1. Balance: The vestibular system: detector of acceleration
- Transduction in vestibular (and auditory) hair cells
- Linear acceleration (gravity: which way is up?): the macula (otolith) organs
- Angular acceleration (head turning): the semicircular canals 1

2. Where the vestibular system is2

3. It contains 5 balance organs each side
3 semicircular canals: angular acceleration (turning)
2 macular organs: linear acceleration (gravity/tilt) 3

4. Detecting linear acceleration: which way is up?4

5. Here’s how molluscs do it
Which way is up?
Here’s how molluscs do it
Statocyst organ from Hermissenda 5

6. Here’s how molluscs do it
Which way is up?
Here’s how molluscs do it
heavy stone
hair cell to detect how the stone moves 6

7. We do it much the same way...Human otolith organ
Otoconia from human ear
Otolith = “ear stone”
Otoconia = “ear dust” 7

8. There are two otolith (macula) organs
Utricular macula organ (utricle): horizontal
Saccular macula organ (saccule): vertical 8

9. Otolith organs detect linear acceleration9

10. Hair cells detect movement of the otolith membrane10

11. Structure of a vestibular hair cell
Kinocilium
“Hair bundle”:
stereocilia
Synapse 11

12. Vestibular hair cell transduction
Move the hair bundle at different angles...
Membrane potential changes depend on angle 12

13. Vestibular hair cell transduction
Depolarisation Hyperpolarisation
Glutamate release No glutamate release 13

14. Vestibular hair cell transduction
What happens when the hair bundle bends? 14

15. “Tip links” between adjacent stereocilia
Bending the hair bundle stretches the tip link 15

16. Vestibular hair cell transduction
Tip link is directly coupled to an ion channel
Stretching the tip link opens the channel
That lets ions flow: but what happens to membrane potential?
For that we need to understand the fluids bathing the hair cell: endolymph and perilymph 16

17. Endolymph and perilymph
Endolymph Perilymph 17

18. Endolymph and perilymph
High K+
+80 mV
Perilymph (extracellular fluid)
Low K+
0 mV
Intracellular
High K+
-80 mV 18

19. What happens when the channel opens?
Endolymph
High K+
+80 mV K+
Ca2+
Intracellular
High K+
-80 mV
Large driving force for K+
K+ enters hair cell and depolarises it 19

20. Vestibular hair cell and nerve activity20

21. Vestibular hair cell responses in the otolith organs21

22. All orientations are represented in the otolith organs
Arrows point towards the kinocilium
So moving the otoconia in that direction stimulates the hair cells
Utricle
Saccule 22

23. Otolith organ summary Detectors of linear acceleration
Major source of linear acceleration is gravity
So they detect head tilt and tell us which way is up
Also stimulated by e.g. starting to walk, car starting to move, plane taking off
All orientations are represented in the otolith organs: so they can detect acceleration/gravity/tilt in any direction 23

24. Angular acceleration 24

25. Angular acceleration is detected by the semicircular canals25

26. How the semicircular canals work
Cupula
Turn
Inertia of fluid 26

27. How the semicircular canals work
All hair cells in the cupula are oriented in the same direction
So a given fluid movement stimulates (or inhibits) all of them
Cupula
Turn
Inertia of fluid 27

28. Effect of a turning motion28

29. Firing of a vestibular nerve fibre
Firing rate increases when we start turning (acceleration)
Firing rate decreases when we stop (deceleration) 29

30. Firing of a vestibular nerve fibre
This is all explained by fluid inertia
(Imagine turning a bowl of soup!) 30

31. Orientation of the semicircular canals
(about 45°) 31

32. Semicircular canal summary
Detectors of angular acceleration
Major stimulus is head turning
Respond only at onset and cessation of a turning movement
Semicircular canals work in pairs and each member of a pair is stimulated by turns in the opposite direction
Cupula has same density as endolymph: so semicircular canals are unaffected by gravity 32

33. Summary: 5 balance organs each side
3 semicircular canals: angular acceleration (turning)
2 macular organs: linear acceleration (gravity/tilt) 33

34. Caloric test 34

35. How warm water affects semicircular canals
convection current
warm
perceived turning motion
external auditory meatus 35

36. How cold water affects semicircular canals
convection current
cold
perceived turning motion
external auditory meatus 36

37. Keeping the eyes on target: vestibulo-ocular reflexes37

38. Vestibulo-ocular reflexes38