Vestibular apparatus

1. Vestibular apparatus
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
R.D. Gardi Medical College, Ujjain, Madhya Pradesh.
Email: dr.goothy@gmail.com

2. Introduction
 In addition to its cochlear-dependent role in hearing, the
inner ear has another specialized component, the
vestibular apparatus, which provides information
essential for the sense of equilibrium and for
coordinating head movements with eye and postural
movements.

4. Introduction
 Removal of vestibular apparatus in animals and birds produces
disturbances in the posture and equilibrium.
A study of vestibular reflexes in animals showed that they
influence posture and equilibrium.
So it is concluded that the vestibular apparatus has a role in the
regulation of posture and equilibrium.

5. Structure
 Vestibular apparatus is a complex sense organ
It is stimulated by
Gravity
Rotation movements

6. Structure
 Vestibular apparatus consists of two types of structures
Semicircular canals
Otolith organs

7. Structure
 There are three semicircular canals
Superior or anterior
Posterior
Lateral or horizontal

8. Vestibular apparatus
 Equilibrium is the sense of body orientation and motion.
The vestibular apparatus detects changes in position and motion of the
head.
As in the cochlea, all components of the vestibular apparatus contain
endolymph and are surrounded by perilymph.
Also, similar to the organ of Corti, the vestibular components each contain
hair cells that respond to mechanical deformation triggered by specific
movements of the endolymph.

9. Vestibular apparatus
 And like the auditory hair cells, the vestibular receptors may be
either depolarized or hyperpolarized, depending on the direction
of the fluid movement.
Unlike information from the auditory system, much of the
information provided by the vestibular apparatus does not reach
the level of conscious awareness.

10. Role of semicircular canals
The semicircular canals detect rotational or angular acceleration or
deceleration of the head, such as when turning the head, starting or
stopping spinning, or somersaulting.
Each ear contains three semicircular canals arranged three-
dimensionally in planes that lie at right angles to each other.
The receptor hair cells of each semicircular canal are situated on top
of a saddle shaped ridge located in the ampulla, a swelling at the base
of the canal.

12. Role of semicircular canals
The hairs are embedded in an overlying, caplike, gelatinous layer, the
cupula, which protrudes into the endolymph and stretches to the roof
of the ampulla.
The force of moving endolymph pushes against the cupula, causing it
to bow so that the embedded hairs are bent.
Acceleration or deceleration during rotation of the head in any
direction causes endolymph movement in at least one of the
semicircular canals because of their three-dimensional arrangement.

13. Role of semicircular canals
As you start to move your head, the bony canal and the ridge of hair
cells embedded in the cupula move with your head.
Initially, however, the fluid within the canal, not being attached to your
skull, does not move in the direction of the rotation but lags behind
because of its inertia.
When the endolymph is left behind as you start to rotate your head,
the fluid that is in the same plane as the head movement is in effect
shifted in the opposite direction from the movement.

15. Role of semicircular canals
This fluid movement causes the cupula to lean in the opposite
direction from the head movement, bending the sensory hairs
embedded in it.
If your head movement continues at the same rate in the same
direction, the endolymph catches up and moves in unison with
your head so that the hairs return to their unbent position.

16. Role of semicircular canals
When your head slows down and stops, the reverse situation
occurs.
The endolymph briefly continues to move in the direction of the
rotation while your head decelerates to a stop.
As a result, the cupula and its hairs are transiently bent in the
direction of the preceding spin, which is opposite to the way they
were bent during acceleration.

17. Role of semicircular canals
The hairs of a vestibular hair cell consist of one cilium, the kinocilium, along with a
tuft of 20 to 50 microvilli—the stereocilia—arranged in rows of decreasing height
from the taller kinocilium.
 As in the auditory hair cell, the stereocilia are linked by tip links. When the
stereocilia are deflected by endolymph movement, the resultant tension on the tip
links pulls on mechanically gated ion channels in the hair cell.
Depending on whether the ion channels are mechanically opened or closed by hair
bundle displacement, the hair cell either depolarizes or hyperpolarizes.

19. Role of semicircular canals
Each hair cell is oriented so that it depolarizes when its
stereocilia are bent toward the kinocilium and hyperpolarizes
when the stereocilia are bent away from the kinocilium.
The hair cells form a chemically mediated synapse with terminal
endings of afferent neurons whose axons join with those of the
other vestibular structures to form the vestibular nerve.

20. Role of semicircular canals
This nerve unites with the auditory nerve from the cochlea to form the
vestibulocochlear nerve.
Depolarization increases the release of neurotransmitters from the hair
cells, thereby bringing about an increased rate of firing in the afferent
fibers;
conversely, hyperpolarization reduces neurotransmitter release from
the hair cells, in turn decreasing the frequency of action potentials in
the afferent fibers.

21. Role of semicircular canals
This nerve unites with the auditory nerve from the cochlea to form the
vestibulocochlear nerve.
Depolarization increases the release of neurotransmitters from the hair
cells, thereby bringing about an increased rate of firing in the afferent
fibers;
conversely, hyperpolarization reduces neurotransmitter release from
the hair cells, in turn decreasing the frequency of action potentials in
the afferent fibers.

22. Role of semicircular canals
When the fluid gradually comes to a halt, the hairs straighten
again.
Thus, the semicircular canals detect changes in the rate of
rotational movement (rotational acceleration or deceleration) of
your head.
They do not respond when your head is motionless or when it is
moving in a circle at a constant speed.

23. Role of Otolith organs
The otolith organs provide information about the position of the
head relative to gravity.
(that is, static head tilt) and detect changes in the rate of linear
motion (moving in a straight line regardless of direction).
The otolith organs, the utricle, and the saccule are saclike
structures housed within a bony chamber situated between the
semicircular canals and the cochlea.

25. Role of Otolith organs
The hairs (kinocilium and stereocilia) of the receptor hair cells in
these sense organs also protrude into an overlying gelatinous
sheet, whose movement displaces the hairs and results in
changes in hair cell potential.
Many tiny crystals of calcium carbonate—the otoliths (“ear
stones”)—are suspended within the gelatinous layer, making it
heavier and giving it more inertia than the surrounding fluid

27. Role of Otolith organs
When a person is in an upright position, the hairs within the
utricle are oriented vertically and the saccule hairs are lined up
horizontally
Let us look at the utricle as an example. Its otolith-embedded,
gelatinous mass shifts positions and bends the hairs in two ways:

28. Role of Utricles
When you tilt your head in any direction so that it is no longer vertical
(that is, when your head is not straight up and down), the hairs are
bent in the direction of the tilt because of the gravitational force
exerted on the top-heavy gelatinous layer.
This bending produces depolarizing or hyperpolarizing receptor
potentials depending on the tilt of your head.
The CNS thus receives different patterns of neural activity depending
on head position with respect to gravity.

29. Role of Utricles
The utricle hairs are also displaced by any change in horizontal linear motion
(such as moving straight forward, backward, or to the side).
As you start to walk forward the top-heavy otolith membrane at first lags
behind the endolymph and hair cells because of its greater inertia.
The hairs are thus bent to the rear, in the opposite direction of the forward
movement of your head.
If you maintain your walking pace, the gelatinous layer soon catches up and
moves at the same rate as your head so that the hairs are no longer bent.

31. Role of Utricles
When you stop walking, the otolith sheet continues to move
forward briefly as your head slows and stops, bending the hairs
toward the front.
Thus, the hair cells of the utricle detect horizontally directed
linear acceleration and deceleration, but they do not provide
information about movement in a straight line at a constant
speed.

32. Role of Saccule
The saccule functions similarly to the utricle, except that it
responds selectively to tilting of the head away from a horizontal
position (such as getting up from bed) and to vertically directed
linear acceleration and deceleration (such as jumping up and
down or riding in an elevator).
Together the otolith organs let you know which way is up and
what direction you are heading.

33. Vestibular reflexes
Signals arising from the various components of the vestibular apparatus are carried
through the vestibulocochlear nerve to the vestibular nuclei, a cluster of neuronal
cell bodies in the brain stem, and to the cerebellum.
Here, the vestibular information is integrated with input from the eyes, skin surface,
joints, and muscles for
(1) maintaining balance and desired posture;
(2) controlling the external eye muscles so that the eyes remain fixed on the same
point, despite movement of the head; and
(3) perceiving motion and orientation.

34. Motion sickness
Some people, for poorly understood reasons, are especially
sensitive to particular motions that activate the vestibular
apparatus and cause symptoms of dizziness and nausea; this
sensitivity is called motion sickness.

35. Ménière’s disease
Occasionally, fluid imbalances within the inner ear lead to Ménière’s disease.
Not surprisingly because both the vestibular apparatus and cochlea contain
the same inner ear fluids, both vestibular and auditory symptoms occur with
this condition.
An afflicted individual suffers transient attacks of severe vertigo (dizziness)
accompanied by pronounced ringing in the ears and some loss of hearing.
 During these episodes, the person cannot stand upright and reports feeling
as though self or surrounding objects in the room are spinning around.

36. Permanent damage of semicircular canals
Permanent damage to the semicircular canals causes poor
balance and shaky, blurred vision when the head is moving
(because
the person cannot keep the eyes on the target during the motion).

37. Test for vestibular function
Caloric test:
Semicircular canals were stimulated by inducing water at 30
degrees or 44 degrees ( 7 degrees above or below the body
temperature) into the external auditory meatus.
It causes nystagmus, vertigo, nausea
Used as diagnostic test

38. Test for vestibular function
Barany’s test:
Barany’s chair- the subject is rotated along with the chair in the
plane of a SCC.
That particular SCC is stimulated and brings about nystagmus,
vertigo and nausea