Vestibular system

1
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

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Vestibular system

• Vestibular portions of inner ear
• Membrane-lined fluid filled cavities in temporal
  bone (contains endolymph. connected with cochelar
  duct through ductus reuniens)

• 3 semicircular canals (anterior, posterior and
  horizontal)
• Respond to angular acceleration (yes, no, tipping
  of head)
• Utricle Saccule
• Respond to linear acceleration gravity

3
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

4
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

5
Semicircular canals

• Respond to angular acceleration
• 3 on each side
• Filled with fluid
• Perpendicular to each other
• Pairs of canals in same plane

6
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

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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

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Utricle and Saccule

• Ovoidal sac of membranous laby-rinth 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

• gelatinous layer shifts with respect to
  underlying epithelium
• deflects haircell bundles
• elicits electrical response

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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

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Utricle and Saccule

• Otoliths

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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.

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Vestibular Information and pathways

• Vestibular information is used in 3 ways
• Control eye muscles so that in spite of changes
  in head position, the eyes can remain fixed on
  same point
• Reflex mechanisms for maintaining upright posture
• 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

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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

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Vestibulo-Spinal Tracts vestibulo-spinal reflexes

• 2 vestibulospinal tracts
• Lateral
• Concerned with goal-directed 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

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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

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Projections from Vestibular Nuclei
vestibulo-spinal reflexes

• Vestibular connections (postural control medial
  and limb movement lateral) to the cerebellum

• Thalamic information relayed to cortex - allow
  for conscious perception of head position and
  movement

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Central vestibular connections

• Afferent fibers relay through 4 vestibular nuclei
  (superior, lateral, medial and inferior)
• 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

• Strong input to cerebellum

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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

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Nystagmus vestibulo-ocular reflexes
Stabilize eyes when head moves
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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)

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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

Post-rotatory nystagmus
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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)

Post-rotatory nystagmus