Vestibular System

1. The Vestibular System
Maintaining Balance

2. Maintaining Balance
•The vestibular system determines the position and
motion of your head in space.
•There are two components to monitoring motion:
•Detecting rotation
–what happens when you shake or nod your head.
–This is called angular acceleration.
•Detecting motion along a line
–what happens when the elevator drops beneath you or
when your body begins to lean to one side
–This is called linear acceleration.
•The vestibular system has two receptor organs to
accomplish these tasks.

3. Elements of the Vestibular
Labyrinth
•Continuous with the cochlea
•Three semicircular canals
–Detect angular acceleration
•Two otolith organs:
–Utricle & saccule
–Detect linear acceleration
•Vestibular nerve fibers
–synapse with hair cells
–have cell bodies in Scarpa's ganglion

4. The Vestibular Labyrinth

5. Semicircular Canals
•Detect angular acceleration
•There are 3 canals
•correspond to the three dimensions in which
you move
•each canal detects motion in a single plane.
•Paired on opposite sides of the head
•Each canal is a continuous endolymph-filled
hoop.
•The actual hair cells sit in a small swelling at
the base called the ampula.

6. View of Semicircular Canals

7. Semicircular Canal Cross-Sections
Semicircular duct inside
Semicircular canal

8. The Ampulla
•Bulge at end = ampulla
•Contains a sheet of cells = cristae
•The hair cells are arranged as a single
tuft that projects up
•Cilia of hair cells extend upward into a
gelatinous cupula

9. How it Works
•Angular acceleration (moving your head
in the plane of the canal) causes the
endolymph fluid to move.
•This pushes the cupula . . .
•which stimulates the hair cells . . .
•which synapse onto vestibular nerve
fibers.

10. Around & Around!
•If you keep turning in circles, the fluid
catches up with the canal.
•There is no more pressure on the cupula.
•If you stop spinning, the moving fluid will
slosh up against a suddenly still cupula
•You feel as though you are turning in the
opposite direction!

11. Left and Right Input
•Each tuft of hair cells is polarized
–If you push it one way, it will be excited; if
you push it the other way, it will be
inhibited.
•The same arrangement is present on
both sides of the head.
•The canals on either side of the head
operate in a push-pull rhythm.
–When one is excited, the other is inhibited.

12. Picturing Left & Right Input

13. Disagreement Produces Vertigo
•It is important that both sides agree
what the head is doing.
•If both sides push at once, you will feel
vertigo and nausea.
•Infections of the endolymph or damage
to the inner ear can cause vertigo.
•If one vestibular nerve is cut, the brain
will get used to listening to one side
–this can be a treatment for intractable
vertigo.

14. Interaction with the Visual System
•The semicircular canal system keeps your
eyes still in space while your head moves.
•If you nod and shake and swivel your head,
your eyes stay focused
•The semicircular canals exert direct control
over the eyes, so they can directly
compensate for head movements.
•This compensatory system is called the
vestibulo-ocular reflex (VOR).

15. Vestibulo-ocular reflex (V.O.R.)
•Recall that the eye is controlled by three
pairs of muscles:
–the medial and lateral rectus, the superior
and inferior rectus, and the inferior and
superior oblique.
•Their directions of motion are matched
closely by the planes of the three
semicircular canals
–a single canal (in general) interacts with a
single muscle pair.

16. One Piece of the V.O.R.
•The medial-lateral rectus
pair, coupled to the
horizontal canal, is shown
•The lateral rectus muscle
pulls the eye laterally
•The medial rectus pulls the
eye medially
•Both in the horizontal
plane.
•The horizontal canal
detects rotation in the
horizontal plane.

17. How It Works
•If you move your head
to the left, you will
excite the left
horizontal canal,
•Which inhibits the
right.
•To keep your eyes
fixed on a stationary
point, you need to fire
the right lateral rectus
and the left medial
rectus, to move the
eyes to the right.

18. Pathway of the V.O.R.
•The vestibular nerve enters the brainstem
–synapses in the vestibular nucleus.
•Cells that received information from the left
horizontal canal project to the abducens nucleus on
the right side, to stimulate the lateral rectus.
•They also project to the oculomotor nucleus on the
left side, to stimulate the medial rectus.
•The same vestibular cells also inhibit the opposing
muscles (in this case, the right medial rectus, and
the left lateral rectus).

19. What Happens on the Other Side
•The right horizontal canal is wired to the
complementary set of muscles.
•Since it is inhibited, it will not excite its target
muscles (the right medial rectus and the left
lateral rectus), nor will it inhibit the muscles
you want to use (the right lateral rectus and
the left medial rectus).

20. Medial Longitudinal Fasciculus
•Much of the VOR axon traffic travels via
a fiber highway
•The medial longitudinal fasciculus (MLF)
•The integrity of this tract is crucial for
the VOR to work properly.
•It can be damaged by medial brainstem
strokes.

21. Otolith Organs
•Detect linear acceleration, e.g. gravity
•The utricle and saccule
•Each organ has a sheet of hair cells =
macula
•Cilia of hair cells are embedded in a
gelatinous cap
–like the semicircular canals.
•This gel has small crystals embedded in it =
otoliths (otogonia)
–crystals of biogenic calcium carbonate

22. The Otolith Organs

23. Function of Otolith Organs
•Deflection of cilia on top of the hair cells causes
excitation
•The otoliths provide the inertia.
•Tilting the macula causes the otoliths to pull the
gelatin and bend the cilia
•Once you are moving at a constant speed, such as
in a car, the otoliths come to equilibrium and you no
longer perceive the motion.
•Different hair cells are arranged in different
orientations so that a full range of tilt can be
detected

24. The Macula

25. Range of Motion Detection
•The hair cells in the utricle and saccule are
polarized
•They are arrayed in different directions so that a
single sheet of hair cells can detect motion forward
and back, side to side.
•Each macula can thus cover two dimensions of
movement.
•The utricle lays horizontally in the ear, and can
detect any motion in the horizontal plane.
•The saccule is oriented vertically, so it can detect
motion in the sagittal plane (up and down, forward
and back).

26. Dealing with Gravity
•A major role of the saccule and utricle is to
keep you vertically oriented with respect to
gravity.
•If your head and body start to tilt, the
vestibular nuclei will automatically
compensate with the correct postural
adjustments.
•If you watch someone trying to stand still,
you notice constant small shifts

27. Central Vestibular Pathways
•Connections to medial and lateral vestibular
nuclei
•Also inputs to vestibular nuclei from
cerebellum, visual system, somatosensory
system
•Outputs to cerebellum, extraocular motor
neurons, limb motor neurons, neck motor
neurons
•Integrates body position and movement
information
•Connections to neocortex through ventral
posterior nucleus of the thalamus

28. Vestibular System Problems
•Result in loss of
balance and vertigo.
•Often there is slow
recovery as brain
learns to rely on
visual and
proprioceptive inputs