Second-order vestibular neurons project to the oculomotor nuclei, forming the substrate for the VOR.
Horizontal canals drive motor neurons of the abducens nucleus that innervate the contralateral lateral rectus muscle, and the motor neurons of the oculomotor nucleus that innervate the ipsilateral medial rectus muscle, while producing appropriate inhibition in the antagonist muscles.
The vertical canals drive motoneurons that innervate the superior and inferior recti and oblique muscles.
For example:
- For horizontal head movement to the right, the right horizontal semicircular canal is activated and projects to the right medial vestibular nucleus (MVN), which, in turn, projects to the contralateral abducens nucleus. The abducens nucleus contains projection neurons that innervate the ipsilateral lateral rectus muscle and interneurons that project via the median longitudinal fasciculus (MLF) to the contralateral medial rectus subnucleus of the third cranial nerve.
- With upward head movements during straight-ahead fixation, afferents from each posterior semicircular canal are excited, and eyes move downwards by primary projections to the contralateral inferior rectus and ipsilateral superior oblique muscles.
- Likewise, downward head rotation produces excitation in afferents from the anterior semicircular canals, which is relayed to the contralateral inferior oblique and ipsilateral superior rectus muscles, resulting in an upward eye movement.
Figure 1. The mechanism of action of achieving a horizontal eye movement by activation of the 3rd and 6th cranial nerves.
Head turn to the right, resulting in movement of eyes to the left.
