Vestibular system is clearly explained

1. Physiology of vestibular
system
-DEV

2. Inner ear structures
• Cochlea
• Vestibular apparatus : 1.semi-circular canal
2.vestibule – a)Utricle
b)Saccule

3. Semi-circular canal
• Bony labyrinth – semi circular canal, contains Perilymph.( na+ and k+)
inside
• Membranous labyrinth – semi circular duct, contains endolymph.
inside ( na+ & k+)
• sensory epithelium – Crista Ampullaris
There are three semi circular canals
• Anterior or superior semi circular canal
• Lateral or horizontal semi circular canal
• Posterior semi circular canal
The semi circular canals join the vestibule, at the junction
there is a dilated part called ampulla where the Crista Ampullaris
is located.

4. •Position of the semi circular canals
1. Anterior canal – 41 degree from the sagittal plane.
2. Posterior canal – 56 degree from the sagittal plane.
3. Lateral canal – 30 degree from the naso-occipital plane.
This position allows the semi circular canals to sense the
angular acceleration from all direction, Also in this position
they are perpendicular to each other.
Their main function is to maintain the Dynamic equilibrium.

5. Crista Ampullaris
It is the sensory epithelium of the
semi circular canal.
It has a gelatinous structure called the
cupula, which is attached to the hair
cells.
The hair cells give out the fibers for the vestibular nerve.

6. How does it work ?
Consider a case of rotating head to left
• Due to the endolymph inertia the cupula will shift to right.
• This causes the hair cells of the left semi circular canal to
excite and release action potentials but in the right semi circular
canal it will cause inhibition.
Afferent fibres
vestibular ganglion/Scarpa’s ganglion
vestibular nuclei
right abducens nuclei
left oculomotor nuclei right lateral rectus
Left medial rectus
This helps in fixing our gaze

7. Saccadic movement
• Whenever you rotate to the left the eye
moves to the right very fast
• Afterwards it has to compensate and go back
to left to maintain the gaze to the left.
• This is done by the frontal eye field.
Frontal eye field
Paramedian pontine reticular formation
Left abducens right abducens
Left Lateral rectus right oculomotor
right medial rectus
This causes the eye to move to left, this
movement is called saccadic movement.
The beating to the right and to the left is
called Vestibular Nystagmus. (physiological)

8. Vestibule
• Outer bony part – vestibule - perilymph
inside
• Inner membranous – utricle and saccule - endolymph
inside
• Sensory epithelium – macula or otolith organ

9. Utricle
• In here the macula is present horizontally on the
floor.
• The stereocilia is facing upwards.
• It responds to linear acceleration in horizontal
plane and head tilt.
Saccule
• Macula is on vertical plane .
• The stereocilia facing outwards.
• It responds to vertical acceleration.
Both utricle and saccule maintain the static
equilibrium.

10. Macula
• It is the sensory epithelium of the
vestibule.
• It responds to linear acceleration in
vertical and horizontal plane.
• The otolith membrane attached to
the hair cells move in the opposite
direction of the acceleration due to
the inertia of the otoconia present
on the top of it.
• This causes depolarisation of the
hair cells.

11. • There are two types of vestibular hair cells Type I
and Type II.
• There are stereocilia on them, the longest one is
called kinocilia.
• The K+ channel on the stereocilia are linked to
each other by Tip link.
• When the tip link is stretched the K+ channel
opens and K+ influx occurs which also opens the
voltage gated Ca channel and causes
depolarization.
• Excitatory neurotransmitters like glutamate and
aspartate are released.
Vestibular hair
cells

13. Pathway
Vestibular branch of CNVIII
Vestibular nucleus
Vestibulospinal fastigial nucleus abducens & thalamus
tract oculomotor
Spinal cord cerebellum eye cerebral cortex
Maintains posture posture, coordinate eye higher
and balance muscle tone, movements perception
balance

14. THANKS FOR LISTENING
ref: Diseases of ENT by PL Dhingra, Human physiology by Guyton, Ninja nerdYouTube
channel(pic), c super boys