The vestibular system detects motion and orientation of the head. It consists of five sensory end organs - three semicircular canals and two otolith organs. The semicircular canals detect rotational movement and contain cristae that sense angular acceleration. The utricle and saccule are the otolith organs and detect linear acceleration and gravity. They contain hair cells covered by an otolith membrane with embedded crystals that provide inertia against endolymph fluid. Together, the vestibular system works with vision and proprioception to maintain balance and spatial orientation.

1. Anatomy of vestibular
system
Dr. Ghada M. Wageih Felfela
Audio-Vestibular medicine Specialist
Health Insurance

2. Balance of our body during static or dynamic positions is
maintained by:
1. Vestibular apparatus (inner ear) [Fastest system].
2. Vision (Eye)
3. Somatosensory/ proprioceptive system

3. Vestibular function contributes to:
- Perception of Self-motion
- Head position in relation to motion
- Stabilizing body position to avoid falling
- Spatial orientation relative to gravity
- posture correction
- Adaptation to new balance tasks &
- Central Compensation after vestibular diseases.

4. Types of movement:
1. Translational (Linear)
2. Rotational

6. Vestibular system is formed of bony & membranous
labyrinth with 5 sensory end organs:
1. 3 crista ampularis in 3 SCC → for angular velocity.
2. 2 otolith organs: Utricle & Saccule → for linear acceleration

7. Fluid Dynamics:
- Perilymph is a filtrate of CSF and from blood vessels in the
ear.
- Endolymph is from marginal cells of the stria vascularis of the
cochlea and the dark cells of the vestibular labyrinth.
Endolymph Circulation:
a) The longitudinal  endolymph flows toward endolymphatic
sac, where it is resorbed.
b) The radial theory  a local transverse and active diffusion
process between endolymph and perilymph.
c) The dynamic theory.

9. CELLULAR MORPHOLOGY OF VESTIBULAR SENSORY
EPITHELIUM:
- The sensory epithelium is composed of:
o Hair cells,
• Type I cells: Flask-shaped cells”, enclosed by a nerve calyx → Phasic
receptors (fire when move).
• Type II cells: Cylindrical body → Tonic receptors (fire all the time).
o Supporting cells,
• Extend from the basement membrane to the apical surface,
with basal nuclei just above BM and below the hair cells.
• Supporting cells can differentiate into new hair cells following
destruction of the sensory epithelium as after acoustic trauma.
o Afferent nerve fibers
o Efferent nerve fibers

11. o TIP-LINKS:
o RIBBON SYNAPSE:
o Relationship between the Direction of Force and Hair
Cell Activation:
- The adequate stimulus for hair cell activation is a force
acting parallel to the top of the cell, resulting in
bending of the hairs.
- Deflection of the hairs toward the kinocilium causes
depolarization.
- While, bending in the opposite direction produces the
reverse effect (hyperpolarization).

12. Semi Circular Canals
• Each canal forms two-thirds of a circle.
• provide sensory input about angular head velocity.
• Head motion frequencies (0.5–7 Hz).
• each canal is maximally sensitive to rotations that lie in
the plane of that canal.
o Lateral canal inclined to 30 degrees  sensitive to rotations
about a vertical axis.
o Superior/posterior canals 45° off of sagittal plane

13. END ORGAN ANATOMY:
Ampulla, contains the
o Neuroepithelium (crista ampullaris),
o Cupula,
 gelatinous mass
 consists of mucopolysaccharides within a keratin
meshwork,
 extends from the surface of the cristae to the roof and
lateral walls of the membranous labyrinth
 form a fluid-tight partition,
 dividing the interior of the ampulla into two approximately
equal parts.
o Supporting cells,
o Connective tissue, blood vessels, and
nerve fibers.

14. • In horizontal SCC cristae, the kinocilia are located on the side of the hair
cell, closest to the utricle.
• In vertical SCC cristae, the kinocilia are located on the side of the hair cell
furthest from the utricle, the canalicular side.
• The ampullae of HSCC and ASCC lie close together, just above the oval
window, but the ampulla of the posterior canal opens on the opposite side
of the vestibule

16. SCC & Detection of angular acceleration: dynamic
equilibrium:
A. Each afferent neuron has a baseline firing rate “Resting
potential”:…….
 Morphological polarization…..
 coplanar arrangement…..
B. “Push-Pull organization”:……….
A. Angular Velocity:
1. Within first few seconds of rotation, endolymph lags behind
and rotates within SCC in a direction opposite that of the
head.
2. After about 5 to 20 seconds of constant speed, the
cupula slowly returns to its resting position by its own
elastic recoil.
3. When the rotation suddenly stops - the cupula bends in the
opposite direction, causing the hair cell to stop discharging
entirely.

18. Ewald’s laws:
1) the eye and head movements always occurred in the plane
of the canal being stimulated and in the direction of
endolymph flow
2) ampullopetal endolymph flow in the horizontal canal
caused a greater response and ampullofugal endolymph
flow in the vertical canals caused a greater response than
did ampullopetal endolymph flow.

19. Otolith Organs
“Gravity receptors”

20. Macula:
• Definition: is a thickened ectodermal elliptical patch, elongated “J-
shaped”, in which the hair cells of each organ is localized, is 2 mm
in diameter
• In the utricle the macula projects from the anterior wall on the
floor of that tubular sac, contains about 30,000 hair cells.
• In the saccule the macula is in the vertical plane and directly
overlies the bone of the inner wall of the vestibule. & contains
some 16,000.

21. Each macula consists of neuroepithelium:
- supporting cells and
- sensory cells,
- a basement membrane,
- nerve fibres and nerve endings,
- underlying connective tissue………………
- otolithic membrane, that covers the entire sensory
macula.
- Otoconia (“ear dust”) or “statoconia”:
• fine, dense particles
• rhombohedral crystals (0.5-20 μm long)
• Consisting of calcium carbonate
• embedded within and lying on the otolithic membrane.
• SG is almost 3 times that of the membrane and the endolymph—
and thus add considerable mass to it.

22. Abnormal or reduced otoconia have been produced in
humans by:
 Aminoglycosides, phenytoin, carbonic anhydrase inhibitors,
prostaglandins, and ethacrynic acid.
 Aging also lead to loss of otoconia.
 Alteration in endolymphatic ion concentration may be the cause of both
hair cell and otoconial degeneration.

23. • In the saccule and utricle, the striola divides the hair cells
into populations with opposing hair bundle polarities.

24. MECHANISM OF STIMULATION
• Detection of linear acceleration:
 static equilibrium & linear acceleration