• Movement of your head through space stimulates the vestibular system. The vestibular system is designed to detect both angular and linear acceleration and thus maintain coordination of eye/head movements and the postion of the body in space.
  • Vestibular periphery
  • Labyrinth
  • Endorgans
  • Crista ampullares
  • Macular (otolith) organ
    
• Hair cells, afferents and efferents
• How hair cells do their job
  • As with the auditory system, external signals are transduced by the deflection of organized arrays of hair cells within the semicircular canals or vestibule (saccule, utricle) in the inner ear. In the saccule and utricle there are calcium carbonate crystals (otolith = "rocks") that exert a shearing force on the hair cells.
  • The facial and vestibular nerves and their ganglia (geniculate ganglion for the seventh nerve, superior and inferior vestibular ganglion of the eight nerve) come together in the internal auditory meatus. Superior and inferior semicircular canals sit at a 45° angle to the sagittal plane - ideal for detecting angular movement of head. The utricle and saccule are positioned at a 20° angle to the horizontal plane and are perpendicular to each other - thus situated for detecting linear acceleration in both horizontal and vertical planes.
  • Hair cells contain a predominant kinocilium followed by a descending array of stereoclia. In general, application of a shearing force from the shortest to the longest hair (i.e. toward the kinocilium) induces an increase in the firing rate of the hair cell. Application of a force in the opposite direction decreases the firing rate.
  • There are two different types of hair cell:
    • Type II - These cells are more primitive (i.e. simpler), They have a regular firing rate and have bouton-like afferent nerve endings. The firing rate of these cells is proportional to the velocity of the head movement.
    • Type I - These cells are more complex. They are surrounded by a calyx and have a more variable firing rate due to the fact that the rate is proportional not to velocity but to acceleration.
    • In general, all hair cells within a canal are oriented in the same direction.
  • Physiological context
  • The detection of movement by the CNS results from coupling of vestibular excitation/inhibition from opposite sides of the head (i.e. with a given movement, excitation of hair cells on one side of the head is matched by inhibition of hair cells in the complementary canal on the other side of the head). This decoded information is translated bilaterally into coordinated eye movements designed to maintain fixation of specific focused objects on the retina.
• Combining morphology and physiology
  
• There are four major vestibular nuclei:
  • Superior, medial - receive information regarding angular rotation and drive extraocular muscles via projections through the ascending medial longitudinal fasciculus.
  • Lateral, inferior (Spinal) - these nuclei are somatotopically organized and receive information regarding linear acceleration. The axons of the large neurons of the lateral nucleus form the lateral vestibulo-spinal tract which are involved in the maintenance of balance and position of the body in space. Neurons of the anterior part lateral nucleus also contribute to the descending medial longitudinal fasciculus which descends to cervical spinal cord levels and is involved in the coordination of head-eye movements.Vestibular-related tractsMedial vestibulospinal tract (MLF)
    
• Vestibular-related tracts
  • Medial vestibulospinal tract (MLF)
  • Lateral vestibulospinal tract
  • Vestibulo-thalamo-cortical tract
  • Vestibulo-cerebellar tract Vestibulo-cerebellar fibers pass directly to archicortical cerebellum (flocculo-nodular lobe) via the juxtirestiform body.
• The vestibular system works to stabilize retinal orientation during saccadic (rapid) eye movements toward a stimulus. As the head rotates toward a fixated object, eye movement must coordinate with those head movements in order to stabilize the focus on that position.
  
• Peripheral structures
  • Extraocular muscles
  • Intraocular muscles
  • Eyelid
    
• Central brainstem nuclei
  • Oculomotor nucleus
  • Edinger-Westphal nucleus
  • Trochlear nucleus
  • Abducens nucleus
    
• Eye movements and reflexes.
  • Eye movements for stabilizing vision
    • fixation
    • vestibulo-ocular reflex (VOR)
    • optokinetic reflex (OKN)
  • Eye movements for changing the direction of gaze
    • VOR cancellation
    • smooth pursuit
    • saccades
    • vergence (conjugate) movement
• Other eye-related reflex movements
  • Pupillary light reflex
  • Blinking and other eyelid movements
• Clinical disorders
  • Nystagmus
  • Strabismus
  • Ptosis
  • Ophthalmoplegia
• Clinical tests
  • Rotation test
  • Caloric or thermal test

Consider the Following Questions ..

• Vestibular nuclei connect directly to the spinal cord by way of two tracts which are called? Which one traverses the length of the spinal cord and which one to the cervical cord?

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• How do vestibular afferents play a role in the movement of the eyes?

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Copyright © 1997- 2014 [University of Illinois at Chicago, College of Medicine, Department of Anatomy and Cell Biology].  Last revised: December 30, 2013.