vestibular apparatus, choclear process, process of hearing and balance in human, function and component of vestibular apparatus, types of cells present in vestibular apparatus

1. Vestibular Apparatus
By: MEENALI MISHRA
A13156216002
M.Sc. Medical Physiology
(3rd Semester)

2. Vestibular Apparatus
• Vestibular Apparatus is present in the inner ear
• Provides a sense of balance and orientation in space
• Responsible for coordination of movements
• Provides other control systems information about the position of
head in space
• Cannot accomplish postural adjustments on its own

3. Functions Of Vestibular Apparatus
It has 3 primary functions:
 Plays a dominant role in subjective sensation of motion & spatial
orientation of the head
 Adjusts muscular activity & body positions to maintain posture
 Stabilizes in space the fixation point of the eye when the head
moves, providing a stable image upon the retina

4. Vestibular Anatomy
 Consist of a tube ( membranous labyrinth) within a tube (bony
labyrinth) construction
 Both tubes are fluid filled: Endolymph in Membranous labyrinth
and Perilymph in Bony labyrinth
 Two sac-like Utricle & Saccule (otolith organs) and three
semicircular canals are joined together in the same fluid filled
environment

6.  The Otolith Organs
• Saccule & Utricle are 2 otolith organs
• They are dilations of the labyrinth that contain sensory epithelia
• Receptors called Maculae are patches of hair cells topped by small
calcium carbonate crystals called Otoconia
• Membrane containing otoconia are called otolithic membrane
• Saccule & Utricle lie at 90° to each other
• Utricle is an oval tube
• Saccule is flattened & irregular

7. • Utricle is most sensitive to tilt when head is in
upright position
• Saccule is most sensitive to tilt when the head
is in horizontal position
• Sensory epithelium of the utricle (Utricular
Macula) is situated parallel to the ground when
the head is upright
• The Saccular Macula is oriented vertically in an
upright position

8. Simulation of Utricle & Saccule in different
Movements of the head:
 In Erect (motionless) posture, Saccules are stimulated as otolith
membrane is pulled down by gravity. Utricles are unaffected
In Supine (motionless) posture, both utricle & Saccule get stimulated
Up & down movement of the head lead to stimulation of saccules
Both utricles & saccules respond to anterior-posterior movement of
the head
Only utricles respond to side to side head movement

9.  The Semicircular Canals
• There are 3 Semicircular canals:
I. Anterior Canal
II. Posterior Canal
III. Horizontal Canal
• The anterior semicircular canal of one side and
posterior canal of opposite side are in the same
plane
• Dilation is present at the junction of each canal
and the utricle called the Ampulla

10. • Receptors are present in the ampulla
• Receptors are called Proprioceptors
• Receptors are mechanoreceptors in nature
• Sensory epithelium of semicircular canal is Cristae Ampullaris or Crista
acustica
• Hair cells of each crista are polarized in the same direction
• Crista is a folded structure
• Cilia of hair cells project into the Cupula (Gelatinous substance)

12. Vestibular Receptor Cells- Hair Cells
• Can be classified as type I or type II
• Situated on the cristae
• Innervated by afferent & efferent fibres of the vestibular division of
vestibulocochlear (8th) nerve
• The hair cells have cilia on them.
• Cilia are of 2 types- one large Kinocilium and several smaller
Stereocilia

13. Stereocilia:
• Non motile & rigid
• Consist of actin filaments and
other cytoskeletal proteins
• Vary in height but are graded with
reference to the kinocilium
• The tallest stereocilia being close
to the kinocilium
• Stereocilia of the hair cells are
embedded in the gelatinous
cupola present on the tip of the
crista of semi-circular canals
Kinocilia:
• They are weakly motile
• Each hair cell is morphologically
polarized with respect to the
location of kinocilium
• Bending of stereocilia towards
kinocilium depolarizes the cell &
results in increased afferent
activity
• Bending of stereocilia away from
the kinocilium hyperpolarizes the
cell & results in decreased
afferent activity

15. Vestibular Physiology
In the Otolith Organs:
• When head tilts forward, gravitational force on otolith membrane &
hair cells changes which displaces the cilia
• Maximal discharge occurs when the cilia are bent to one side
• Minimal discharge occurs when they are bent to the other side
• Hair cells are polarized in different directions in various regions of the
macula, hence some hair cells are always excited by movement of
head in any direction
• Fibres innervating utricular macula signal Linear Acceleration

17. In the Semicircular canals:
• Semicircular canals are positions & structured to sense Angular
Acceleration
• Relative movements of the endolymph & the labyrinth causes the
bending of the cilia in the cristae
• When head is rotated, canals move with it, but endolymph lags behind
due to inertia
• Results in bending of the cupula in direction opposite to rotation & nerve
cell discharge increases or decreases depending on the movement
• If rotation continues, endolymph catches up with the movement, cupula
returns to original position & nerve fibres discharge restored to normal

18. • If rotation suddenly stops, canals immediately stop their rotation, but
endolymph does not.
• Cupula is bent to the other direction so effect on the discharge of the
nerve is opposite to that of original rotation
• Semicircular canals have orthogonal orientation
• Rotation in any direction are sensed by one or more canals
• Any direction of rotation can be signalled uniquely by combined
discharge from the three canals

20. Vestibular Nuclei
• Vestibular Nuclei lies on the floor of the Fourth
Ventricle
• Most afferent fibres from hair cells terminate here
• Four major group of cell bodies may be identified
here
i. Superior Vestibular Nucleus (SVN) of Bechterew
ii. Lateral Vestibular Nucleus (LVN) of Dieter
iii. Medial Vestibular Nucleus (MVN) of Schwalbe
iv. Descending Vestibular Nucleus (DVN)

21. Projections From the Vestibular
Nuclei:
Projections from the vestibular nuclei extend to the cerebellum,
extraocular nuclei & spinal cord
Cells of SVN project to the nuclei of the extraocular muscles ( III & IV)
LVN is the only source of fibres to the Vestibulospinal tract which are
responsible for limb & trunk reflexes
DVN is related to the cerebellum

22. Projection Pathways
Vestibular nuclei on either side of the brainstem exchange signals
regarding movement & body position.
These signals are sent down the following projection pathways:
To the Cerebellum : Signals sent to cerebellum are relayed back as
muscle movements of head, eye & posture
 To Nuclei of Cranial Nerves III, IV & VI: Signals sent to these cause
Vestibulo-ocular reflex
 To the Spinal Cord: Signals sent to these allow quick reactions to
both the limbs & trunk to regain balance

23. To the Reticular Formation: Signals sent to these informs about the new
posture the body has taken on & how to adjust circulation & breathing
due to body position
 To the Thalamus: Signals sent to the thalamus allow for head & body
motor control as well as being conscious of body positions
 Via the Ventral Pathway: Contributes to vertical orientation &
perception of the direction of gravity

25. Motion Sickness
• While travelling in bus or ship, the
jerky movements stimulates the
vestibular apparatus
• When the person sees that he is in
rest with respect to the bus he feels
stabilised
• This conflicting stimuli lead to
confusion & results in Motion Sickness
• It occurs in susceptible individuals
• Characterised by: headache, vertigo,
pallor, perspiration, nausea, vomiting,
etc.

26. Nystagmus
• Characterised by jerky, conjugate movement of
the eyeball
• Maybe horizontal, vertical or rotatory
• In horizontal, eyeball moves slowly to one side &
quickly to other side
• Occurs due to attempt to keep the gaze fixed at
one point when normal fixation is disturbed
• May occur when the head is rotating, when one
is in a moving vehicle, etc.
• Also seen in cerebellar & vestibular diseases

27. Caloric Test
• Hot or cold water is poured to the external ear
• Convection current produced within the endolymph due to
temperature difference
• Movement of endolymph stimulates cupula to produce nystagmus
with healthy vestibule apparatus
• This is Caloric Test

28. Thank You