The Human Vestibular System

1. The VESTIBULAR SYSTEM
MAINTENANCE OF BALANCE AND EQUILIBRIUM

2. VESTIBULAR SYSTEM
the sensory system that provides the leading
contribution about movement and sense of balance.
The brain uses information from the vestibular system in
the head and from proprioception throughout the
body to understand the
body's dynamics and kinematics (including its position
and acceleration) from moment to moment.

3. Vestibular apparatus
Composed of: vestibule housing the utricle and
saccule, and the semicircular canals housing
the semicircular ducts for the sense of
equilibrium.
generate nerve impulses along the vestibular
branch of the vestibulocochlear (VIII) nerve.

5.  Semicircular ducts: Contain cristae ampularis in its
ampullae, site of hair cells for dynamic equilibrium
(maintenance of body position, mainly the head, in
response to rotational acceleration and deceleration).
 Utricle: Contains macula, site of hair cells for static
equilibrium (maintenance of body position, mainly the
head, relative to the force of gravity).
 Saccule: Contains macula, site of hair cells for static
equilibrium (maintenance of body position, mainly the
head, relative to the force of gravity).

8. Components of Sense of Equilibrium
 STATIC EQUILIBRIUM – associated with the vestibule and
is involved in evaluating the position of the head relative
to gravity.
 KINETIC EQUILIBRIUM – associated with the semicircular
canals and is involved in evaluating changes in the
direction and rate of head movements (angular
acceleration).

9. Vestibular stimulus
Angular acceleration (rotation)
SEMICIRCULAR CANALS
Linear acceleration (head position)
UTRICLE AND SACCULE

11. SEMICIRCULAR CANALS
 three semicircular, interconnected tubes that are almost
perpendicular to one another so that they represent all 3
planes in space
 the horizontal semicircular canal (also known as the lateral
semicircular canal),
• superior semicircular canal (also known as the anterior
semicircular canal),
• and the posterior semicircular canal.

13. SEMICIRCULAR CANALS

14. Ampulla: Enlargement at the base of each SCC
Crista ampullaris: Elevation or crest in the
epithelium of within each ampulla containing a
group of hair cells and supporting cells
Cupula: curved gelatinous mass suspended over
the crista ampullaris

15.  Stereocilia: apical modifications of the cell, which are
distinct from cilia and microvilli
 a.k.a stereovilli
 longer than typical microvilli; contains actin filaments
 length (distinguishing from microvilli)
 lack of motility (distinguishing from cilia).
 Kinocilium: a special type of cilium on the apex of hair
cells; the longest cilium located on the hair cell next to
40-70 sterocilia.

20.  During movement of the body, the hair cell is
depolarized when the sterocilia move toward the
kinocilium.
 The depolarization of the hair cell causes
neurotransmitter to be released and an increase in firing
frequency of cranial nerve VIII.
 When the sterocilia tilt away from the kinocilium, the hair
cell is hyperpolarized, decreasing the amount of
neurotransmitter released, which decreases the firing
frequency of cranial nerve VIII

22. Detection of Head Rotation
Relative fluid flow of endolymph opposite to
head rotation (by virtue of inertia)
 hair cells emits tonic discharge of about 100
impulses per second even when cupula is at
rest
 when head rotates, hair bends to one side
and rate of discharge increases greatly
 with continued rotation, excess discharge of
the hair cell gradually subsides back to
resting level during the next few seconds.

26. Detection of Head Rotation
Within the first few seconds of rotation, back
resistance to the flow of fluid causes the
endolymph to begin rotation as rapidly as
the semicircular canal itself
 in another 5 to 20 seconds the cupula slowly
returns to its resting position-- elastic recoil
 when rotation suddenly stops, endolymph
continues to rotate while the semicircular
duct stops  Cupula bends in the opposite
direction  hair cell stop discharging entirely
After another few seconds, endolymph stops
movingcupula gradually to its resting
position back to normal tonic level

31. Predictive Function of the SCD in the
Maintenance of Equilibrium
 SCD system predicts disequilibrium that is about to
happen so that the equilibrium centers can make
appropriate anticipatory adjustments
 Macule of utricle and saccule can not “predict”

32. Otholitic organs: UTRICLE AND SACCULE
 A.k.a ‘Gravity organs’
 UTRICLE- larger than the saccule and is of an oblong
form, compressed transversely, and occupies the
upper and back part of the vestibule
 UTRICULAR MACULA- allows a person to perceive
changes in longitudinal acceleration as well as
effects of gravity.
 SACCULE- smaller of the two vestibular sacs;
globular in form and lies near the opening of
the scala vestibuli of the cochlea; connected to the
utricle by the utricosaccular duct
 SACCULAR MACULA - allows a person to perceive
changes in horizontal acceleration and head tilts in
the vertical plane.

33.  Macula: 2 by 3 mm patches of hair
cells and supporting cells
Within each maculae, the
stereocilia are embedded in a
gelatinous mass known as the
otolithic membrane, which
contains small stonelike calcium
carbonate particles called
otoconia or otoliths
 Striola: curved dividing ridge that
runs through the middle of
the macula

34. The macula
 Bottom layer - made of sensory hair cells embedded in
bottom of a gelatinous layer. Each hair cells consists of 40
to 70 steriocilia and a kinocilium, which lies in the middle
of the steriocilia and is the most important receptor.
 Otolith layer -On top of the bottom layer. The otoliths are
relatively heavy, providing weight to the membrane as
well as inertia allowing for a greater sense of gravity and
motion.
 Otolithic membrane- refers to the gelatinous layer and
the otholiths together, where the tips of the stereocilia
and kinocilium are embedded.
 When the head is tilted such that gravity pulls on the
statoconia the gelatinous layer is pulled in the same
direction also causing the sensory hairs to bend.

35. when the body is in anatomical
position:
the patch of hair cells in the UTRICLE is
nearly horizontal, with the stereocilia
oriented vertically
the sensory epithelium is vertical in
the SACCULE, with the stereocilia
oriented horizontally

36.  in contrast with the semicircular canals (where
directionality is inherent in the structure), in the otolith
organs, directionality is conferred solely by the
orientation of the hair cell stereocilia
 orientation of the stereocilia within the sensory
epithelium is determined by the STRIOLA
 UTRICLE- kinocilia are oriented TOWARD the striola
 SACCULE- kinocilia are oriented AWAY the striola
 in any position, some hair cells will be depolarized and
others hyperpolarized in BOTH otolith organs

40. Vestibulo Ocular Reflex (VOR)
a.k.a Oculocephalic reflex
 reflex eye movement that stabilizes images on the retina during head
movement by producing an eye movement in the direction opposite to
head movement, thus preserving the image on the center of the visual
field.
 e.g when the head moves to the right, the eyes move to the left, and vice
versa.
 patients whose VOR is impaired find it difficult to read using print, because
they cannot stabilize the eyes during small head tremors.
 does not depend on visual input and works even in total darkness or when
the eyes are closed. However, in the presence of light, the fixation reflex is
also added to the movement.[1]

44. Caloric Testing= COWS

46. Disclaimer:
The following are shizz I can not
fully explain but are good-to-
knows to must-knows. Please listen
with discretion. Learning Issue
identification and ISP is advised.