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Physiology 3120 Lecture Notes - Vestibular System, Vestibular Nerve, Vestibular Nuclei

Course Code
Tom Stavraky

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Human Physiology
Monday, November 9, 2009
“Neuro VIII”
Vestibular System
Adjacent to the cochlea
Function is to position & orient the head in space
No conscious awareness of vestibular system in everyday life (compared to other sensory systems)
If it fails, can experience. . .
Nausea (ill feeling, vomit)
Essential for everyday life
“Tuned” by the cerebellum, which is adversely affected by alcohol
2 vestibular system
1. Semicircular system (3 in total at right angles to each other)
Hair cells embedded in the cupula (gelatinous wedge)
Responds to angular acceleration of the head in any direction; NOT head position, NOT
angular velocity
As the head turns, the endolymph lags behind due to inertia, and applies pressure to the
cupula as it moves
Causes movement of hair cells & increase in discharge of vestibular nerve
2. Otolith organ system (utricle & saccule)
Hair cells can be of different orientations
Has crystals of calcium carbonate embedded in the gelatinous matrix on the hair cells;
crystals provide inertia to the system
Detect to (I) linear acceleration, a dynamic stimulus, and (II) gravity, a static stimulus
Transduction of action potential in vestibular system
Large cilia = kinocilium; small cilia = stereo cilia
Record discharge to the VIIIth cranial nerve from hair afferents
When stereo cilia bend towards kinocilia, receptor is depolarized, so get more APs; when
kinocilia bend towards stereo cilia, receptor is hyperpolarized, so get fewer APs
The two vestibular responses on either side of the head are complementary (i.e. if one increases, the
other will decrease)
Pathway of vestibular information
Vestibular nerve vestibular nuclei in the brain stem; also get some visual input to the
vestibular nuclei due to the optokinetic system
Vestibular nuclei project to. . .
Somatosensory cortex/posterior parietal cortex through the thalamus
Spinal cord limbs & their muscles (vestibular postural reflexes); allows us to maintain
Brainstem eyes & eye muscles (vestibular ocular reflexes)
Has nothing to do with vision
Compensatory eye movements in response to head acceleration to keep an object
of interest on the fovea
Cerebellum, which can “tune-up” VORs or VPRs
Decerebrate frog
No brain, but brain stem remains intact (i.e. “brain dead”)
Will always re-establish normal position “righting reflex”
Will turn over if placed on his back because vestibular system remains intact
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