Control of Eye Movements

The medial and lateral rectus muscles move the eyeball horizontally, causing the cornea to look medially or laterally. The actions of the superior and inferior rectus muscles and those of the oblique muscles are more complicated. The superior and inferior rectus muscles move the eyeball upward and downward, respectively. Because they are disposed at an angle of about 20 degrees to the sagittal plane (due to the long axis of each orbit being directed slightly outward), they also impart a minor degree of rotation to the eyeball (intorsion for the superior rectus and extorsion for the inferior rectus). When the eyeball is abducted, the superior and inferior rectus muscles purely elevate and depress the eyeball. The inferior oblique muscle rotates the eyeball outward (excyclotorsion) and elevates the eyeball when it is adducted. However, an exact idea of the actions of the extrinsic eye muscles cannot be obtained by considering each muscle separately because, under normal circumstances, none of the six extraocular muscles acts alone. Consequently, all eye movements are the result of highly integrated and delicately controlled agonist and antagonist activities. The actions of individual muscles have been determined from studies of congenital defects or from functional disturbances caused by disease or injury to the nerve supply.

VESTIBULAR PROJECTIONS IMPORTANT FOR VISUAL FIXATION

The vestibular projection is important for the maintenance of visual fixation during head movements. To effect smooth movement, tracking, and proper visualization, the contraction of one eye muscle must be accompanied by the relaxation of its antagonist. The action of turning the head excites vestibular afferent fibers from semicircular canal receptors. Fibers from an individual semicircular canal excite two specific groups of relay neurons in the vestibular nuclei. One group excites the extraocular motor neurons that cause the eyes to move in the direction opposite to the head movement, and the other group inhibits motor neurons that activate movement of the eyes in the same direction as the head. For example, turning the head to the right will excite fibers from the right horizontal semicircular canal, which, in turn, will activate neurons in the right medial and lateral vestibular nuclei. Some of these vestibular neurons will then excite motor neurons controlling the right medial and left lateral rectus muscles. Other vestibular neurons will inhibit motor neurons controlling the right lateral rectus and internuclear neurons controlling the left medial rectus. The result will be a compensatory movement of both eyes to the left. The vestibulocerebellum modulates the vestibulo–extraocular reflex in such a way that the resulting eye movement precisely compensates for the head movement and thus keeps the gaze fixed on the same point.

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