The vestibular system provides balance and orientation. It contains semicircular canals and otolith organs. The semicircular canals detect rotational head movements and the otolith organs detect linear acceleration and gravity. Hair cells in the organs transduce mechanical stimuli into neural signals. Disorders like Meniere's disease, BPPV, and injuries can damage the vestibular system and cause dizziness and vertigo.

1. Dr.Lakhan.M.S

3. Functions
 Provide general orientation of the body with respect to
gravity
 Enable balanced locomotion and body position
 Readjust autonomic functions after body reorientation
 Gaze stabilization

4. The vestibular organ
Horizontal canal
Anterior vertical canal
Posterior vertical canal
Vestibular Nerve
Facial Nerve
Vestibulocochlear
(VIII) Nerve
Cochlea
Cochlear NerveCochlear Nerve
UtricleSaccule

5. Vestibular nerve
 The human vestibular nerve contains approximately
18,000 afferent nerve fibres.
 The superior branch innervates the cristae of the
superior and lateral canals, the macula of the utricle
and the anteriosuperior part of the saccule.
 The inferior branch supplies the crista of the posterior
canal and the main portion of the macula of the
saccule

6. Vestibular Hair cells
 The sensory epethilia contain two types of sensory
cells characterized by Wersall.
 Type I cells correspond to the inner hair cells of the
organ of Corti.
 Type II cell resemble outer hair cells of the organ of
Corti.

7. Mechanotransduction
Stereocilia towards the kinocilium-
 This stretches the tip links.
 This increases the ion permeability of the channel,
resulting in an influx of cations-depolarization of the
cell.
Stereocilia away from the kinocilium-
 This shortens the tip links.
 It results in closure of the transduction channels-
hyperpolarization.

8. Adaptation
 The transducer's sensitivity is maintained.
 The position at which the hair bundle displays
maximal sensitivity, changes from the resting position
towards that at which the bundle is displaced with
sustained stimulation.
 The stimulus/response relationship of the hair cells
shifts in the direction of the applied stimulus.
 This causes a return of the channel open probability to
its resting value.

9. Semicircular Canals
 SCC-3 in no: two vertical(anterior/superior and posterior
SCC),one horizontal (lateral SCC).
 Each canal - one ampullated end and non ampullated end
 The non ampullated ends of posterior and superior canals
unite CRUS COMMUNAE.

10. The Otolithic Organs
Utricle- It is oblong, irregular and slopes.
 It lies superior to the saccule
 The macula utriculi, which is the largest, lies mostly in
the horizontal plane located in the dilated superior
portion of the utricle.
 The right and left macula lie in the same plane.

11.  The Saccule- It lies in a spherical recess in the medial
wall of the vestibule.
 It is hook-shaped and lies virtually in a vertical
position.

12. Otoconial Layer
 The otoconial membrane consists of a gelatinous layer,
a subgelatinous space and otoconia.
 Otoconia overlies the neuroepithelium which is a
calcareous material.
 These otoconia are anchored and partially embedded
in a gelatinous substance forming the otoconial
membrane.

15. Motion decomposition and Orientation
 The semicircular canals- rotations.
 The macules of the utricle and saccule- translations.
 Orientation of vestibular system in head is such that
 Right Anterior parallel to Left Posterior(RALP)
 Left Anterior parallel to Right posterior(LARP)

17. Movement detection
 Otolith organs
 Semicircular canals

18. Otolith organs
 Linear head motion on acceleration
 Static tilt.
 This is enabled by means of an otolith membrane.
 Hair cells embedded at the base of gelatineous
membrane and any movement cause deflection which
can in turn send signals to the brain.

19.  Two organs respond to respective accelerations or tilts in
their respective planes
 Saccule has vertical orientation of maculae
 Utricle has horizontal orientation of maculae

21. The otolith organs sense linear acceleration.
Hair cells lie in the macula.
When the head tilts the hair cells
are distorted by the shift of the
otolithic membrane
Otoconia (ear dust)
Otolithic
Membraine

23. Semicircular Canals
 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)

24.  Depolarization of the ipsilateral hair cells occurs
during angular head movements
 Hyperpolarization of contralateral hair cells occurs at
the same time

26. Biomechanics
 When the canal is rotated about an axis, three forces
act upon the endolymph and cupula in the canal:
1. the inertial force, proportional to the mass of the
endolymph and cupula;
2. the elastic restoring force of the cupula that
positions the cupula back to the central position after
stimulation;
3. the viscous forces that act upon the fluid when
sliding past the internal wall of the tube. This viscous
force is dependent on the speed of relative movement of
the endolymph with respect to the wall.

27. Principle of VOR generation
 During head rest, hair cells in both SCC have a resting
discharge rate of 90 spikes per second.
 Head rotation is to the right.
 Endolymph fluid lags behind, i.e. moves relative to the
left within each SCC due to inertia.
 The cupula bends to the left in each canal.
 In the (leading) right SCC, the stereocilia bend
towards the kinocilium.

28.  In the (following) left SCC, the stereocilia bend away
from the kinocilium.
 The discharge rate increases in the leading right ear
(e.g. from 90 to 300 spikes per second).
 The discharge rate decreases in the following left ear
(e.g. from 90 to 20 spikes per second).
 The vestibular nuclei interpret the difference in
discharge rates between left and right SCC as
movement to the right, and therefore trigger the
oculomotor nuclei to drive the eyes to the left to
maintain gaze stabilization.

29. Three arc Neuron Representation of VOR

30. Schematic canal stimulation and concomitant eye
muscle contraction and relaxation.

31. Head Impulse Thurst Test
Principle- It detects severe unilateral loss of
semicircular canal function clinically .
 It is more sensitive and specific than the traditional
Romberg test.
 This test can distinguish between vestibular neuritis
and cerebellar infarction.
 The head-thrust test is based on the fact that
inhibition of primary and secondary vestibular
neurons cannot produce fewer than 0 spikes per
second. Excitation can drive the discharge rate from 90
to 300 or more spikes per second.

32. How to Perform

33. The lateral vestibulospinal tract
 Originates in the lateral vestibular nucleus,
predominantly an otolith signal.
 Projects to cervical, thoracic, and lumbar segments via
the ventral funiculus.
 Entirely ipsilateral.
 Allows the legs to adjust for head movements.
 Provides excitatory tone to extensor muscles.
 Decerebrate rigidity is the loss of inhibition from
cerebral cortex and cerebellum on the LVST, and
exaggerates the effect of the tonic signal in the LVST.

34. The Medial Vestibulospinal Tract
 Originates in the medial vestibular nucleus,
predominantly a canal signal.
 Predominantly projects to cervical segments via the
medial longitudinal fasciculus.
 Predominantly ipsilateral.
 Keeps the head still in space – mediating the
vestibulo-colic reflex

35. Cervicoocular reflex
 When the head is fixed but the body is rotated,
nystagmus may be observed.
 This reflex is based on the stimulation of neck
receptors, rather than vestibular receptors.

37. Ototoxic Drugs
 Derangement of the sensory hairs resulting in hair
fusion.
 The cell bodies change in shape and structure.
 The sensory hair cells subsequently die and
disintegrate leaving a 'scar' in the epithelium.
 Loss of nerve fibres supplying the vestibular sensory
epithelia may occur at a later stage.

38. Vestibular
Neuronitis
 There is diffuse lymphocytic infiltration with areas of
gliosis in the vestibular nerve.
 The number of fibres in the vestibular nerve is also
diminished on the affected side.
 Changes in the vestibular sensory epithelia with loss of
sensory hair cells.
 Degenerative changes such as collapse of the
ampullary walls over the neuroepithelium.

39. Benign Paroxysmal Positional
Vertigo
 Detached otoconia, from the macula of utricle, may
float freely in the endolymph.
 These freely floating particles, upon rotatory
movements, affect the cupula, in particular the cupula
of the post semicircular canal.

40. Inner Ear Trauma
 Concussion of the inner ear results in haemorrhage
into the perilymphatic and/or endolymphatic spaces.
 A violent blow to the head can cause otoconia from the
utricule to dislodge from the otoconial membrane
 In Transverse fracture, which may run through some
weak portions of the petrous bone, such as the fossa of
the geniculate ganglion area, as well as the vestibule of
the inner ear.

41. Meniere’s Disease
 Distension of endolymphatic system-mainly cochlear
duct,saccule & utricle & SCCs.
 Reissners membrane bulges-herniaters through
helicotrema to scala tympani-dialation of scala media
filling the scala vestibule
 Swelling of the endolymphatic sac or other tissues in
the vestibular system of the inner ear, which is
responsible for the body's sense of balance.

42.  Dehiscent bony superior semicircular canal
 Congenital- CHARGE association manifests multiple
malformations and aberrations in the vestibular
labyrinth
(a) an absent oval window
(b) hypoplastic endolymphatic sac,
(c) absent bony and membranous semicircular canals
and ampullae
(d) reduced number of Scarpa's ganglion cells.

43.  Benign and malignant tumours in the vestibular
labrynth.
 Infections- Bacterial and viral labrynhtitis
 Ageing

44. Thank you