment due to it’s inertia.
• This causes diplacement of the steriocilia towards the Kinocilium causing depolarisation and thus (all aligned one way in each canal)
• increased discharge of the vestibular nerve.
• a displacement away from the kinocilium cause hyperpolarisation

Question 12

How do the otolithic organs function?

Answer 12

• The macula utriculus and sacculus are arranged in “button-like” sheets
• Stereocilia project into the otolithic membrane
• Otoconia (CaCO₃ crystals) give this membrane mass meaning it is effected by gravity providing info about static head position as well as linear acceleration in the verticle and horizontal planes.

Question 13

Polarisation of hair cells in vestibular organs?

Answer 13

All of the sensory cells of the vestibular organs are polarised and arranged so that they are aligned in the same direction and hence polarise and depolarise the same.

Question 14

Stimulation of hair cells and nerve fibres?

Answer 14

• Constant low-level current flowing through hair cells causing discharge in vestibular nerve
• Stimulation towards te kinocilium leads to cell depolarisation adn increased nerve activity
• Stimulation of hair cells away from kinocilium leads to hyperpolarisation and decreased activity in vestibular nerve.
• The brain relies on the comparison in discharge patterns from both sides to determine direction of movement.

Question 15

Stimulation of the otolithic organs/hair cells of the macula?

Answer 15

• Hair cells of each macula are oriented in different directions so that te tilt of the head will depolarise some cells and hyperpolarise others in the same side.

Question 16

Projections from vestibular nuclei?

Answer 16

1. Vestibulo-oculomotor pathways via medial longitudinal fasciculus : Vestibular-Ocular reflex (VOR)
2. Vestibulo-spinal pathways : Lateral throughout spinal cord or medial LF mostly to cervical and thoracic regions innervating muscles of neck
3. Hippocampus

Question 17

Reflexes of vesibular organs?

Answer 17

Central projections act to:

1. Maintain equilibrium and gaze with movement (vestibulo-ocular reflex)
2. Maintain posture (Vestibulo-cervical and spinal reflex)
3. Maintain muscle tone

Question 18

How does vertigo occur?

Answer 18

At rest the RHS and LHS have the same discharge rate and so interpret no movement. However, if you have acute peripheral damage then the discharge rates change and there is an imbalance between the sides.