Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Vision
Eye Movements

The directions of Eyeball movements driven by the extraocular Muscles are illustrated in Fig. 8-30. Because the oblique muscles pull in a medial direction, The Effect of their action depends on THE POSITION OF the eyes. When the eye is turned toward the Nose, the oblique muscles elevate and depress it, whereas the superior and inferior rectus muscles produce torsional movement; when the eye is turned toward the temple, the superior and inferior rectus muscles elevate and depress it, while the oblique muscles rotate it.

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Fig. 8-30. Extraocular Muscles responsible for the six primary directions of gaze. Adduction is produced by the medial rectus, and abduction by the lateral rectus. An adducted eye is elevated by the inferior oblique and depressed by the superior oblique; an abducted eye is elevated by the superior rectus and depressed by the inferior rectus (Reproduced with permission from Simon RP, Aminoff MJ, Greenberg DA: Clinical Neurology, 4th ed. McGraw-Hill, 1999).

A significant portion of the visual field is binocular; therefore, maintaining continuous alignment of images on both retinas and avoiding diplopia clearly requires exceptionally precise coordination of eye Muscle Action.

There are four MAIN TYPES OF eye movements, each controlled by a distinct neural system that nevertheless shares common final common pathways—the motor Neurons of the extraocular muscles (Fig. 8-31). Saccadic, or rapid, jerky movements occur when shifting gaze from one object to another. They ensure foveal fixation on new objects and reduce visual adaptation that develops during prolonged viewing of a single stationary object. Smooth pursuit movements enable tracking of moving objects. Vestibular movements compensate for HEAD movements by adjusting gaze in response to stimuli from the semicircular canals of the Inner ear. Vergence movements bring the visual axes together when shifting gaze to closer objects. There is a certain analogy between eye movements and human-made tracking systems mounted on moving platforms, such as ships. Saccadic movements are used to search for a target; smooth pursuit movements track its motion; and vestibular movements stabilize the tracking system by making necessary adjustments for displacements of the platform on which they are mounted. In primates, vestibular movements depend on an intact visual cortex. Saccadic movements are programmed in the superior colliculi, whereas pursuit movements are controlled by the Cerebellum.

Fig. 8-31. Types of eye movements (modified and reproduced with permission from Robinson DA: Eye movement control in primates. Science 1968;161:1219).

Superior Colliculi

The superior colliculi, which direct saccadic movements, are innervated by retinal M-fibers. They also receive substantial input from the Cerebral Cortex. Each superior colliculus contains a topographic map of visual space, complemented by a body surface map and an auditory map. There is also a motor map that projects to Brainstem areas controlling eye movements. In addition, projections run via the tectopontine tract to the cerebellum and via the tectospinal tract to areas associated with reflex Head and Neck movements. The superior colliculi continuously and actively adjust eyeball position; they exhibit one of the highest rates of Blood flow and METABOLISM in the Brain.

Strabismus

Disruptions in the coordination mechanisms of eye movements can stem from various causes. If visual images fail to fall on corresponding retinal points, strabismus (squint) results. Some forms of strabismus are successfully treated by surgical shortening of individual extraocular muscles, targeted muscle exercises, and The Use of prismatic lenses that refract light to compensate for the abnormal position of the eyeball. Even so, minor deficits in depth perception typically persist; congenital Anomalies of the oculomotor mechanisms likely underlie both strabismus and impaired depth perception.

If visual images are persistently projected onto non-corresponding points of both retinas in children under six years of age, one of the images is gradually suppressed (suppression scotoma), and diplopia disappears. This suppression is cortical in origin and is generally not seen in adults. It is crucial to initiate Treatment for unilateral visual suppression before the child reaches the age of six, because the eye whose image is suppressed will progressively lose visual acuity. Similar image suppression followed by a loss of visual acuity can develop in children when one eye has a refractive or light-perceptive defect. Vision loss in these cases is termed amblyopia ex anopsia—a term reflecting uncorrectable impairment of visual acuity not directly attributable to organic eye disease. Observations on newborn monkeys have demonstrated that patching one eye for three months leads to the disappearance of ocular dominance columns; inputs from the active eye expand to occupy all Cells of the visual cortex, rendering the patched eye functionally blind. Similar changes likely occur in children with strabismus.