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Vestibular Apparatus and
Equilibrium
Dr Than Kyaw
December 2011
Functional Structure of Ear
External ear
Middle ear
Inner ear
Cochlea and Hearing
Vestibular appratus and Balance
Structure of Ear
External ear
Pinna: cartilagenous
Movable
Localizing and picking up sound
Tympanic membrane (ear drum)
separate middle ear from external ear
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
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
Cochlear portion
(sensory for hearing)
Cochlear: spiral shaped
Its base at the level of oval window
Cochlear
(Cross section)
Cochlear duct
(Scala media)
endolymph
Cochlear duct
(Cross section)
endolymph
Scala vestibuli
(above) filled with
perilymph
Scala tympani
(below) filled with
perilymph
Cochlear duct
(Scala media)
endolymph
Organ of Corti - along with the cochlear duct
convert sound waves to nerve impulses
Mechanism of hearing
Sound
(Air pressure
waves)
Scala
vestibuli
Captured by
Pinna
Tympanic
membrane
Vibratory
Perilymph
Vestibular
window
Auditory
ossicles
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.
Base
Hearing Frequency limit
• Human - 20 to 20,000 cps
• Dog – up to 50,000 cps
Vestibular apparatus/system
Control body stability
Movement
Posture
Balance/Equilibrium
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).
• 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
Mechanism of equilibrium
Linear acceleration
(Macular sensors)
- Pull of gravity
- Position of the head
- 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
1- Supporting cells
4- Membrane otolithique
2- Hair cell
5- nerve fibers
3- Cilia
6- Otolithes
Linear acceleration
(Macular sensors)
• Maculae in the:
– Saccule : is responsible for
vertical acceleration
– Utricle: Is responsible for
horizontal acceleration
Saccule
Utricle
Maculae in Saccule & utricle
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
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
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
Ampulla
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
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
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.