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Vestibular System: Anatomy and Physiology (with Practical Applications)

Author: Beth Christy, SPT , Oversight by Jeff Walter

What is the vestibular system and what does it do for you?

The vestibular system is made up of:

 Provides information about head motion and orientation in respect to gravity.

Generates eye movements to promote gaze stabilization and postural righting responses involving the head and trunk.

ANATOMY:

Vestibule

  1. Semicircular canals
  1. 3 bony canals in each ear – Superior/Anterior, Posterior, & Horizontal
  2. The canals are positioned at a 90° angle from one another, with the horizontal canal tipped backwards 20-30 degrees

  1.  The parts of the canals include:
  1. Endolymph – fluid that fills the canals 
  2. Ampulla - dilated space at the end of each canal
  3. Cupula - gel-like bud, embedded with sensory hair cells, that sits within the ampullated (dilated) portion of each canal
  1. The semicircular canals detect angular accelerations of the head through displacement of the cupula

Illustration compliments of Tim Hain MD: www.dizziness-and-balance.com 

  1. Otolith Organs (Utricle and Saccule)
  1. These organs make up the medial portion of the vestibule
  2. The semicircular canals originate from the utricle
  3.  Sensory hair cells are embedded within the membrane (macula) of each organ
  4. Calcium carbonate crystals called otoconia are attached to both the medial wall of the saccule and floor of the utricle
  5. Otoconia enable the otoliths to detect tilts and translations of the head, because they respond primarily to linear acceleration forces like gravity

Illustration compliments of Tim Hain MD: www.dizziness-and-balance.com 

The Brain

  1. Brainstem Vestibular Nuclei
  1. Primary input comes from the vestibular portion of CN VIII (vestibular-cochlear)
  2. There are 4 Vestibular Nuclei:

 

Function

Lateral/Deiter’s Nucleus

Help the body maintain a desired posture (ie. vestibulospinal reflexes)

Medial/Superior

Coordinates eye, head, and neck movements

Inferior

Integrate information from the cerebellum and other sensory systems

  1. Cerebellum
  1. Midline (vermal) regions regulate balance and eye movements
  2. Lateral regions control muscles of the extremities.
  3. The cerebellum plays a central role in modulating ocular motor reflexes with the goal of maximizing visual performance

The Blood Supply and Innervation

  1. Vascular Supply

  1. Nerve Supply
  1. CN VIII is divided into 2 parts:
  1. Superior portion innervates anterior and horizontal canals and utricle
  2. Inferior portion innervates posterior canal and saccule

PHYSIOLOGY

  1. Canal/Otolith Excitation
  1.  The spontaneous firing rate of vestibular nerve is 90 pulses per second
  2. The capacity to stimulate the nerve is far greater than capacity to inhibit it (Ewald’s 2nd Law)
  3. The canals function in pairs via a “push-pull” relationship – the excitation of one canal will inhibit another
    Some Examples:   
  1. Tip your chin forward just a little and then turn your head right. This stimulates your right horizontal canal and inhibits your left horizontal canal
  2. Touch your nose to your left knee. This stimulates your left anterior canal and inhibits your right posterior canal
  1. Vestibular-ocular Reflex (VOR)
  1. The VOR generates compensatory eye movements in order to stabilize gaze during head motion (i.e. Rotation of head to the left results in rightward compensatory eye movement)

  1. Types of eye movement:
  1. Abduction/adduction
  2. Elevation/depression
  3.         Torsion
  1. There are two phases of nystagmus:
  1. Slow phase (“Drift”) – vestibular activity is responsible for this phase
  2.  Fast phase (“Beat”) – brainstem centers generate this “reset” phase
  1. Canal specific eye movements: slow phase component of VOR
  1. Right posterior canal (RPC) = Left torsion & Down
  2. Right horizontal canal (RHC) = Left
  3. Right anterior canal (RAC) = Left torsion & Up
  4. Left posterior canal (LPC) = Right torsion & Down
  5. Left horizontal canal (LHC) = Right
  6. Left anterior canal (LAC) = Right torsion & Up

Balance and postural control

  1.  The brain uses vestibular input to help it stabilize the head and body in space through neck, trunk and hip muscle activation
  2. Activation of distal muscles is primarily the responsibility of the somatosensory system
  3. Vestibulo-spinal Reflex (VSR)
  1. Maintains vertical alignment of the trunk
  2. When the head tips in one direction, the body elongates to that side and shortens on the other
  1. Vestibulo-collic Reflex (VCR)
  1. Activates the neck musculature to stabilize the head in space
  2. Compensates for displacements of the head that occur during gait

References

  1.  Walter, J. Vestibular Rehabilitation: Practical Management of the Patient with Dizziness. Powerpoint presentation.
  2. Dizziness-and-balance.com  BPPV link. Available at: http://www.dizziness-and-balance.com. Accessed July 1, 2010.