Anatomy and Physiology of Balance

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Anatomy and Physiology of Balance
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Vestibular Hair Cells

• Type I
• (aka inner)
• Type II
• (aka outer)
• With Kinocilium

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The Semicircular Canals

• posterior canal shares plane with contralateral
  anterior canal.
• horizontal canals share plane.

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Stimulated by Angular Acceleration

• greatest when fulcrum is within head
• induces relative motion of endolymph
• crista is displaced by fluid motion

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Responses of the Cristae

• All kinocilia are oriented in the same direction
• Crista in each pair of canals respond inversely
  to each other

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The Otolithic Organs

• Saccule roughly
• vertical orientation,
• responds to acceleration components within
  saggital plane
• Utricle horizontal ( 30 deg.) orientation

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Excitation Patterns in the Utricle
STRIOLA
Anterior
Posterior
Medial
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Cranial Nerve VIII
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Vestibular Portion of C.N. VIII

• superior division utricle, anterior part of
  saccule, and horiz anterior canals
• inferior division posterior part of saccule, and
  posterior canal
• to vestibular nuclei
• to cerebellum

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Vestibulocochlear Nerve

• Exits temporal bone near its medial edge.
• Enters lateral face of brainstem at the level of
  the lower pons.
• Synapsing in (Cochlear and) Vestibular Nuclei

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Responses of Vestibular Neurons

• To changes in acceleration, but onset and fade
  slowly
• For most normal head movements firing rates are
  in phase with head VELOCITY.

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Other inputs to vestibular nuclei

• Cerebellum primarily inhibitory
• Spinal cord
• Pontine reticular formation
• Contralateral vestibular nuclei

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From the Vestibular Nuclei

• Vestibulo-Oculomotor Pathways
• Direct to oculomotor nuclei.
• Indirect via reticular formation to oculomotor
  nuclei (III IV and VI)
• Vestibulo-Spinal Pathways
• Lateral V-S-throughout spinal cord
• Medial V-S-cervical thoracic
• Reticulospinal tract-via brainstem reticular
  formation

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Median Longitudinal Fasciculus

• A tract linking Vest. Nuclei to nuclei of CN III,
  IV, VI
• Supports conjugate eye movement during movement
  of the head.
• Continuous with the medial vestibulospinal tract.
  
• The mlf runs near midline ventral to ventricle IV
  and the periaqueductal gray matter of the
  midbrain

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In the brainstem

• Vestibular inputs undergo integration
• Integrated signal is combined with original
  (velocity driven) signal
• Processing to reset spatial map for eye
  musculature

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Integration Leaky Integration
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The VOR
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Central Nervous System Will Adapt to Peripheral
Damage
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Eye Movements

• Saccadesrapid shift in gaze
• Pursuitstabilize image of moving object
• Fixationstabilize image of still object
• VORstabilize image during head motion
• OKNbackup for when VOR decays to contd head
  rotation
• Vergent movementschange depth of focus

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Saccades

• Pause cells inhibit
• Burst Neurons
• which stimulate
• III VI (horizontal)
• or
• III IV (vertical)

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Compare, Select Combine Senses
Visual System
Vestibular System
Somato- Sensation
SENSORY INPUTSVisionVestibularSomatosensory
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SOMATOSENSORY RECEPTORS
Compare, Select Combine Senses
Somato- Sensation
JointsPositionKinesthesia
MusclesLengthTension
SkinTouchPressure
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VISUAL RECEPTORS
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Visual System
CentralOrientation Navigation
PeripheralMotion-sensitive Body-sway
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VESTIBULAR RECEPTORS
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Vestibular System
Semi-circular canalsOrientation Navigation
Utricle SacculeHorizontal vertical accelerati
on deceleration
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SENSORY ORGANIZATION
Determination of Body Position
Compare, Select Combine Senses
Visual
Vestibular
Somatosensory

• Processing of inputs from the periphery

• Selection based on
• Availability Accuracy
• Value for the task at hand

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Functional Balance Navigating in our Environment
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A Quiz

• Vestibular Pathways Quiz