Vascular supply The vascular supply for the peripheral and central portions of the vestibular system arises from the vertebral-basilar arterial system. The two vertebral arteries travel up the right and left side of the brainstem to give off branches at the level of the medulla, called the posterior inferior cerebellar artery (PICA), which supplies the inferior aspect of the cerebellar hemispheres as well as the inferior aspect of the vestibular nuclear complex. As the vertebral arteries become the vertebral-basilar artery at the level of the pons, the anterior inferior cerebellar arteries (AICA) branch out, supplying the peripheral vestibular system, via the labyrinthine artery, and ventrolateral aspects of the cerebellum.
The labyrinthine artery bifurcates into the superior or anterior vestibular artery and the common cochlear artery, which eventually splits off to give rise to the posterior vestibular artery. The anterior vestibular artery supplies the anterior and horizontal semicircular canals and the otoliths, whereas the posterior vertebral artery supplies the posterior semicircular canals and the saccule. It is important to appreciate that the AICA is the sole circulatory supply for the peripheral vestibular system; an AICA infarct would result in complete unilateral loss of peripheral vestibular function.
MOTOR OUTPUT OF THE VESTIBULAR SYSTEM
As previously noted, the purpose of the vestibular system is to maintain head-eye coordination to stabilize gaze during head movement, such as when we walk, run, and turn, and to maintain an upright position during motion. It does this through Vestibulo-ocular pathways and vestibulo-ocular reflex The VOR is responsible for generating rapid reflexive eye movement in response to head movement to maintain gaze stability. To maintain stable gaze, the VOR must generate eye movements in the opposite direction of head movement and with equal velocity. This 1:1 relationship of head-eye movement is expressed as the “gain” of the VOR system. The motor pathways of the VOR originate at the level of the vestibular nuclei, carrying information to the nuclei of the ocular motor neurons (CN III, CN IV, CN VI) via two white- matter tracts to control head-eye coordination. These vestibulo-ocular tracts are the ascending tract of Deiters and the medial longitudinal fasciculus (MLF). The ascending tract of Deiters carries output from the horizontal canals to the abducens nuclei (CN VI) to drive motor activity of the ipsilateral lateral rectus muscles of the eyes during horizontal head motion. Recalling the push- pull neurodynamics of the coplanar pairs of the semicircular canals, when the head is rotated to the right, excitation occurs in the mechanoreceptors of the right canal (increased firing), and hyperpolarization (decreased firing) occurs in the mechanoreceptors of the left canal. The VOR pathways carry excitatory inputs generated in the right semicircular canal to the left lateral rectus and right medial rectus to elicit leftward eye movement corresponding to rightward head movement (see Figure 5). At the same time, the VOR pathways also carry inhibitory inputs generated from the left horizontal canal to the right lateral and left medial rectus muscles. This antagonistic arrangement of extra-ocular muscle activity is referred to as the Law of Reciprocal Innervation (Leigh & Zee, 1991) and describes the correlate relaxation (inhibition) of a muscle while the opposing muscle is contracting. In this example, the medial and lateral rectus muscles of each eye are undergoing reciprocal inhibition. This response results in lateral eye movement that is equal and opposite to lateral head rotational movement. Outputs from the anterior and posterior semicircular canals are carried to the other nuclei of the extra-ocular muscles (CN III and CN IV) via the MLF, mediating reciprocal excitation and inhibition of the superior and inferior rectus and oblique muscles. This results in reflexive rotary eye movement with an upward or downward component motion. In this instance, the contralateral posterior and anterior canals are working as coplanar pairs. The superior rectus muscles direct eye movement in a vertical plane (up and down). The oblique muscles drive rotational eye movement, with the superior oblique generating movement down and in, and the inferior oblique generating eye movement up and out. When head movement occurs in the plane of the right posterior canal (head tipped back with rotation to the right) VOR pathways from the right posterior canal carry excitatory responses to the ipsilateral superior oblique muscle and contralateral inferior rectus muscle, and simultaneous inhibition to the ipsilateral inferior oblique muscle and contralateral
neural pathways to the ocular muscles as well as the spinal cord that mediate the VOR and the vestibulospinal reflex (VSR), respectively.
superior rectus muscle. At the same time, the VOR pathways for the contralateral anterior canal, being the coplanar pair of the posterior canal, mediate the same pattern of muscle activity. In other words, head tilt in the plane of the right posterior canal, and subsequent left anterior canal, results in excitation of the right superior oblique and superior rectus muscles, and the contralateral correlate of left inferior oblique and inferior rectus muscles. Inhibition occurs in the opposite pairs of eye muscles. The net effect is a downward and left torsional eye movement. Thus, eye movement occurs in the opposite direction of head movement to maintain eye position fixed on the target. We will revisit this phenomenon when discussing examination of canalithiasis. Figure 5: Vestibulo-Ocular Reflex (VOR) Pathways from Horizontal Canals
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