Medical neuroscience

1. Domina Petric, MD
Peripheral vestibular
mechanisms

2. Vestibular function
 Vestibular labyrinth is an extension of the inner ear designed to
sense the motions of the head movements and inertial effects due
to gravity.
 Static head position and linear accelerations of the head are sensed
by hair cells in the otolith organs.
 Rotational accelerations are sensed by hair cells in the semicircular
canals.
 Vestibular signals are relayed to integrative centers in the brainstem
and cerebellum: adjusted postural reflexes and eye movements.

3. Vestibular function
Vestibular signals also reach parts of the parietal
cortex.
In the parietal cortex our normal sense of
orientation in three dimensional space is
constructed (space awareness).

4. Movements of the
head: rotational
movements or
rotational accelerations
Rolling movement: rotation
around the x axis.
Pitch movement: rotation
around y axis.
Yaw movement: rotation
around z axis.
http://www.3dcadbrowser.com
X axis
Y axis
Z axis

5. There are also linear movements of the head:
back-forward
left-righ
up-down

6. Otic placode and labyrinth
 Otic placode is embriologycal precursor both for the cochlea and the
labyrinth.
 Semicircular canals of the labyrinth are filled with endolymph.
 Endolymph is rich with potassium ions (+80 mV).
 Perilymph surrounds vestibular membranes.
 Otolithic organs of labyrinth are the utricle and the saccule.
 There are three semicircular canals in the labyrinth: superior,
inferior and horizontal.
 Hair cells are present both in the otolithic organs and semicircular
canals: sensory transduction.

7. Vestibular nerve
Hair cells are innervated by the
peripheral processes of ganglion
cells of the Scarpa´s ganglion from
wich the vestibular nerve arises.

8. Sensory transduction
The longest stereocilium of the hair cell is close to the kinocilium
which might be present in adult life (sometimes degenerates).
When there is deflection of stereocilia from the shortest to the
longest one, there will be depolarisation of the hair cell.
When there is deflection of stereocilia from the longest to the
shortest one, there will be hyperpolarisation of the hair cell.

9. Stereocilia move in the direction from the smallest towards the largest.
Tip link protein streches and opens the potassium channels: influx of potassium ions
causes the depolarisation of the hair cell.
Calcium channels open: calcium
ions influx causes the exocytosis
of vesicles. Released neurotransmitter
generates the action potential
in the axons of the vestibular nerve.
Neurotransmitter is probably glutamate.

10. Sensory transduction, otolith organs
In otolith organs the hair cells are arranged with certain axis
of symmetry that divides the sensory epithelium within each
of the otolith organs (utricle, saccule).
Sensory spot of otolith organs is called MACULA.
Saccular macula has to be rotated along the y axis to cause
depolarisation-hyperpolarisation of its hair cells: pitch of the
head in the forward or the backward tilt and linear
accelerations up and down (for example in elevator).
Saccular macula is mostly in the vertical plane.

11. Sensory transduction, otolith organs
In the utricular macula the longest stereocilia are
directed towards the line of the symmetry.
Utricular macula is mostly in the horizontal plane.
Utricular macula is sensitive to linear accelerations of
the head in the horizontal plane (left-right movement).

12. Otolithic organs maculas
Saccular macula Utricular macula
 Hair cells stereocilia are directed
towards the line of the symmetry
so the longest stereocilium is close
to the line of symmetry.
Hair cells stereocilia are directed away
from each other: the longest stereocilium is
close to the periphery.

13. Otolith organs
 There is a gelatinous membrane that sits on the top of the apical
surface of the sensory epithelium in otolith organs (in maculas).
 On the top of the gelatinous membrane are calcium carbonate
crystals that are called OTOCONIA (˝ear stones˝).
 When we tilt our heads, gelatinous membrane with otoconia on the
top of it, tilts and slips in one direction relative to the fixed sensory
epithelium with hair cells.
 Movement of head in one direction causes depolarisation of half of
hair cells and hyperpolarisation of the other half of hair cells that
are directed in the opposite direction (axis symmetry).

14. Semicircular canals
Hair cells of the semicircular canals are located in the
ampullae.
Within the ampulla there is sensory epithelium on the
structure called CRISTA.
In each crista of the semicircular canals hair cells are
arranged in one direction (there is no axis of symmetry).

15. Semicircular canals
 In the ampulla of semicircular canal there is crista.
 Overlying that crista is a gelatinous mass through which protrude
the stereocilia of the hair cells.
 Gelatinous mass is called CUPULA.
 There is only one axis of depolarisation for the hair cells in one
semicircular canal.
 The cupula creates a barrier to the flow of endolymph.
 When we turn our heads in one direction, the force of inertia is
going to cause a deflection of the cupula.

16. Semicircular canals
Moving the head for example in the right direction causes the tilt of
sensory epithelium to the right and moving of the endolymph to the
left with left deflection of cupola with deflection of hair cells:
 if the stereocilia move towards the longest stereocilium, there will
be depolarisation and vice versa.
In one crista of one semicircular canal all hair cells depolarise or
hyperpolarise because there is no axis of symmetry.
Moving the head in right direction will activate (depolarise) hair cells
in the right semicircular canal and deactivate (hyperpolarise) hair
cells in the left semicircular canal.

17. Semicircular canals
Both horizontal semicircular canals (on the left and right side of
the head) operate in the same plane (horizontal plane).
The superior semicircular canal in one side of the head is
operating in the plane of the inferior semicircular canal in the
opposide side of the head.
For example, the right superior canal is a functional pair of the
left inferior canal.
Forward tilt of the head in the right direction will activate right
superior canal and deactivate left inferior canal.

18. Semicircular canals
 Backward tilt of the head in the right rotation (right direction) will
activate the left inferior canal and deactivate the right superior
canal.
 Forward tilt of the head in the middle (median line) will activate
both superior canals and deactivate both inferior canals.
 Forward tilt of the head in the left direction will activate left
superior canal and deactivate right inferior canal.
 Backward tilt of the head in the left direction will deactivate left
superior canal and activate right inferior canal.

19. Head
moves
forward
in the
right
direction.
Right
superior
canal is
activated.
Left
inferior
canal is
deactivated.
Head
moves
backward
in the
right
direction.
Right
superior
canal is
deactivated.
Left
inferior
canal is
activated.

20. Head
moves
forward
in the left
direction.
Left
superior
canal is
activated.
Right
inferior
canal is
deactivated.
Head
moves
backward
in the left
direction.
Left
superior
canal is
deactivated.
Right
inferior
canal is
activated.

21. Head moves forward
in the middle
direction.
Left
superior
canal is
activated.
Right
superior
canal is
activated.
Right
inferior
canal is
deactivated.
Left
inferior
canal is
deactivated.
Head moves
backward in the
middle direction:
both inferior canals
are activated and
superior canals are
deactivated.

22. Semicircular canals
When we move our heads in the z axis to the
right, that will activate right horizontal canal and
deactivate left horizontal canal.
When we move our heads in the z axis to the left,
that will activate left horizontal canal and
deactivate right horizontal canal.

23. Vertigo
If there is damage to the left vestibular system,
there will be right vertigo (feeling like the head is
spinning to the right).
If there is damage to the right vestibular system,
there will be left vertigo (feeling like the head is
spinning to the left).

24. Literature
https://www.coursera.org/learn/
medical-neuroscience: Leonard E.
White, PhD, Duke University
http://www.3dcadbrowser.com