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Ocular movement and vision are virtually inseparable. A moving object evokes movement of the eyes and almost simultaneously arouses attention and initiates the perceptive process. To look searchingly, i.e., to peer, requires stable fixation of the visual image on the center of the two retinas. One might say that the ocular muscles are at the service of vision.
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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 highly specialized neural mechanisms that enable the eyes to move together (supranuclear and internuclear palsies). This distinction, in keeping with the general concept of upper and lower motor neuron paralysis, hardly portrays the complexity of the neural mechanisms governing ocular motility. Perhaps more common but not primarily neurologic is a third group of disorders, strabismus, in which there is a congenital imbalance of the yoked muscles of extraocular movement that may lead to a developmental reduction in monocular vision (amblyopia), as discussed at the end of the previous chapter. Knowledge of the anatomic basis of normal movement is essential to an understanding of abnormal 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. Moreover, the entirely predictable and "hardwired" nature of the central and peripheral oculomotor apparatus allows for a very 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. Most of the nuclear structures and pathways concerned with fixation and stable ocular movements are now known and much has been learned of their physiology both from clinical-pathologic correlations in humans and from experiments in monkeys.
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Accurate binocular vision is actually achieved by the associated action of all the ocular muscles. The symmetrical and synchronous movement of the eyes is termed conjugate movement or gaze (conjugate meaning yoked or joined together). The simultaneous movement of the eyes in different directions, as in convergence, is termed dysconjugate or disjunctive. These two forms of normal ocular movements are also referred to as versions (versional) and vergence, respectively. Vergence movements have two components—fusional and accommodative. The fusional movements are convergence and divergence, which maintain binocular single vision and depth perception (stereopsis); ...