1. Vestibular analyzer

4. Function of vestibular system
Vestibular system - organ of position and balance sense - placed in the semicurcular canals in petrous bone lying in three mutually perpendicular planes. The canals start in utricle, which is connected with sacculus. Both parts are placed in vestibulum communicating with ductus cochlearis.
One outlet of each canal is transformed in ampulla, divided by the ampullary crist into two parts. Macula utriculi is in the lower part of utricle, the macula sacculi in sacculus. The crists and ampullae are covered by sensory epithelium composed of hair-cells. There are also gelatinous cupulae on ampullary crists and the statoconia membranes in maculae. Their function is to stimulate stereocilia of sensory cells. The statoconia are crystals of CaCO3 - it increases the mass of gelatinous membranes.

5. Function of vestibular system
The semicircular canals allow analyse the rotational motion of the head. Receptors of ampullary crists react on angular acceleration. The cupulas of crists work as valves, which are deflected by streaming endolymph and stimulate the hairs of sensory cells by bending – depolarisation or hyperpolarisation takes place.
The receptors of utricle and sacculus react on linear acceleration and gravitation. When changing the head position, the membrane with statoconia shifts against hairs of sensory cells - excitation arises. Important for keeping erect position - static reflexes.

6. The Vestibular System generates compensatory responses to head motion
postural responses
ocular-motor responses
visceral responses
To achieve this the vestibular system measures
Head rotation
Head acceleration
Einstein’s equivalency theorem states that an accelerometer cannot distinguish between translational accelerations and tilts

7. SSC Coplanar Pairing
Spatial arrangement of the 6 SSC cause 3 coplanar pairings
R & L lateral, L anterior and R posterior; l posterior & R anterior; R & L horizontal
Allows for a Push-Pull arrangement of the two sides (e.g., as head turns right, right SSC will increase firing rate & the left SSC will decrease firing rate)
Advantages
sensory redundancy
common mode rejection/noise
assist in compensation for sensor overload

8. Inhibitory Cutoff
Depolarization of the ipsilateral hair cells occurs during angular head movements
Hyperpolarization of contralateral hair cells occurs at the same time
Hair cells are only able to hyperpolarize to what they were at rest = cut off of inhibitory influences from the movement going in the opposite direction even if the ipsilateral hair cells continue to spike higher firing rates

9. Purves 2001

10. Otoliths Utricle and saccule Otolith sensory structures
Maculae
Otolithic membrane
Otoconia
Movement of gel membrane & otoconia cause a shearing action to occur over the hair cells → sensitivity of otoliths

11. Otolith Function Respond to: Linear head motion on acceleration
Static tilt
Two organs respond to respective accelerations or tilts in their respective planes
Saccule has vertical orientation of maculae
Utricle has horizontal orientation of maculae

12. Bear 1996

13. Hair Cells 2 types: kinocilium & stereocilia
Sensory structures for the peripheral end organs (maculae and ampula)
Hyperpolarized or depolarized depending upon the direction of deflection of the stereocilia (movement of stereocilia towards the kinocilium causes depolarization of the hair cell)
Affect the firing rate of the primary vestibular afferents to the brainstem

14. Bear 1996

15. Striola of the Macula Striola serves as a structural landmark
Contains otoconia arranged in narrow trenches, dividing each otolith
Orientation of the hair cells change over the course of the macula
Allows otoliths to have multidirectional sensitivity

16. Principles of the Vestibular System
Tonic firing rate
Vestibular Ocular Reflex
Push-pull mechanism
Inhibitory cutoff
Velocity storage system

17. Vestibular Apparatus
Vestibular apparatus and cochlea form the inner ear
Vestibular apparatus – provides sense of equilibrium
consists of otolith organs (utricle and saccule) and semicircular canals

18. Vestibular Apparatus
Sensory structures located within membranous labyrinth
filled with endolymph and located within bony labyrinth

19. Vestibular Apparatus
Utricle and saccule provide info about linear acceleration
Semicircular canals, oriented in 3 planes, give sense of angular acceleration

20. Vestibular Apparatus Hair cells – receptors for equilibrium
Each contains stereocilia (hair-like extensions)
1 of these is a kinocilium---a true cilium

21. Vestibular Apparatus
Stereocilia bend toward kinocilium – hair cell depolarizes
releases NT that stimulates CN VIII
When bent away from kinocilium – hair cell hyperpolarizes
In this way, frequency of APs in hair cells carries information about movement

24. When stereocilia are bend away from kinocilium, hair cell is hyperpolarized, i.e. inhibited. It occurs because acceleratory force acts to flow of fluid in semicircular canals during circular motion of the head or whole the body.
Hair cells are located along crista ampularis and protect their cilia in cupula. Hair cells are secondary sensor cells, which synapse with neurons. Axons of these nerve cells compose vestibular nerve.

25. Statoconia membrane in sacculus
cellbio.utmb.edu/.../Ear/ organization_of_the_inner_ear.htm.

26. Utricle and Saccule Have a macula that contains hair cells
Hair cells embedded in gelatinous otolithic membrane
contains calcium carbonate crystals (otoliths) that resist change in movement

27. Utricle and Saccule Utricle sensitive to horizontal acceleration
Hairs pushed backward during forward acceleration
Saccule sensitive to vertical acceleration
Hairs pushed upward when person descends

28. Semicircular Canals Provide information about rotational acceleration
Project in 3 different planes
Each contains a semicircular duct
Crista ampullaris – where sensory hair cells are located
10-42

29. Semicircular canal function
Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia.
Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.

30. Semicircular Canals
Hair cell processes embedded in cupula of crista ampullaris
When endolymph moves cupula moves
Sensory processes bend in opposite direction of angular acceleration

32. From Kandel and Schwartz

34. Ascending Pathways Vestibular nerve Vestibular nuclei Cerebellum
Oculomotor complex
CN 3, 4, and 6
Along with vestibulospinal reflexes coordinate head and eye movements

35. Relay Centers Thalamus Vestibular Cortex
Connection with vestibular cortex and reticular formation → arousal and conscious awareness of body; discrimination between self movement vs. that of the environment
Vestibular Cortex
Junction of parietal and insular lobe
Target for afferents along with the cerebellum
Both process vestibular information with somatosensory and visual input

37. Tonic Firing Rate
Vestibular nerve and vestibular nuclei have a normal resting firing rate ( cycles/sec)
Baseline firing rate present without head movement
Tonic firing is equal in both sides; if not, a sense of motion is felt e.g., vertigo, tilt, impulsion, spinning
Excitation and inhibition of the vestibular system can then occur from stimulation of the hair cells
Spontaneous recovery with light

38. Vestibular-Ocular Reflex (VOR)
Causes eyes to move in the opposite direction to head movement
Speed of the eye movement equals that of the head movement
Allows objects to remain in focus during head movements

39. Compensatory Eye Movements
VOR
Optokinetic reflex
Smooth pursuit reflex, saccades, vergence
Neck reflexes
combine to stabilize object on the same area of the retina=visual stability

40. Vestibular Processing Gain
Keeps eye still in space while head is moving
Ratio of eye movement to head movement (equals 1)

41. Vestibular Processing Velocity Storage Mechanism
Perseveration of neural firing in the vestibular nerve by the brainstem after stimulation of SSC to increase time constant (10sec.)
SSC respond by producing an exponentially decaying change in neural firing to sustained head movement
Otolith & somatosensory input also drive mechanism

42. VOR Dysfunction Direction of gaze will shift with the head movement
Cause degradation of the visual image
In severe cases, visual world will move with each head movement

43. Oscillopsia
Visual illusion of oscillating movement of stationary objects
Can arise with lesions of peripheral or central vestibular systems
Indicative of diminished VOR gain
motion of images on fovea
diminished visual acuity

44. Cerebellum Monitors vestibular performance
Readjusts central vestibular processing of static & dynamic postural activity
Modulates VOR
Provides inhibitory drive of VOR (allows for VORc)

45. Descending Pathways
Provide motor output from the vestibular system to:
Extraocular muscles (part of VOR)
Spinal cord & skeletal muscles (generate antigravity postural activity to cervical, trunk & lower extremity muscles)
Response to changing head position with respect to gravity (righting, equilibrium responses)

46. Vestibulospinal Reflex (VSR)
Generates compensatory body movement to maintain head and postural stability, thereby preventing falls

48. Neural Pathways for Equilibrium and Balance

49. Nystagmus and Vertigo
Vestibular nystagmus – involuntary oscillations of eyes
occur when spinning person stops
Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline
Vertigo – loss of equilibrium
Natural response of vestibular apparatus
Pathologically, may be caused by anything that alters firing rate of CN VIII
Often caused by viral infection

50. Otolith Ambiguity Translation Tilt
Gravity
Acceleration
The otolith organs transduce acceleration
Thus, translational accelerations and tilts of the head cause similar activity on otolith afferents.
Otolith information is therefore ambiguous (Einstein’s equivalency principle) and must be resolved in order to provide proper stabilizing and orientation responses.
Both Angles are equal.

51. Vestibular System Consists of three semi- circular canals
Monitors the position of the head in space
Controls balance
Shares fluid with the cochlea
Cochlea & Vestibular system comprise the inner ear