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Ocular movement and vision are virtually inseparable. Normal visual experience depends not only on the integrity of the afferent visual pathways, but also on the mechanisms controlling various types of eye movements. Coordinated eye movements interspersed with periods of stable fixation are essential to stabilize visual images on the two retinas. Specialized networks mediate volitional and reflexive fast eye movements (saccades), tracking of a moving object (pursuit), and reflexive eye movements in response to movements of the body or head (vestibulo-ocular reflexes).
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Abnormalities of ocular movement are of three basic types. One category can be traced to a lesion of the extraocular muscles themselves, the neuromuscular junction, or to the cranial nerves that supply them (nuclear or infranuclear palsy). The second type is a derangement in the specialized neural mechanisms that enable the eyes to move together (supranuclear and internuclear palsies). Perhaps more common but not primarily neurologic is a third group of disorders, congenital strabismus, in which there is ocular misalignment relating to an imbalance of the yoked muscles of extraocular movement. This early ocular misalignment is one cause of a developmental reduction in vision (amblyopia), as discussed at the end of the previous chapter.
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SUPRANUCLEAR CONTROL OF EYE MOVEMENT
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Anatomic and Physiologic Considerations
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In no aspect of human anatomy and physiology is the sensory guidance of muscle activity more instructively revealed than in the neural control of coordinated ocular movement. Knowledge of the central and peripheral mechanisms subserving normal eye movements allows for precise localization of lesions within these pathways. To focus the eyes voluntarily, to stabilize objects for scrutiny when one is moving, to bring into sharp focus near and far objects—all require the perfect coordination of six sets of extraocular muscles and three sets of intrinsic muscles (ciliary muscles, sphincters, and dilators of the iris). The neural mechanisms that govern these functions reside mainly in the midbrain and pons, but are greatly influenced by centers in the medulla, cerebellum, basal ganglia, and the frontal, parietal, and occipital lobes of the brain. Much of the anatomy and physiology of the pathways concerned with fixation and stable ocular movements has been learned both from clinical–pathologic correlations in humans and from experiments in monkeys.
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Accurate binocular vision is achieved by coordianted actions of all the ocular muscles. The term duction denotes the movement of one eye in a single direction, whereas the synchronous movement of both eyes is a version. The commonly used term conjugate gaze indicates that the eyes are aligned as they move in the same direction. Convergent movements bring the eyes toward each other, as required when one looks at a near object. At the same time, the pupils constrict and the ciliary muscles relax to thicken the lens and allow near vision (the accommodative-near reflex, or triad). Divergence is required for distant ...