1. Central vestibular processing
Domina Petric, MD

2. Vestibular pathway
Scarpa´s ganglion provides axons that create vestibular nerve.
Next station is vestibular nuclei complex.
Vestibular nuclei are found in the lateral tegmentum of the upper medulla and caudal part of the pons.
There are some vestibular afferents that grow directly into the cerebellum: the floccular nodular lobe in the inferior part of the cerebellum (vestibular cerebellum).

3. Vestibulo-occular reflex
One of the major functions of the vestibular nuclei is to coordinate the movements of the eyes so that we can mantain the stable fixation while our head and body are in motion.
Vestibulo-occular reflex: if we fixate our eyes in the spot that is located in the midline (between our eyes) and turn the head to the left, our eyes are going to move to the right.
So in this case, there is activation of the left horizontal semicircular canal and inactivation of the right horizontal semicircular canal.

4. Vestibulo-occular reflex
While we turn our heads to the left, turning our eyes to the right activates the right lateral rectus muscle and the left medial rectus muscle.
The tone of the antagonistic muscles has to be reduced.
There is activation of the right abducens nerve (for right lateral rectus muscle) and of the left occulomotor nerve (for left medial rectus muscle).

5. Vestibulo-occular reflex: turning the head to the left and eyes to the right
FIRST, there is activation of ganglion neurons on the left side that innervate the left horizontal semicircular canal.
SECOND, some neurons from the vestibular nuclei will send the activation signal via their axons to the abducens nucleus on the opposite side of the brainstem: activation of the right lateral rectus muscle.
THIRD, the internuclear interneuron sends excitatory signal from the right abducens nucleus to the left occulomotor nucleus for the activation of the left medial rectus muscle.

6. Vestibulo-occular reflex: turning the head to the left and eyes to the right
Inhibitory interneuron that grows from vestibular nucleus inhibits the firing of the left abducens nucleus so the tone of the left lateral rectus muscle is reduced.
Internuclear interneuron is then inhibited and sends the inhibitory input to the right occulomotor nucleus so the tone of the right medial rectus muscle is reduced.

7. L R Tonus of the left lateral rectus muscle and
Left medial rectus is activated.
Right occulomotor nucleus is deactivated. L
Right lateral rectus muscle is activated.
activates left occulomotor nucleus.
Inhibitory input travels via the internuclear interneuron to the right occulomotor nucleus.
Internuclear interneuron R
to the left abducens nucleus.
to the right abducens nucleus.
Inhibitory signal travels
excitatory signal travels
From the left vestibular nuclei
Tonus of the left lateral
rectus muscle and
the right medial rectus
muscle is reduced.
AND at the same time...
causes activation of the left horizontal semicircular canal.
Eyes looking to the right.
Turning head to the left

8. Vestibular nistagmus
Nystagmus is a rhythmic form of reflexive eye movements composed of slow component in one direction interruped repeatedly by fast saccadic-like movements in the opposite direction.
Vestibular nistagmus is normally driven by persistent rotation of the head.
The slow component of the nistagmus is driven by the vestibulo-occular reflex and the fast saccadic component resets the eye position.

9. Vestibular nistagmus
The balance between the activities of the two vestibular nerves afferents that arise from the functional pairs of semicircular canals determines the type and direction of nystagmus expressed.
If there is hypofunction of the right vestibular system, slow fase of the vestibular nistagmus will be to the right side and fast saccadic component will be to the left side.
If there is hypofunction of the left vestibular system, slow fase of the vestibular nistagmus will be to the left side and fast saccadic component will be to the right side.

10. R L R L Fast (yerk) fase is to the left.
Fast fase (nystagmus) is to the right.
Fast (yerk) fase is to the left.
Slow fase is to the left.
Slow fase is to the right.
Hypofunction of the left vestibular system
Hypofunction of the right vestibular system

11. Vestibular nuclei regulate posture
Vestibular nuclei produce reflexive adjustments of the posture that compensate for movements of the head.
Medial system of descending axons regulates posture in the most medial parts of the spinal cord white matter.
Medial vestibulospinal tract is the most medial part of the medial descending system.
Lateral vestibulospinal tract is the lateral part of the medial descending system.

12. Medial vestibulospinal tract
Medial vestibulospinal tract arises from the medial vestibular nucleus and it gives rise to bilateral projections that innervate mainly motor neurons in the cervical cord.
Those motor neurons are concerned with movements of the neck and head.
Vestibulocervical reflex connects the vestibular system and the spinal cord: if we fall forward, there will be bacward tilt of the head (vestibular part) and extension of both of our hands forward (spinal part).

13. Lateral vestibulospinal tract
Lateral vestibulospinal tract originates in the lateral vestibular nucleus.
It gives rise to descending projections that run through the anterior medial spinal cord white matter.
They innervate mostly neurons in lumbosacral enlargement ipsilaterally: maintaining the extensor tone of the anti-gravity muscles of the lower extremities.

14. Lateral vestibulospinal tract
If the body is falling to the right, there will be activation of the right vestibular system and via the lateral vestibulospinal tract the activation of motor neurons that will extend the muscles of the right leg.

15. Higher vestibular processing
Lateral and superior vestibular nuclei project to the ventral posterior complex of the thalamus and from the thalamus to the parietal cortex where is vestibular cortex.
Vestibular cortex: region near face representation of somatosensory cortex (Brodmann´s area A3 concerning proprioception) and posterior parietal cortex (Brodmann´s area 5 concerning vestibular signals).

16. Additional vestibular connections
Vestibular nuclei recieve inputs from somatic sensation, audition and vision.
Vestibular nuclei are connected to the parts of reticular formation (integration of visceral sensory signals): feelings of illness, dizziness, for example, sea sickness.

17. Literature
Leonard E. White, PhD, Duke University