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Review of The vestibular system
Two classes of sensory subsystems:
Otolith organs: encode linear acceleration
3 crista: encode angular acceleration
• Located in the ampulla at the terminus of
each of 3 fluid-filled semi circular canals
• The inertial force of the fluid provides the
basis for moving hair bundles
Two types:
• Utricle (Horizontal acceleration)
• Saccule (vertical acceleration)
• Located in the utricle
• The inertial force of the otoliths
provides the basis for moving
hair bundles
In both subsystems the hair cell is used to transduce head movement information
At rest there is a continuous intermediate level of NT release
Hair cells depolarize or hyperpolarize depending direction of cilium movement
•Towards kinocilium depolarized (increases NT release)
•Away from kinocilium hyperpolarizes (decreases NT release)
•Note that all output from the vestibular organs exit the boney labyrinth via the VIII
cranial nerve (vestibular-cochlear nerve)
An interesting link:
http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2005/Cowell/index.htm
The vestibular system pathways: Overview
Vestibular system output travels the VII CN splits and terminates in the brain stem:
• Cerebellum (movement coordination)
• Vestibular nuclei. Four nuclei:
• Superior
• Lateral
• Medial
• Inferior
Output from the vestibular nuclei project
• Up ipsilaterally and contralaterally to optic motor control areas
(compensatory eye movements)
• Down the spinal chord in two pathways to control motor output (balance and
posture control)
The vestibular system pathways
Location of primary and secondary brainstem nuclei
•Vestibular nuclei
•Project to cerebellum, PPRF and spine
•Paramedian pontine reticular formation
• projects to abducens
•Abducens
•Project to lateral rectus muscle and
contralateral oculomotor nucleus
•Trochlear nucleus
•superior oblique muscle of the eye
•Oculomotor nucleus
•Several extra ocular muscles
Vestibular nuclei
Vestibulospinal projections: The lateral vestibular nucleus/spinal tract
•Descends the ipsilateral spinal chord
•Terminates at all levels of the spine.
•Excitatory pathway activates postural
muscles (proximal to spine) to correct
for leftward listing.
Vestibulospinal projections: The medial vestibular nucleus/spinal tract
•Descends ipsilateral and contralateral
spinal chord but asymmetrically
•Terminates in the cervical and thoracic
spine.
•Excitatory pathway activates postural
muscles (proximal to spine) mainly in
the neck.
Ascending projections to oculomotor nuclei
•Both direct contralateral and indirect ipsilateral control of
extra ocular muscles
•Counters head movements to keep image stable on retina.
•Damage of vestibular nerve/nuclei causes nystagmus a
oscillation of horizontal eye movement.