1. ANATOMY OF VESTIBULAR
APPARATUS
AND
PHYSIOLOGY OF EQUILIBRIUM
By
Dr. Syed Salman Hussaini

2. 
The inner ear has five different organs which are
functionally related and establish neural connections in
the brain with the oculomotor and postural control
systems.

The vestibular system plays a lesser role for the control of
posture and balance, but is of great importance clinically
due to its common and dramatic involvement during
pathological condition affecting the inner ear.

Hearing and vestibular organs contain mechanoreceptors
or sensory cells with directionally sensitive ciliary bundles
or hairs.

The vestibular organ consists of a delicate system of
membranous ducts containing sensory epithelia or
mechanoreceptors important for the sense of gravity and
balance.

3. 
The sensory epithelium is located in the three ampullae of the
semicircular canals and in the maculae of the saccule and utricle.

The organ contains endolymph and is surrounded by perilymphatic
fluid.

The membranous system, or 'labyrinth' due to its complexity, is
connected to the cochlea through the thin reunion duct.

In addition, the various portions of the vestibular organ are
interconnected by small canals: the utricular duct and the saccular
duct which merge and form the utriculosaccular duct.

This canal runs posteriorly and enters the internal aperture of the
bony vestibular aqueduct.

Here, it forms the endolymphatic duct which widens triangularly
within the bone of the posterior surface of the petrous bone into the
endolymphatic sac.

The endolymphatic sac ends blindly within a dural duplicate in the
posterior cranial fossa in close association with the sigmoid sinus and
venous drainage of the brain.

The endolymphatic duct and sac, which are closely related to the
brain, are thought be involved in the reabsorption of endolymph and
regulation of endolymph pressure.

6. VESTIBULAR SENSORY SYSTEM

The vestibular sensory organ is a so-called 'open' system
that rests on a balance of function between the left and
right ear.

There is a continuous influx of nerve impulses (over one
million per second) from each vestibular nerve to the
central nervous system (CNS).

This resting discharge is the source of vestibular tonus.

A viral infection, fracture or a vascular disturbance
(enhancement or impairment) in one of the two organs will
result in an imbalance between tht sides and give rise to
clinical symptoms often perceived as violent by the patient.

Typically is a rotatory type of vertigo.

7. THE VESTIBULAR NERVE

The vestibular nerve contains approximately 18,000
afferent nerve fibres whose size varies between 2 and 15
microns.

The number of ganglion cells has been estimated to be
approximately 18,000 in young adults, decreasing to
around 12,000 at the age of 80 years.

The vestibular ganglion or Scarpa's ganglion consists of
bipolar neurons located in the lateral part the internal
auditory canal.

It consists of a superior and an inferior group of cells
associated with the superior and inferior vestibular nerve.

The superior branch innervates the cristae of the superior
and lateral canals, the macula of the utricle and the
anterosuperior part of the the saccule.

The inferior branch supplies the crista of the posterior
canal and the main portion of the macula of the saccule.

9. 
The nerves contain both large and small nerve fibres;

The larger nerve fibles are predominantly involved in the
innervation of type I hair cells located in the central region
of the maculae (striola) or the summit of the crista.

The small fibres are more abundant at the slope and the
peripheral region of the maculae.

The vestibular ganglion cells are unmyelinated and
surrounded by a thin sheath of Schwann cell.

In the internal auditory meatus, the vestibular and cochlear
nerves merge.

During their course to the brainstem, the facial nerve
becomes located further up the brain.

A small arterial branch from the anterior inferior cerebellar
artery (AICA) runs between the VlIth and VIII nerve on the
brainstem.

10. 
The vestibular nerve contains efferent nerve fibres
terminating in both the cochlear and vestibular sensory
organs.

Inner ear effererents of central origin arise from the
general visceral efferent cell column of the facial nerve and
are parts of the nervus intermedius having autonomic
functions.

The human vestibular labyrinth is supplied by
approximately 200-300 efferent nerve fibres.

There is both an ipsilateral and contralateral origin of
vestibular efferents in the brainstem.

12. 
There is a bimodal type of innervation where the
efferents only make direct contact with the type II cells,
while on the type I cells the efferent terminals always
contact the afferent nerve fibre or terminal.

Thus a higher or central control of afferent sensory activity
could act both at a presynaptic level on the afferent fibres,
but also as a direct modulation on the peripheral receptor
as can be observed in other sensory systems.

It may allow the primary afferent receptors to both increase
and decrease their activity as a response of stimulation.

13. VESTIBULAR SENSORY CELLS, TYPE I AND TYPE II

The sensory epithelia contain two types of sensory cells.

The cells are classified as type I and type II sensory cells.

The type I cells are flask-shaped and surrounded by a
nerve chalice formed by the terminal end of the afferent
nerve fibre of the vestibular nerve.

Many of the calyces have collateral extensions that end on
type II hair cells.

Type II hair cells synapse with these collaterals and also
directly with the outer membrane of calyces surrounding
type I hair cells.

Efferent nerve endings also terminate on the afferent nerve
calyce and on type II hair cells.

14. 
Type I cells, correspond to the inner hair cells of the organ
of Corti.

Type II cells, resemble outer hair cells of the organ of Corti.

They are cylindrical in shape, but have the same
arrangement of stereo- and kinocilia as the type I cells.

The upper surface of the hair cell contains approximately
70 stereocilia and one kinocilium arranged with the longest
stereocilia positioned adjacent to the kinocilium.

The stereocilia in the macula are a few microns long, while
in the crista they measure up to over 35 microns.

16. 
The stereocilia contain actin filaments.

All the cilia within a bundle move together as if joined to
one another.

The upper surface of the cell contains a thicker region,
called the 'cuticular plate', into which the stereocilia
extend their rootlets.

17. SACCULE AND UTRICLE

The utricle is oblong, irregular and slopes anteriorly
upwards.

It lies superior to the saccule .

The macula utriculi, lies in the horizontal plane located in
the dilated superior portion of the utricle.

The mean area in adults and children is 4.30 mm 2

The macula utriculi contains approximately 33,000 hair
cells.

The right and left macula lie in the same plane.

The human saccule lies in a spherical recess in the medial
wall vestibule, is hook-shaped and lies in vertical plane.

The mean area in adults is 2.4 mm 2.

The saccular macula contains approximately 18,000 hair
cells.

20. 
Overlying the neuroepithelium is a calcareous material
consisting of a multitude of small cylindrical and
hexagonally shaped bodies with pointed ends, usually
named otoconia.

These otoconia are anchored and partially embedded in a
gelatinous substance forming the otoconial membrane.

The hair processes of the sensory cells project into the
otoconial membrane and are displaced mechanically by
the otoconial mass relative to the sensory epithelium.

21. 
Each macula is be divided into two areas by a narrow
curved line in its centre termed the 'striola'.

The striola area contains a higher concentration of type I
cells.

The sensory cells change orientation along this line so that
cells are often 'mirror-shaped' with opposite polarity as to
the location of the kinocilium.

In the utricle, the kinocilia face the striola.

In the saccule the kinocilium is polarized away from the
striola.

22. OTOCONIAL LAYER

The otoconial membrane consists of a gelatinous layer, a
subgelatinous space and otoconia.

The otoconia measure 3-19 microns in length.

They consist of an organic protein matrix together with
inorganic calcium carbonate crystallized in the form of
calcite.

Secretion of organic material occurs from the apical
cytoplasm of adjacent supporting cells.

The enzyme carbonic anhydrase is located in the
epithelium which catalyzes the chemical reaction H2O +
CO2 ---> H2C03.

The carbonic acid combines with calcium to form crystals
of CaC03 which is trapped in the matrix leading to the
formation of otoconia.

24. STRUCTURE AND FUNCTION OF THE AMPULLA
AND CUPULA

The three semicircular canals (superior, posterior and
lateral) provide a complete coordinate system.

The horizontal canal generally lies in a horizontal plane
when the head is oriented in an active position.

The superior opening of the horizontal and superior canal,
and the inferior opening of the posterior canal widen into
an ampulla which contains the crista ampullaris.

The crest is covered by sensory cells whose kinocilia are
directed in the same direction.

In the crista of the lateral semicircular canal, the
polarization is towards the utricle, while it is polarized away
from the utricle in the superior and posterior canal cristae.

26. 
The afferents in the horizontal canal are stimulated when
endolymph flows against the utriculus, while the two
vertical canals are stimulated when endolymph moves
against the ampulla.

The sensory epithelium on the crista is covered by a
gelatinous mass called the cupula.

The cupula in the ampulla of the semicircular canal helps
in transferring endolymph fluid movement stimuli to the
hair cells.

This gives rise to kinetic reflexes.

It contains proteoglycans arranged in a filamentous
network.

28. SUPPORTING AND 'DARK' CELLS

The sensory cells are surrounded by supporting cells
whose function is partly secretory in nature.

They provide support and insulation for the sensory cells
and also form precursor cells for sensory hair cells.

The membranous labyrinth contains so-called 'dark' cells in
all sensory epithelia, except the saccule.

The cells have an irregular surface and resemble the
marginal cells of the stria vascularis.

In addition, pigmented cells or melanocytes are often
associated with the dark cells, which is similar to the
situation in the stria vascularis in the cochlea.

29. 
These cells are important for the development and
maintenance of the endolymph adjacent to the vestibular
mechanoreceptors thereby playing a role for the proper
function of the electric activity of the sensory cells and
initiating conductive neural responses of afferent nerves.

Degrading otoconia can often be seen on the surface of
the dark cells, suggesting that these cells are involved in
the degradation and resorption of dislodged otoconia.

30. VASCULAR SUPPLY OF THE VESTIBULAR ORGAN

The inner ear blood supply derives from the labyrinthine
artery, which is a branch of the AICA and vertebrobasilar
artery system.

It is an end artery.

The labyrinthine artery divides into the superior
vestibular artery, which supplies the vestibular nerve,
utricle and parts of the semicircular canals, and the
common cochlear artery which divides into the cochlear
artery and the vestibulocochlear artery.

This artery divides into vestibular branches at the basal
turn of the cochlea and supplies the saccule and the
semicircular canals.

The vestibular labyrinth is drained both through a
labyrinthine vein running through the meatus and through
the vein of the vestibular aqueduct.

32. PHYSIOLOGY OF EQUILIBRIUM

33. ROLE OF THE VESTIBULAR SYSTEM: INTRODUCTION

The main function of the mammalian vestibular system is
to generate information for the central nervous system with
a four-fold purpose:
1. provide general orientation of the body with respect to
gravity;
2. enable balanced locomotion and body position;
3. readjust autonomic functions after body reorientation; and
4. ensure gaze stabilization.

Several input systems provide information to the brain,
such as the visual, vestibular and proprioceptive systems.

35. 
Central processing of all this information in the brain leads
to specific outcomes:
• the vestibuloocular reflex (VOR) to ensure gaze
stabilization;
• the vestibulospinal reflex (VSR) and the vestibulocollic
reflex (VCR) to ensure maintenance of an upright
position of the body and trunk and head stabilization in
space;
• orientation and navigation and the perception of self-
position with respect to the surroundings and gravity,
mediated by the vestibular cortex;
• autonomic function adjustments after alterations of body
orientation.

36. Motion decomposition and orientation in the head

Every motion in space can be broken down into three
rotational degrees of freedom (yaw, pitch and roll) and
three translational degrees of freedom (left-right, up-down,
for-aft).

No event in one degree of freedom can be described by
the others, hence every movement is uniquely and
appropriately described by a combination of all six degrees
of freedom.

The anatomical design of the motion sensors in the
peripheral vestibular system in the inner ear reflects these
six degrees of freedom.

The semicircular canals measure predominantly rotations,
whereas the macules of the utricle and saccule detect
mainly translations.

38. 
The left and right canals are parallel systems, i.e. the left
posterior canal is parallel with the right anterior canal
which both lie in a plane denoted as the right anterior-left
posterior (RALP) plane.

With the right posterior canal, the left anterior constitutes
the left anterior-right posterior (LARP) plane.

Both horizontal canals are also parallel with each other in
the lateral plane.

The horizontal canal makes an angle of approximately 30°
with respect to the horizontal axis of the upright head.

The angles of the vertical canal are approximately 45° with
the saggital plane of the head.

42. Movement detection

Basic laws of physics are responsible for the detection of
movement or orientation by the vestibular system.

The first law of Newton states that 'objects in motion will
stay in motion until acted upon by a force.

This force can either change the objects direction or
speed.

The second law states that a force acting on an object
equals mass times acceleration (F = m x a).

43. 
The movements are sensed in the vestibular organ by a
rigid coupling of sensory epithelium to the bony structure.

Thus, a fluid-filled system is attached to the skull and the
inertial forces (upon head acceleration, as well as gravity)
drive the fluid.

Motion of the fluid lags behind any motion of the head and
this relative displacement is the trigger for movement
detection.

To limit the movement of the hair cells triggered by
accelerations that drive head movements, the canal
system is designed such that the deflection of the hair cells
is proportional to the head velocity.

44. OTOLITH ORGANS

The gravitational force acts permanently, hence gravitational
acceleration needs to be detected continuously, and this
function is served by the otolith organs, which detect all linear
accelerations.

The orientation of the otolith organs, the saccule and utricle, is
relatively orthogonal to each other, with the utricle being
horizontal and the saccule predominantly vertical.

Consequently, motion in the horizontal plane triggers
predominantly the utricle, whereas vertical movements trigger
mainly the saccule.

This is enabled by means of an otolithic membrane, i.e. a
gelatineous membrane stacked with otoconia (calcium
carbonate crystals), which are characterized by a density that is
considerably higher than that of the surrounding endolymph.

Hair cells are embedded at the base of the gelatineous
membrane, so that any movement of the membrane results in a
deflection of the hair cells, which in turn codes for a signal to the
brain.

46. NYSTAGMUS

The eye response to a head rotation consists of a
combination of a slow phase or drift until the eye reaches
the edge of the outer canthus, and a fast phase to reset
the eye in its initial position.

This saw-tooth pattern is called nystagmus.

The direction of the nystagmus is defined by the fast reset
phase, since this is the most easy for the clinician to
identify.

The slow phase, however, represents the actual vestibular
output and is quantified.

47. An upward excursion, by convention on
electronystagmography or video nystagmography,
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.
It is quantified by measurement of the slope of the upward
trace, which indicates the speed of the eye movement
(degrees/second).

48. 
The semicircular canals and the otoliths provide the inputs
for the vestibuloocular reflex.

Horizontal VOR compensates for both horizontal rotation
and horizontal translation.

The former is due to the canal system where the latter is
due to the utricular system.

It is therefore more convenient to use the angular VOR
(aVOR) and linear VOR (lVOR).

49. VELOCITY STORAGE

The semicircular canal system is not designed for
sustained rotations, and even rotation at relatively high but
constant speeds (> 200 degrees/second) is not sensed by
the SCC.

Only the acceleration or deceleration phase is detected.

During constant speed rotations, elastic forces pull the
cupula again into its centre position.

However, after sudden cessation of the rotation (e.g. with a
deceleration of 400 degrees/second2), the endolymph
continues to move within the SCC and a nystagmus in the
opposite direction becomes evident for another 20-30
seconds.

The subject has the impression of being spun around in
the opposite direction.

50. 
The time constant during which the cupula repositions
itself is determined as approximately five to seven
seconds, which implies that after 12 seconds the cupula is
almost entirely restored to its central position.

This gradual decrease in cupular deviation results in a
decrease in firing rate of the vestibular neurons to the
brain, suggesting erroneously a progressively lower head
velocity.

The fact that the nystagmus outlasts this mechanical
phenomenon is due to the so-called velocity storage
mechanism (VSM), which is a neurophysiological
process, taking place mainly in the nucleus prepositus
hypoglossus and the adjacent medial vestibular nucleus.

The VSM serves to maintain the VOR at low frequencies
(below about 0.02 Hz).

51. 
In conditions where visual information of the surrounding
rotating world is present during sustained rotations, the
optokinetic reflex comes into play and, although slower in
response, this takes over the fading performance of the
vestibular system.

The transition between both reflexes is facilitated by the
VSM.

The VSM not only regulates the dynamics of the VOR, but
also accounts for optokinetic (after) nystagmus (OKN), i.e.
the reflexive eye movements that are induced by a moving
background (e.g. while looking outside the window of a
moving train).

52. 
The velocity storage mechanism coordinates these two
oculomotor responses that are related to self-motion.
Whereas the VOR reflects high-frequency behaviour, the
OKN reflects low-frequency characteristics.

Matching of their time constants in the brain assures
smooth transition from the quick-onset vestibular response
into the slow-onset optokinetic response.

Together, the VOR initially, and the OKN subsequently,
matched and fine-tuned by the VSM, ensure gaze
stabilization.

53. Semicircular canal stimulation and VOR generation

A normal resting discharge rate of approximately 90 spikes
per second is modulated, such that increase of this rate
corresponds with an excitation and decrease with
Inhibition.

The polarization of the hair cells in the horizontal
semicircular canal is such that deflection of the stereocilia
in the cupula towards the kinocilium (ampullo or
utriculopetal) results in hair cell depolarization and
consequently the activity of the primary afferent neurons
increases.

Deflection of the stereocilia away from the kinocilium
(ampullo or utriculofugal) results in hair cell
hyperpolarization and decreased primary afferent neuron
activity.

The left and right semicircular canals are oriented in the
head such that any movement always induces an
antagonistic response in both canals.

55. In first approximation, this VOR process is a three arc neuron
reflex (first order vestibular neurons, second order
vestibular neurons and oculomotor plant neurons).
The simplified principle of VOR generation (yaw plane
rotation and horizontal SCC) is as follows:
1. During head rest, hair cells in both see have a resting
discharge rate of 90 spikes per second.
2. Head rotation is to the right.
3. Endolymph fluid lags behind, i.e. moves relative to the left
within each see due to inertia.
4. The cupula bends to the left in each canal.
5. In the (leading) right SCC, the stereocilia bend towards the
kinocilium.
6. In the (following) left SCC, the stereocilia bend away from
the kinocilium.

56. 7. The discharge rate increases in the leading right ear (e.g.
from 90 to 300 spikes per second).
8. The discharge rate decreases in the following left ear (e.g.
from 90 to 20 spikes per second).
9. 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.

58. 
Similar to the horizontal canal, a push-pull principle also
governs the vertical canal excitatory and inhibitory
functions.

For example, the left anterior canal is excited, while the
right posterior canal is inhibited for the same movement.

Also, the vertical canals are direction sensitive, but at this
stage ampullopetal movements result in a decreased firing
rate.

Thus, the ampullar deflection in the vertical canals,
corresponding to excitation, is in the opposite direction to
the horizontal sec.

For the horizontal canal, excitation is elicited upon
ampullopetal endolymph movement (towards the
vestibulum-utricle), whereas for both the vertical posterior
and anterior SCC ampullopetal flow is inhibitory.

59. 
Every stimulation in whichever direction always evokes the
push-pull principle.

When the head is tilted sidewards, a torsional nystagmus
is generated, as both posterior and anterior canals are
stimulated.

When the head is pitched forward, both left and right
anterior canals are stimulated, whereas both posterior
canals are inhibited.

This produces an upward eye movement.

The same applies when the head is tilted backwards, but
now both posterior canals are stimulated producing a
downward compensatory eye movement.

61. NYSTAGMUS INDUCED BY ACUTE UNILATERAL
DEAFFERENTIATION

The push-pull principle is very convenient to explain the
origin of the nystagmus observed during acute peripheral
lesions.

In the case of an acute unilateral vestibular deafferentiation
(u VD) on, for example, the right side, all three right canals
and the otoliths cease spontaneous activity (from 90
spikes per second to zero), while the spontaneous activity
of the left SCC remains at 90 spikes per second.

Although the head is not moving, the brain perceives an
apparent imbalance (R, 0 spikes/second versus L, 90
spikes/second) similar to, for example, when the left
system is triggered during head movements towards the
left.

62. 
This tonic imbalance drives the vestibular and,
consequently, the ocular motor nuclei to move the eyes
towards the right, as would be appropriate for a head
movement towards the healthy side (left).

The brain erroneously interprets the abruptly decreased or
absent firing rate of the ipsilateral affected peripheral
system as a relative increase of the contralateral system,
resulting in a nystagmus that beats away from the acute
lesion.

63. CENTRAL PROJECTIONS OF THE PERIPHERAL
VESTIBULAR SYSTEM

In the brainstem, four vestibular nuclei (VN) are identified:
the superior vestibular nuclei (SVN), lateral vestibular
nuclei (LVN), medial vestibular nuclei (MVN) and
descending vestibular nuclei (DVN).

From there, several projections are found to the
oculomotor nuclei, the lateral and medial vestibulospinal
tract, the vestibulocerebellum (floculus, nodulus), the
parapontine reticular formation (PPRF), the nucleus
prepsositus hypoglossus, nucleus tractus solitarius, and
the parietoinsular vestibular cortex (PIVC).

The vestibulocerebellum plays a predominant role in the
fine tuning of the vestibular functions.

64. PROJECTIONS TO THE CENTRAL NUCLEI

Two types of neurons triggered by the lateral SCC input
have been traced to the medial vestibular nucleus, namely
type 1 and type 2 neurons;

Their resting discharge rates are approximately 80 to 90
spikes/second.

Increased rates, however, can exceed 300 spikes/second.

Some type 1 neurons are excitatory and some are
inhibitory.

Primary vestibular afferents synapse with inhibitory, as well
as excitatory type 1 neurons.

66. 
Upon head rotation to one side, denoted as the ipsilateral
side, the ampulla of the ipsilateral horizontal canal is
stimulated followed by an immediate increase in firing rate
of the neurons, proportional to the velocity of the head
turn.

These signals project mainly on to the ipsilateral MVN, but
other parts of the vestibular nuclei are also involved.

After passing along the primary afferent, the vestibular
signals are relayed into the VN.

From there, axons decussate on to the contralateral
abducens nucleus which innervate the lateral rectus of the
contralateral eye through the VIth nucleus.

Additionally, the interneurons of the contralateral abducens
nucleus project through the longitudinal medial fasciculus
to the ipsilateral medial rectus subnucleus in the
oculomotor nucleus (III) that activates the medial rectus
muscle of the ipsilateral eye.

68. 
There is also an accessory pathway that originates from
projections of the ipsilateral horizontal canal ampulla on to
the ipsilateral magnocellular part of the medial vestibular
nucleus.

From there, increased activity is transmitted via the
ascending tract of Deiters (ATD) on to the medial rectus
subnucleus of the ipsilateral oculomotor nucleus, activating
the medial rectus muscle of the ipsilateral eye.

This drives both eyes to rotate towards the side opposite to
the direction of the head to stabilize the image on the
retina.

69. 
In addition to the contraction of the appropriate eye
muscles (ipsilaterally the lateral rectus and contralaterally
the medial rectus), a relaxation of the antagonist eye
muscles has to be initiated in the context of the push-pull
principle.

This is generated by inhibitory type 1 neurons in the
ipsilateral MVN.

These inhibitory type 1 neurons project on to the ipsilateral
abducens neurons and interneurons and, by inhibition,
relax the lateral rectus on the ipsilateral eye through the
Vlth nerve.

Likewise, the medial rectus of the contralateral eye is
relaxed by inhibition of the contralateral oculomotor
neurons where the signals come from the ipsilateral
superior vestibular neurons and pass through the medial
longitudinal fascicle (MLF).

71. 
To enhance this mechanism even further, at the same
time, the contralateral ampulla is deflected such that the
firing rate of the primary afferents is decreased.

This inhibits the contralateral medial vestibular nuclei,
resulting in an opposite effect for the antagonist eye
muscles, again optimizing gaze stabilization during
movement.

To summarize, generation of the VOR is mediated through
a combination of signals corning from both labyrinths.

During rotation to the ipsilateral side, the ipsilateral
horizontal SCC with ampullopetal endolymph movement
will transmit an increased firing rate to the central VN,
whereas the contralateral SCC will transmit a decreased
firing rate to the contralateral VN due to the ampullopetal
endolymph movement.

72. Commissural pathways

In the VN, there are also type 2 secondary vestibular
neurons, which behave exactly in an opposite manner to
type 1 neurons.

Activating the inhibitory type 2 neurons silences the
neighbouring type 1 neurons.

Whereas type 1 neurons increase their discharge rate
upon ipsilateral head acceleration, inhibitory type 2
neurons decrease their firing rate.

For movement towards the contralateral side, ipsilateral
type 1 neurons decrease their firing rate and ipsilateral
type 2 neurons increase it.

73. 
The VOR-generating mechanism is enhanced in a positive
feedback loop by commissural pathways where the excited
type 1 neurons in the ipsilateral MVN excite the type 2
neurons on the contralateral side that in their turn also
silence the contralateral type 1 neurons.

Excitation of the ipsilateral type 1 neurons is increased as
the silenced type 1 neurons on the contralateral MVN
inhibit the ipsilateral type 2 neurons, so that their inhibitory
effect is reduced on the ipsilateral type 1 neurons.

Therefore, type 1 cells receive direct input from the
ipsilateral SCC, as well as indirect input from the
contralateral SCC.

74. Principle of the head impulse (thrust) test

The head-impulse or head-thrust test, has become
established in clinical examination of the vertiginous
patient.

It detects severe unilateral loss of semicircular canal
function clinically.

The head-thrust test is positive for the side that causes the
refixation saccade upon thrust.

77. Vestibulospinal reflex

This head movement is actually a head nystagmus meant
for gaze stabilization

The extraocular muscles are the effector organs for the
VOR, while the extensor muscles of neck, trunk, arms and
limbs are those for the vestibulospinal reflex.

Similar to the VOR, the same push-pull mechanisms are
used for controlling the balance between extensor and
flexor muscles.

Any change in movement, detected by the vestibular
organ, is compensated by a series of contractions and
relaxations of several muscles.

Since the freedom of motion of the body is much larger
than that of the eye, a multitude of muscles are involved in
the reaction chain to maintain upright position and stability.

78. 
These reflexes are mediated through projections of the
vestibular nuclei on to the medial and lateral
vestibulospinal tract.

These pathways project to the lower limb and neck
muscles to maintain an upright position and balanced
locomotion.

The driving input here is mainly gravity detected by the
otolith system.

However, proprioceptive and visual information are also
necessary to provide the correct body position.