The Vestibuloocular System

The vestibulo-ocular system maintains image stability on the fovea in the presence of head movement. The afferent arm of this reflex is initiated by acceleration receptors in the inner ear. The equivalent of mathematical integration is performed on the acceleration signal by the dynamic properties of the semicircular canal and cupula (see Ch§pie.Ll2.). The result of this integration is a signal proportional to head rotational velocity in space that is supplied to the vestibular nuclei by way of the eighth cranial nerve. The information is then relayed to brain stem gaze centers, and slow eye movements with velocity equal to and direction opposite head rotation are generated. The arrangement of vestibular connections in the brain stem has been thoroughly reviewed. y

Because vertically acting rectus and oblique muscles exert mixed vertical and torsional rotations, the nystagmus resulting from vertical canal stimulation has mixed vectors and depends on the position of the eye in the orbit (abducted or adducted). The activation of the anterior semicircular canals produces upward and torsional slow phases, whereas activation of posterior canals induces downward and torsional movements. Each vestibular nucleus interacts with two motoneuron pools, one for each eye. Excitatory projections from vestibular nuclei cross the midline, but inhibitory connections do not. The pathways for upward and downward VORs differ.

Vestibular nuclei project to ocular motor neurons and to other nuclear groups that are important in various aspects of integrating the numerous inputs to eye movement. These include the nucleus prepositus hypoglossi and the nucleus of Roller, where integration of burst and step

(holding) discharge patterns are assembled into a final oculomotor command. Outputs of the nucleus prepositus hypoglossi to burst cells initiate saccades and produce the fast phase of nystagmus.

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