Lecture 21: Balance 2

Question 1

Describe stereocilia organisation of the sensory cells;

Answer 1

Sensory cells are polarised by the cilia, which are organized so that the tallest cilia are on one side.

The tall side is the direction of depolarisation

Question 2

What are otolithic organs?

Answer 2

Utricle
Saccule

Contains otoconia( Ca carbonate crystals),in a otolith membrane which the stereocilia project into.

Crystals provide mass

Question 3

What sensory cells are in the vestibular system?

Answer 3

Type 1 and 2 hair cells, but differing arrangements

Question 4

What is the region of membrane where the otoconia are sparse?

Answer 4

Striola, seperates the utricle and saccula into two parts.

Polarisation of hair cells is determined by their relationship to the striola.

Question 5

Describe polarisation of hair cells in the utricle;

Answer 5

Hair cells are polarised towards the striola, so movement of hair cells towards the striola leads to depolarisation.

Question 6

Describe the polarisation of hair cells in the saccule;

Answer 6

Hair cells are polarised away from the striola, so movement of hair cells away from the striola leads to depolarisation.

Question 7

What sort of fluid is found in the utricle and saccule, surrounding the otricle?

Answer 7

Endolymph, high K mechanism

Question 8

Whats important about the hair cell polarisation and bilateral organs

Answer 8

Bilateral pairing and polarisation of vestivular organs are key for function.

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

Question 9

Describe the resting potential of these sensory cells;

Answer 9

Constant low level current flowing through hair cells causing resting discharge in vestibular nerve.

Discharging spontaneously at rest! The Hz of depolarisation changes with stimulation. (basilar rate).

Question 10

Describe stimulation of hair cells and what happens;

Answer 10

Stimulations towards kinocelium leads to cell depolarisation and increased nerve activity

Stimulation hair cells away from kinocelium leads to hyperpolarisation and decreased activity in the vestibular nerve

Fluid moves and causes stimulation based on head movement.

In the case of onoconia, their mass in influenced by gravity and causes movement and thus sensory transduction of hair cells

Question 11

What does the frequency of vestibular nerve encode for?

Answer 11

The rate of firing determine direction and rate of movement based on bilateral inputs.

Question 12

Describe hair cell transduction in the vestibular system;

Answer 12

Mechanically gated ion channel process, same as seen on cochlea (tip links)

Background channels exist too

Tip links, open K channels, leads to Ca channel opening and neural discharge

Question 13

What does Ca entry into the cell do?

Answer 13

• Neural discharge
• Activates K channel, K exits the cell.

Therefore there is some oscillating activity between K and Ca channels.

Question 14

Describe the adaptation and tuning of sensory cells;

Answer 14

Certain head movements cause the stereocilia to move together, tip links (springs) lose tension (mechanically gated Ca channels close). This is rectified by motor proteins that climb up the actin filaments in the sensory cells and retension the tip links (springs). This causes the Ca channels to reopen.

adaptive mechanism that prevents it from being constantly activated…. (Ca closes K channels??)

Question 15

Describe the rate of vestibular nerve firing in the cristae (semicircular canals)

Answer 15

Cristae detects acceleration with increased firing.
Inertia overcome to steady state (firing) - no differential velocity between fluid in ear and head movement)
Deceleration results in firing returning to rest.

Question 16

Describe otolithic organ influence on vestibular nerve firing;

Answer 16

Otolithic organs detect head placement. Firing is constant for change in position.

i.e firing rate is not for acceleration but for head position, so only returns to normal when your head is in the correct position, not when you stop moving.

Question 17

What is actually sensed by the ampullae?

Answer 17

The differential velocity between the bone and the endolymph.

This causes distortion of the capulla by the force of the fluid (push-pull)

Question 18

What is the physiology significance of paring and cell polarisation in the semicircular canals;

Answer 18

Movement of the head will cause endolymph movement. i.e head turning

In this instance the endolymph will move in opposite directions for each ear therefore it will depolarise one ear and hyperpolarise the other, because the sensory cells are polarised in the same direction.

The differential between these two will indicate to the brain head movement.

i. e lateral semicircular canals
- Hair cells polarised in the ant direction.
- Turn head left.
- Right LSC endolymph moves post. = cells hyperpolarise
- Left LSC endolymph moves ant = cells depolarise.

fluids appear to move… in reality they dont b/c inertia, but eventually they do start to move at same velocity as head.

Question 19

What does hair cell polarisation lead to in head movement?

Answer 19

Hair cell polarisation leads to stimulation on one side and decreased activity on the other with head turning.

Bilateral pairing.

Activity changes from basal rate.

Question 20

Why does vertigo occur from acute peripheral damage?

Answer 20

The brain sense differential information between L and R vestibular systems.

At rest basal rates are equal. If one gets damage/ doesnt fire then the brain sense movement as there is a differential in firing.

Question 21

What happens eventually to compensate in vertigo?

Answer 21

Eventually compensated by increased standing discharge at right side vestibular nucleus to match left side

“Central compensation”

Question 22

Where does the vestibular nerve go to?

Answer 22

The vestibular nuclei;
divided into
- Lateral, medial
- Superior, inferior

Question 23

What are some other inputs to the vestibular nucleus?

Answer 23

Cerebellum
Spinal cord
Reticular formation
Contralateral vestibular nucleus

Descending control?

Question 24

What are some projections from the vestibular nuclei?

Answer 24

• Vestibulo-occulomotor pathways via medial longitudinal fasiculus (vestibular-occular relfex)
• Vestibulo spinal pathways
  
  • lateral throughout spinal pathways
  • Medial LF mostly to cervicle and thoracic regions innervating musculature of the neck
• Hippocampus

Question 25

What are the functions of the reflexes?

Answer 25

Central projections act to;

• Maintain equilibrium and gaze with movement (VOR)
• Maintain posture

Question 26

Which reflexes maintain posture?

Answer 26

VCR (vestibulo cervicle reflex), MVN to neck muscles via MLF and cervicle spine. Influenced by cerebellar inputs

VSR vestibulo spinal reflexes. LVN and MVN project to excite extensor motor neurones and inhibit flexor neurones. Effect on trunk and limb muscles to maintain posture and balance.

Question 27

What is VOR, nystagmas?

Answer 27

As you move your head, your eyes can fixate a position, moving slowly, but only up to a certain point, then they will rapidly flick back to centre focus (i.e if you do a 360 and try fixate on a point, once you no longer can your eyes will move very rapidly to centre.)

Question 28

What can the physiological nystagmas give information on?

Answer 28

how well the vestibular organs are working.

Question 29

How can you measure vestibular function?

Answer 29

You can use convection i.e warm or cold air/water to stimulate (warm) or inhibit (cold) firing of the vestibular organs, which causes eye movement.

(water in ear canal) water irrigation or ear canal. Effect of gravity of ear canal.

Measure eye movement through eye lids

COWS

Cold Opposite
Warm Same

for fast flick in nystamas.

Question 30

Whats a new test for semicircular canal function?

Answer 30

vHITS

Video Head Impulse Test

Question 31

What is vHITS?

Answer 31

• Test all semicircular canals
• Measures relationship between head and eye velocity. (VOR)
• Uses goggles to measure velocity and eye movement.

Can identify uni and bilateral canal lesions

Head and eye velocities should be very similar during movement.