1. Vestibular Apparatus and Equilibrium
Dr Than Kyaw
December 2011

2. Functional Structure of Ear
External ear
Middle ear
Inner ear
Cochlea and Hearing
Vestibular appratus and Balance

3. Structure of Ear
External ear Pinna: cartilagenous Movable Localizing and picking up sound Tympanic membrane (ear drum) separate middle ear from external ear

4. Structure of Ear
Middle ear Three Ossicles: malleus, incus, stapes (also k/s hammer, anvil, stirrup) Veatibular (oval) window Cochlear (round) window Auditory tube (eustachian tube) - communicate with the pharynx

5. Structure of Ear
Internal ear 1. Cochlear portion (sensory for hearing) 2. Vestibular portion (sensory for equilibrium - Both organs in the temporal bone (bony cavity: osseous labyrinth) - Supplied by 2 branches of vestibulocochlear nerve

7. Cochlear portion (sensory for hearing) Cochlear: spiral shaped
Its base at the level of oval window

8. Cochlear duct (Scala media) endolymph Cochlear (Cross section)
Scala vestibuli
(above) filled with
perilymph
Cochlear duct
(Cross section)
endolymph
Scala tympani
(below) filled with
perilymph

9. Cochlear duct (Scala media) endolymph
Organ of Corti - along with the cochlear duct
convert sound waves to nerve impulses

10. Mechanism of hearing Sound (Air pressure waves) Tympanic membrane
Captured by Pinna
Vibratory
Scala vestibuli
Vestibular window
Auditory ossicles
Perilymph
The wave is transmitted to the scala media and from there to the scala tympani. Displacement of hair cell cilia against tectorial membrane caused by oscillations of basilar membrane (resulting from dissipation of sound waves) causes the hair cells to depolarize and create a nerve impluse.
Organ of Corti
The impulse is transmitted to auditory cerebral cortex through vestibulocochlear nerve.

11. Base

13. Hearing Frequency limit
Human - 20 to 20,000 cps
Dog – up to 50,000 cps

14. Vestibular apparatus/system
Control body stability
Movement
Posture
Balance/Equilibrium

15. Vestibular System Vestibule 3 semi-circular canals
- anterior, lateral, posterior
- perpendicuar to each other
The utricle
The saccule
Ampulla
These organs contain the sensory hair receptors:
the maculae (for the utricle & saccule)
and cristae (ampullae).

16. Macuae and cristae hair cells embedded in and otolithic membrane
Otolithic membrane – gelatinousc material
- contain otoliths: calcium carbonate crystals
relatively heavy
- utricle receptors – horizontal plain
- saccule receptors – vertical plain

19. Mechanism of equilibrium
Linear acceleration
(Macular sensors)
- Gliding stress to the hair cells
This force register position of the head
Due to weight of otoliths – sufficient inertia to sense linear acceleration or deceleration of the head
- Pull of gravity
- Position of the head

20. 1- Supporting cells 4- Membrane otolithique
2- Hair cell nerve fibers
3- Cilia Otolithes

21. Maculae in Saccule & utricle
Linear acceleration
(Macular sensors)
Saccule
Utricle
Maculae in the:
Saccule : is responsible for
vertical acceleration
Utricle: Is responsible for
horizontal acceleration
Maculae in Saccule & utricle

22. Linear Acceleration Stimuli
When the head starts or stops moving in a linear
acceleration otolothic membrane slides backward or forward over hair cells the hair cells will bend

23. Linear Acceleration Stimuli
When the hair bends towards the kinocilium the hair cell depolarize faster steam of impulse is sent to the brain
When the hair bends in the opposite direction the hair cells hyperpolarize Slower impulse generation
NOTE: It is important to understand that the maculae is responsible for the change in acceleration only. Because the hair cell can adapt it quickly
Nerve Action Potential

24. Rotational acceleration
(Ampullary crista)
- Ampullary crista detect any plane of rotational acceleration or deceleration
- Hair cells of cristae are stimulated when the head is moved.
- Mechanical action through the endolymph

25. Ampulla

26. Rotational acceleration
The receptors for Dynamic equilibrium are the ampulla which is found in the semicircular canals.
In each ampulla is a small elevation called a crista. Each crista is made up of hair (receptor)
cells and supporting cells, and covered by
a jelly-like material known as the cupula. Movement of the cupola stimulates the
hair cells
Ampulla

27. Rotational acceleration
The ampulla is responsible for the change in rotational movement, as continuous rotation does not stimulate the ampulla.
when the head starts moving in a rotationally the endolymph in the semicircular ducts move in the direction opposite to the body’s direction deforming the crista in the duct causes depolarization
If the body continues to rotate at a constant rate The endolymph moves at the same direction and speed as the body and stop the movement of hair cells

28. Rotational acceleration
When we suddenly stop moving, the endolymph keeps on moving in the opposite direction hyperpolarization of the hair cells that will tell the brain that we have stopped movement.