PHYSIOLOGY OF BALANCE BY DR MARJAN

1. PHYSIOLOGY OF
BALANCE
DR.MARJAN

3. Main functions of vestibular system is to generate
information to CNS for
1. To ensure gaze stabilisation
2. To enable balanced locomotion and body position
3. To provide general orientation of the body with
respect to gravity
4. To readjust autonomic functions after body
reorientation

4. • For this we need a proper functioning - visual, propioceptive
system along with vestibular system

5. VESTIBULAR SYSTEM
• PERIPHERAL-
which is made up of membranous
labyrinth[(horizontal(lateral),
superior(anterior) & posterior
SCC ,utricle and saccule] and
vestibular nerve.
• CENTRAL-
Consists of vestibular nuclei and their projections to thalamus,
cerebellum, cortex, descending spinal cord extraocular nuclei
(nuclei and fibre tracts in the CNS to integrate vestibular
impulses with other systems to maintain body balance

6. • Every motion in space can be broken down into 6 degrees of
freeom
 3 rotational (roll,pitch and yaw) &
 3 translational (left–right, up– down,front–back).
• The semicircular canals (SCCs) measure predominantly
rotations whereas the maculae of the utricle and saccule
detect mainly translations.

7. PERIPHERAL RECEPTORS
They are of two types:
1. Cristae
They are located in the ampullated ends of the three
semicircular ducts. These receptors respond to angular
acceleration.
2. Maculae
They are located in otolith organs (i.e. utricle and saccule).
Macula of the utricle- in the floor in a horizontal plane.
Macula of the saccule-in the posterior wall in a vertical
plane.
They sense position of head in response to
gravity and linear acceleration.

8. Cristae
It is a crest like mound of connective
tissues on which lie the sensory
epithelial cells.
The cilia of the sensory hair cells project
into the cupula, (gelatinous mass
extending from the surface to the ceiling
of the ampulla and forms a water tight
partition)
It get displaced to one or the other side
like a swing door, with movements of
endolymph.
The gelatinous mass of cupula consists of
polysaccharide and contains canals into
which project the cilia of sensory cells.

9. Hair cells are of two types
Type I cells-flask-shaped with a
single large cup-like nerve terminal
surrounding the base.
Type II cells -cylindrical with
multiple nerve terminals at the base.
From the upper surface of each
cell, project a number of cilia, 20–
300, the stereocilia
The kinocilium is thickest &largest
among them and is located on the
edge of the cell.
Sensory cells are surrounded by
supporting cells which show
microvilli on their upper ends

11. MACULA
A macula consists mainly of two parts:
(i) a sensory neuroepithelium, made
up of type I and type II cells.
(ii) an otolithic membrane, which is
made up of a gelatinous mass and on
the top, the crystals of calcium
carbonate called otoliths or otoconia
The cilia of hair cells project into the
gelatinous layer.
The linear, gravitational and head tilt
movements cause displacement of
otolithic membrane and thus stimulate
the hair cells

12. • Potassium flows into
cell
• Calcium flows into cell
• Voltage shifts to a less
negative value
• More neurotransmitter
is released

13. ;;;;;

14. OTOLITHORGANS(UTRICLE&SACCULE)
• The utricle lies in the posterior part of bony vestibule.
• It receives the five openings of the three semicircular ducts.
• The saccule also lies anterior to the utricle and opposite the
stapes footplate. Connected to utricle - utriculosaccular duct.
• Utricle lies relatively horizontal & Saccule –vertical

15. • Otolith organs-detects gravitational acceleration continuously
• Also sense linear accelerations caused by translational
movements of the head as well as static tilts of the head
• Macules are the receptors

16. SEMICIRCULARCANALS
• 3 in number
• These canals lie at right angles to
each other
• Respond to angular acceleration
and deceleration.
• The canal which lies at right angles
to the axis of rotation is stimulated
the most.
• Thus horizontal canal will respond
maximum to rotation on the
vertical axis.
• Due to this arrangement of the
three canals in three different
planes, any change in position of
head can be detected.

17. • SUPERIOR/ANTERIOR
SCC- Head nodes up
and down as in YES
motion(pitch)
• POSTERIOR SCC- When
head tilt towards
shoulder(roll)
• LATERAL SCC- When
head shakes side to
side in ‘NO’
motion(yaw)

18. Vestibulo-ocularreflex
• It is a reflex , where activation of vestibular system of the ear
causes eye movement, which is to stabilize image on the
centre of the retina
• The left and right SCCs are oriented in the head such that
any movement always induces an antagonistic response
in both canals

19. • The left and right canals as
parallel systems,
• i.e. the right anterior (RA)
canal is parallel with the left
posterior (LP) - RALP plane.
• Right posterior (RP) canal the
left anterior (LA) constitutes
the LARP plane.
• Both horizontal canals are also
parallel with each other in the
lateral plane
• The hair cells of both right
and left canals as a mirror
image, the deflection on the
‘leading’ side induces a
movement of the stereocilia
towards the kinocilium,
whereas on the opposite
‘following’ ear the
movement of the stereocilia
is away from the kinocilium.
• V;

20. The simplified principle of VOR generation – PUSH-PULL PRINCIPLE
“ (yaw-plane rotation - horizontal SCC) is as follows:
1. During head rest, hair cells in both SCCs have a resting discharge
rate of 90 spikes per second.
2. Head rotation is to the right.
3. Endolymph fluid lags behind due to inertia.
4. The cupula bends to the left in each canal.

21. 5. In the (leading) right SCC the stereocilia bend towards the
kinocilium.- discharge rate increases
6. In the (following) left SCC the stereocilia bend away from the
kinocilium- discharge rate decreases

22. The vestibular nuclei
interpret the difference in
discharge rate between
left and right SCCs as
movement to the right,
and trigger the oculomotor
nuclei to drive the eyes to
the left to maintain gaze
stabilization

23. Three arc Neuron Representation of VOR

25. • Thus, nystagmus is
horizontal-horizontal canal,
vertical -posterior canal
Rotatory -superior canal

26. NYSTAGMUS
• The eye response to a head rotation –
combination of a slow phase/drift(until the eye
reaches the edge of the outer canthus) and a fast phase to
reset the eye in its initial position- is called nystagmus
• The direction of the nystagmus is defined by the fast reset
phase, since that is easiest identified by the clinician.
• The slow phase, however, represents the actual vestibular
output and is quantified.
• It is quantified by measurement of the slope of the upward
trace, which indicates the speed of the eye movement
(degrees/s).

27. • In electronystagmography or video-nystagmography, An
upward excursion-represents eye deviation to the right.
• If the slope of the sawtooth is upward to the right, i.e. a
positive slope, this corresponds to a slow drift of the eye to
the right, followed by a quick leftward reset saccade, as
represented by a steep downward trace.
• This is defined as a left nystagmus.

28. The quick component of nystagmus is always opposite to the
direction of flow of endolymph.
Thus, if a person is rotated to the right for sometime and then
abruptly stopped, the endolymph continues to move to the right
due to inertia,the nystagmus will be directed to the left
Remember nystagmus is in the direction opposite to the
direction of flow of endolymph.

29. Head Impulse Test/ Thrust test
• It is a simple, reliable, bedside test to detect unilateral
loss of semicircular canal function clinically
• This test can distinguish between vestibular neuritis
and cerebellar infarction

30. whentheeyesrotatetogetherwiththeheadandarenolongerfocused
onthetarget,theSCConthesidetowardswhichtheheadwasturnedisnotfunctioningproperly

31. VESTIBULARNERVE
Vestibular or Scarpa’s ganglion is situated in the lateral part of the
internal acoustic meatus. It contains bipolar cells.
The distal processes of bipolar cells innervate the sensory
epithelium of the labyrinth while its central processes aggregate
to form the vestibular nerve.

32. Vestibular Portionof C.N. VIII
superior division: utricle, anterior part of saccule, and
horizontal & anterior canals
inferior division: posterior part of saccule& posterior
canal

33. CENTRALVESTIBULARCONNECTIONS
The fibres of vestibular nerve end in vestibular nuclei and
some go to the cerebellum directly.
Vestibular nuclei are four in number, the superior,
medial, lateral and descending.
Afferents to these nuclei come from:
1. Peripheral vestibular receptors (SCC, utricle and saccule)
2. Cerebellum
3. Reticular formation
4. Spinal cord
5. Contralateral vestibular nuclei

34. Efferents from vestibular nuclei go to:
1. Nuclei of CN III, IV, VI via medial longitudinal bundle
(Vestibulo-Oculomotor Pathways)
It is the pathway for vestibulo-ocular reflexes and this explains the
genesis of nystagmus.
2. Motor part of spinal cord (vestibulospinal fibres).
This coordinates the movements of head, neck and body in the
maintenance of balance.
Lateral V-S-throughout spinal cord
Medial V-S-cervical & thoracic
Reticulospinal tract-via brainstem reticular formation

35. 3. Cerebellum (vestibulocerebellar fibres). It helps to coordinate
input information to maintain the body balance.
4. Autonomic nervous system. This explains nausea, vomiting,
palpitation, sweating and pallor seen in vestibular disorders (e.g.
Ménière’s disease).
5. Vestibular nuclei of the opposite side.
6. Cerebral cortex (temporal lobe). This is responsible for
subjective awareness of motion

37. In the brainstem
• Vestibular inputs undergo integration
• Integrated signal is combined with original (velocity
driven) signal
• Processing to reset spatial map for eye musculature

40. *VESTIBULOSPINAL REFLEX
The lateral vestibulospinal tract.
• Projects to cervical, thoracic, and lumbar segments via the
ventral funiculus
• Entirely ipsilateral
• Originates in the lateral vestibular nucleus,predominantly
an otolith signal
• 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.

41. VESTIBULOCOLLICREFLEX
The Medial Vestibulospinal Tract
• Originates in the medial vestibular nucleus,
predominantly a canal signal
• Projects to cervical segments via the medial
longitudinal fasciculus
• Predominantly ipsilateral.
• Keeps the head still in space – mediating the
vestibulo-collic reflex

44. MAINTENANCE OF BODY
EQUILIBRIUM
• In static neutral position, each side contributes equal sensory
information, i.e. push and pull system of one side is equal to
that of the other side.
• If one side pulls more than the other, balance of the body is
disturbed.
• During movement, i.e. turning or tilt, there is a temporary
change in the push and pull system, are corrected by
appropriate reflexes and motor outputs to the eyes (vestibulo-
ocular reflex), neck (vestibulocervical reflex), and trunk and
limbs (vestibulospinal reflex)

45. THANK YOU