Central Vestibular Pathways

The vestibulo-ocular reflex

• Explicitly: the excitatory projections from the horizontal canals.

Each of the semicircular canals has its own projection pattern.

• The vestibular nuclear complex excites the contralateral abducens nucleus
  -It stimulates the right lateral rectus muscle to produce eye abduction: it drives the right eye to the right.
• The abducens nucleus also projects fibers across midline that ascend the medial longitudinal fasciculus and excite the contralateral oculomotor nucleus.
  -It causes the left medial rectus muscle to produce eye adduction: it drives the left eye to the right.
• The vestibular nuclear complex also sends direct projections to the ipsilateral oculomotor nucleus via the ascending tract of Deiters.

The vestibulo-ocular projections

• The vestibular nuclear complex simultaneously inhibits the ipsilateral abducens nucleus and the contralateral oculomotor nucleus.
• Of the four vestibular nuclei (drawn later), the medial and lateral nuclei have the most robust ocular projections.

For a simple way to remember this circuitry at the bedside:

• Point your index fingers towards midline to demonstrate that the left vestibular nuclear complex drives the eyes toward the right and the right vestibular nuclear complex drives the eyes toward the left.
• When one vestibular nuclear complex is damaged (for instance the right), the intact vestibular nuclear complex (in this case, the left) drives the eyes in the opposite direction (toward the damaged side) (in this case, toward the right).

The vestibulo-spinal projections

Divide the vestibular complex into medial and lateral divisions.

• The top of the medial division is the superior nucleus, the rest is the medial nucleus.
• The top half of the lateral division is the lateral nucleus, the bottom half is the inferior nucleus.

The lateral nucleus is also called Deiter's; nucleus and the inferior nucleus is also called the descending nucleus.

Note that although not shown as such in our diagram, the lateral and inferior nuclei actually overlap in the low pons and the medial and superior nuclei overlap in the mid-pons.

• The lateral vestibular nucleus projects uncrossed descending fibers of the lateral vestibulospinal tract, which descend through the ventral spinal cord and innervate extensor motor neurons throughout the spinal cord.
• The medial vestibular nucleus projects crossed and uncrossed medial vestibulospinal tract fibers that descend the medial spinal cord to innervate extensor motor neurons in the upper spinal cord, only.
• The medial vestibulospinal tract also receives minor inputs from the lateral and inferior vestibular nuclei, as well.
• These vestibulospinal pathways provide antigravity movements for the maintenance of posture, as do the reticulospinal tracts.

additional central vestibular functions

• The vestibular nuclei project to the thalamus for somatosensory detection of vestibular movement.
• The vestibular nuclei project to the reticular formation, which induces nausea and vomiting.
• The vestibulocolic reflex maintains the head at a level position during movement.