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Vestibular system
• Part of the membranous labyrinth of the inner ear
• Involved in a form of proprioception
• The vestibular apparatus detects head movements and the position
of the head in space
- requires two sets of sensory epithelia to transduce angular and
linear acceleration of the head
- together they from five receptor organs (3 semicircular canals; as
well as utricle and saccule)
• Receptive organs are ensheathed by connective tissue
3/04/09
Subconscious motor
Vestibular
system
• Vestibular
portions of
inner ear:
• Membranelined fluid filled
cavities in
temporal bone
• 3 semicircular canals (anterior, posterior and horizontal)
– Respond to angular acceleration (yes, no, tipping of head)
• Utricle & Saccule
– Respond to linear acceleration & gravity
3/04/09
Subconscious motor
(contains
endolymph.
connected with
cochelar duct
through ductus
reuniens)
Vestibular Components
• A and P canals are
oriented in vertical
planes perpendicular to
each other
• H canal is oriented
horizontally
• sense events in 3
dimensions of space
•Utricle & Saccule
respond to linear
acceleration & gravity
• vestibular component
of 8th cranial nerve;
20,000 myelinated
axons
3/04/09
Subconscious motor
Receptors = Hair Cells
- Depolarize when stereocilia are bent towards kinocilium (non-motile cilium;
results in functional polarity of hair bundle)
-Endolymph is high in K+; girdle of tight junctions separates endolymph from
perilymph (like extracellular fluid; high in Na+)
- afferents fire both tonically and phasically – firing can persist or adapt – resulting
in mechanisms to signal sustained stimulation (acceleration from gravity) and
abrupt changes in acceleration
3/04/09
Subconscious motor
Semicircular canals
• Respond to angular acceleration
• 3 on each side
– Filled with fluid
– Perpendicular to each other
– Pairs of canals in same plane
3/04/09
Subconscious motor
Semicircular canals
Mechanism of stimulation:
• Hair cells (7,000) located
in ampulla - Gelatinous
Cupula covers stereocilia
(each “canal” is a closed tube of ~8
mm in diameter filled with
endolymph)
• During rotation of head in
the plane of a canal:
– Fluid moves around canal
(acceleration detected by
inertia)
– Fluid flow interrupted by
cupula (Tilts the cupula;
Stereocilia bent)
– Afferents excited on one
side & inhibited on the other
3/04/09
Subconscious motor
Semicircular canals
Mechanism of stimulation:
• Fluid presses against one
side of cupula
• Cupula bows, displacing
the haircells
• All hairbundels share common orientation
-Angular acceleration in preferred direction (towards kinocilium) depolarizes haircells and
stimulates afferents, acceleration in opposite direction hyperpolarizes receptors
• three canals are almost precisely perpendicular to one another
representing 3 mutually orthogonal axes
3/04/09
Subconscious motor
Utricle and Saccule
• Ovoidal sac of membranous labyrinth about 3mm long
- Utricle: 30,000 HC, saccule: 20,000 HC
• Respond to linear acceleration &
gravity
• One of each on each side
– Utricle - macular surface horizontal
– Saccule - macular surface vertical
– Proportional activity in 2 channels for
info on acceleration along all axes
• Mechanism of stimulation:
– hair cells in macular surface
– Stereocilia covered by gelatinous
matrix
– Otoliths embedded in gelatin
• Otoliths more dense than water (fine,
dense particles, “ear dust”)
• Mass lags behind movement of head
3/04/09
Subconscious motor
• gelatinous layer shifts with respect to
underlying epithelium
• deflects haircell bundles
• elicits electrical response
Utricle and Saccule
- Linear acceleration or
gravity forces otoliths to
move gelatin and bend
stereocilia
– Utrical signals horizontal forces (utricle has variations in axes in populations
of hair cells; tilt in any direction will depolarize some cells and hyperpolarize
others)
– Saccule signals vertical forces
3/04/09
Subconscious motor
• Otoliths
3/04/09
Subconscious motor
Utricle and
Saccule
Vestibulo-cochlear Nerve
-nerve along which the sensory cells (hair cells) of the inner ear transmit information
- consists of the cochlear nerve (hearing), and the vestibular nerve (balance)
- emerges from the medulla oblongata and enters the inner skull via the internal auditory meatus
in the temporal bone, along with the facial nerve.
3/04/09
Subconscious motor
Vestibular Information and pathways
Vestibular information is used in 3 ways
1) Control eye muscles so that in spite of changes in head position,
the eyes can remain fixed on same point
2) Reflex mechanisms for maintaining upright posture
3) Conscious awareness of the potion and acceleration of body,
perception of space surrounding the body and memory of spatial
information
Pathways
- Information is relayed from vestibular apparatus to nuclei in
brainstem via vestibular branch of cranial nerve VIII
- Transmitted through multineuronal pathway through the thalamus
to vestibular centers in parietal lobe and cerebellum
- descending projections sent to spinal chord to affect postural
reflexes
- vestibular information integrated with info from joints, tendons and
skin 3/04/09
Subconscious motor
Vestibulo-Spinal Tracts; vestibulo-spinal reflexes
• 2 vestibulospinal tracts
(medial and lateral)
• Medial:
– Provides basic postural
control
– receives much input from
semicircular canals
– Causes movement of head
and shoulders to coordinate
head and eye movements
(ends at cervical cord)
– Descend in the ipsilateral
column of spinal cord;
terminate in ventro-medial
spinal gray matter;
innervate axial and
proximal muscles
3/04/09
Subconscious motor
Vestibulo-Spinal Tracts; vestibulo-spinal reflexes
• 2 vestibulospinal tracts
• Lateral:
– Concerned with goaldirected limb movement
such as reaching and
manipulating
– receives much input from
utricle and saccule
– Changes muscle tone in
response to gravity
– Descending pathway
descend to dorsal part of
lateral column of spinal
cord
3/04/09
Subconscious motor
Vestibulo-Spinal Tract: vestibulo-ocular reflexes
• Other vestibular pathways
ascend to oculomotor
nuclei: CN III
(oculomotor nerve; controls
most of the eye's movements,
constriction of the pupil, and
maintains an open eyelid), CN
IV (trochlear nerve;
innervates a single muscle: the
superior oblique muscle of the
eye), CN VI (abducens
nerve; controls the movement of
a single muscle, the lateral rectus
muscle of the eye)
• Cause eye movement in
response to head rotation:
Nystagmus
3/04/09
Subconscious motor
Projections from Vestibular Nuclei; vestibulo-spinal reflexes
• Vestibular connections (postural
control [medial] and limb
movement [lateral]) to the
cerebellum
3/04/09
• Thalamic information relayed
to cortex - allow for conscious
perception of head position
and movement
Subconscious motor
Central vestibular connections
• Afferent fibers relay
through 4 vestibular nuclei
(superior, lateral, medial and
inferior)
• Strong input to cerebellum
• 2 vestibulospinal tracts
• Lateral:
– receives much input from
utricle and saccule
– Changes muscle tone in
response to gravity
• Medial:
– receives much input from
semicircular canals
– Causes movement of head
and shoulders to coordinate
head and eye movements
3/04/09
Subconscious motor
Central vestibular connections
• Together, vestibular reflexes stabilize eyes and body when head
moves
• Vestibulospinal reflexes enable skeletomotor system to compensate
for head movement
• Vestibuloocular reflexes keep eyes still when head moves
3/04/09
Subconscious motor
Nystagmus; vestibulo-ocular reflexes
Stabilize eyes when head moves
3/04/09
Subconscious motor
Nystagmus; vestibulo-ocular reflexes
• Stabilize eyes when head moves
• you can read a book while shaking your head if the book is still
(visual processing slower than vestibular processing for image
stabilization)
• vestibular apparatus signals how fast head is moving, ocular
motorsystem uses info to stabilize eyes (visual image motionless on
retina)
• slow eye movement in opposite direction of head movement (driven
by vestibular system; otholith reflex)
• nystagmus to reset to center of gaze (driven by brain stem circuits)
3/04/09
Subconscious motor
Vestibulo-occular control
• Subject seated on stool and rotated to left
• Initial response (hard to visualize)
– Slow tracking eye movements to right
– Fast eye movements back to left
• Nystagmus: alternate slow (otolith reflex) and
fast eye movement (brain stem)
• Semicircular canals habituate, eyes begin to
move in space
• Response to stopping turning (post-rotatory)
– Head stops but fluid continues moving left
– Eyes track slowly left, quick movement to right
• Nystagmus normal for head rotation and
repetitive moving object (optokinetic)
– Nystagmus without movement = sign of lesion
3/04/09
Subconscious motor
Post-rotatory nystagmus
Vestibulo-occular control
Coffee cup example:
• gently twist your coffee; watch a bubble at
fluid boundary
• at beginning, coffee tends to maintain its
original orientation and thus counter rotates
the cup
• at conclusion of turning, when cup
decelerates, coffee moves in opposite
direction (post rotatory nystagmus)
3/04/09
Subconscious motor
Post-rotatory nystagmus