The vestibular system includes the semicircular canals and otolith organs which help maintain balance and spatial orientation. The semicircular canals detect rotational head movements and provide information about angular velocity to the brain. The otolith organs detect linear acceleration and provide information about the direction of gravity. Together the vestibular system helps control posture and eye movements like the vestibulo-ocular reflex which allows clear vision during head movements. Damage to the vestibular system can cause nystagmus and vertigo.

1. Physiology of Balance and
Vestibular Functions
Prof. Vajira Weerasinghe
Dept of Physiology

3. Major functions
 1. The primary organ of equilibrium
 2. Major role in the subjective sensation of
motion and spatial orientation
 3. Adjustments of muscle activity and body
position to allow for upright posture
 4. Helps stabilise the eyes in space during head
movements

4. Vestibular apparatus
 Semicircular canals
 Utricle
 saccule

5. Vestibular apparatus
 Sense organs of the vestibular system are
mechanoreceptors
 The semicircular canals are so arranged that they
lie in planes orthogonal to one another

6. Semicircular canals
 Semicircular canals sense angular acceleration
 The three semicircular canals have swellings,
called ampullae and within each ampulla is the
sense organ, called the crista
 In the cristae the hairs of the hair cells are
embedded in a gelatinous mass, called the
cupula, which extends across the ampulla

9. During movement
 Rotational angular acceleration of the whole
canal causes fluid to be left behind on account
of its inertia
 Fluid inertia during angular acceleration results
in displacement of the cupula and bending of
the sensory hairs
 This is the adequate stimulus for exciting the
hair cell

11. During movement
movement of the cupula and its embedded hairs during
rotation first in one direction and then in the opposite
direction

12. Transduction in semicircular
canals
 the resultant bending of the cilia causes cell
– excitation when the bending is toward the kino-
cilium
– with a resultant increase in the firing frequency of
vestibular nerve fibres
– inhibition when away from the kino-cilium
– with a resultant decrease in the firing frequency of
vestibular nerve fibres

14. Transduction in semicircular
canals
 the sensory cells exhibit a "resting discharge"
 modified (increased or decreased) depending on the
direction in which the cupula is deflected depending
on the direction of rotation
 the cilia-kinocilium complexes of the hair cells in the
semicircular canals are oriented in one direction
 all the hair cells in each canal one of the three canals
are maximally excited by the same direction of angular
rotation

15. Semicircular canals
 the neural output from ampulla represents the velocity
at which the canal is being rotated over the range of
normal head movements
 the canal mechanism therefore preforms a
mathematical integration of the input signal (the
integral of acceleration = velocity)
 due to the very small size of the canal (diameter of
about 0.3 mm), which results in a large increase in the
viscous properties of the fluid

16. Semicircular canals
 semicircular canal system acts as a
precise angular (rotational) speedometer
 its neural output is directly proportional to
the angular (rotational) velocity of head
movements
 By combining the input from each of the
three canals, the brain can create a
representation of the vector which
describes the instantaneous speed of
head rotation relative to 3D space
: a 3D speedometer!

17. Otolith organs (utricle and
saccule)
 The utricle and the saccule are two sac-like
structures
 each of which contains a specialized region (the
macula)
 which is made up of a ciliated sensory
epithelium (the vestibular hair cells

19.  the hair cells in the vestibular system differ from those
in the auditory system
 Each vestibular cell has a number of thin stereocilia
and one thicker longer kinocilium positioned at one
end of the cell’s hair-bearing surface
 Hair cells exhibit a constant "resting discharge activity"
even in the absence of a stimulus
 Thus, stimulation is sensed by the CNS as a change in
this resting, "spontaneous" discharge rate

20.  The cilia which emerge from
the hair cells are embedded in
a gelatinous matrix (the
otolith membrane) containing
solid CaCO3 crystals (the
otoconia) which overlies the
cells
 During linear acceleration, the
crystals (being denser than
the surrounding fluid) will
tend to be left behind due to
their inertia

21.  resultant bending of the cilia
causes cell
 excitation when the bending
is toward the kinocilium
– with a resultant increase in the
firing frequency of the afferent
sensory fibres
 inhibition when away from
the kinocilium
– with a resultant decrease in the
firing frequency of the afferent
sensory fibres

23.  Macula of the utricle lies in the horizontal plane
– Respond to horizontal linear force
 Macula of the saccule lies in the vertical plane
– Respond to vertical linear force

24. Within each organ,
the cilia-kinocilium
complexes of hair
cells are spatially
arranged such that
all possible
directions of linear
movement are
represented

25.  Since they are sensitive to acceleration, the
otolith organs detect
– the direction and magnitude of gravity
– transient linear accelerations due to movement

26. Central connections
 The maculae and cristae are innervated by
bipolar neurons of the vestibular ganglion
 The central processes of these cells form
the vestibular nerve which enters the brain
stem at the cerebellopontine angle medial to
the cochlear nerve
 The vestibular nerve bifurcates into short
ascending and long descending branches which
are distributed to the vestibular nuclei

27.  Some vestibular nerve fibers continue without
interruption to the ipsilateral cerebellar cortex and one
of the deep cerebellar nuclei
 Most primary vestibular fibers terminate differentially
in the four main vestibular nuclei in the floor of the
fourth ventricle
 The vestibular nuclei give rise to secondary vestibular
fibers which project to specific portions of the
cerebellum, certain motor cranial nerve nuclei and to
all levels of the spinal cord

29. Role of vestibular system

30. Role in posture
 Vestibular postural reflexes
– Tonic labyrinthine reflex
– Labyrinthine righting reflex

31. Postural reflexes
 tonic labyrinthine reflex
 Stimulus: gravitational pull
 Response: contraction of limb extensors
 receptors: vestibular organs
 (work through vestibulospinal tract)
 labyrinthine righting reflex
 Stimulus: gravitational pull
 Response: attempt to maintain head level
 receptors: vestibular organs
Video

32. Vestibulo-ocular reflex
 The is an important mechanism by which unblurred vision is made
possible during the head movements that are generated during
every day life
 If the head is turned to the left, the balance of the afferent neural
information from the two horizontal canals on each side of the head
would cause the eyes turn to the right
– opposite direction to head movement
 The leftward head movement causes
– an increase in the activity of left horizontal canal hair cells and afferent
fibers
– a decrease in the activity in the horizontal canal hair cells and afferent
fibers
 This difference in activity between the left and right afferent fibers
is responsible for generating an oppositely directed eye movement
at the same velocity as the head movement
 This eye movement "reflex" is very important
 It allows us to keep our retina fixed on the same point in visual
space both during and following naturally generated head
movements
Video

34. Eye movements
 Saccadic movement
 Smooth pursuit
movement

35. What is nystagmus?
 Periodic rhythmic
oscillatory movements of
the eyeball

36. Various types of nystagmus
 Jerk nystagmus (with fast and slow phases)
 Pendular nystagmus (like sine waves)
 Horizontal, vertical or rotatory
 Downbeat or upbeat
 Pathological
– Vestibular disorder, cerebellar disorder
 Physiological
– Optokinetic

37. Optokinetic nystagmus

39. Physiological basis of nystagmus
 Visual fixation
 Vestibulo-ocular reflex
 Neural integrator
– cerebellum, ascending vestibular pathways, and
oculomotor nuclei are important components of the
neural integrator

40. Oculocephalic reflex (Doll’s head
phenomenon, Doll’s manoevre)
Doll’s head
phenomenon
•In comatose or severely lethargic patients, the vestibulo-ocular reflex can be used to
test whether brainstem eye movement pathways are intact
•The oculocephalic reflex, a form of the vestibulo-ocular reflex, is tested by holding the
eyes open and rotating the head from side to side or up and down
•this test consists of the rapid rotation of the patient's head in a horizontal or vertical
direction
• the eyes move conjugately in the opposite direction of the head turn
• vestibular nuclei and medial longitudinal fasciculi should be intact

41. Caloric testing
 Instilling cold or warm water into the
external auditory canal can produce
nystagmus
 This produce the same movement of
endolymph in the semicircular canals
produced by rotations of the head
 Cold water induces nystagmus away
from the ear being irrigated
 Water water induces nystagmus
towards the ear being irrigated
Video

42. Vertigo
 Feeling of rotation
 May be positional or non-positional
 Causes
– Inner ear disorders
– Vestibular disorders
– Brain stem lesions