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Control of Eye Movements
Photoreceptors throughout the animal kingdom use G protein–coupled transduction mechanisms for added sensitivity, but they pay a price in speed: images need to stay still on the retina for a tenth of second or so at a time to be seen clearly. And for animals with a fovea (like us), images need to stay still on precisely that small part of the retina. All animals with image-forming eyes alternate between relatively brief periods of gaze shifting (during which vision is poor) and longer periods of image stabilization. Finally, animals with frontally directed eyes (again, like us) need to keep both foveae pointed at the same part of the world to make binocular depth perception possible; if this part of the system breaks down and the two images don't correspond, diplopia (double vision) results.
Two general kinds of movements are required to keep our eyes lined up this way. First, for objects at a constant distance from us we need to move both eyes the same amount in the same direction; these are called conjugate movements. Second, for objects at varying distances we need to either converge or diverge our eyes; these are appropriately called vergence movements. There are two distinctly different kinds of conjugate movements: fast ones called saccades , used to shift gaze or when something moves too fast to track, and slow ones that are used to stabilize images while we move or objects move.
Six Extraocular Muscles Move the Eye in the Orbit
We need to move each eye in various combinations of six directions. Four of them are obvious—medially ( adduction ), laterally ( abduction ), up ( elevation ), and down ( depression ). The two others are torsional movements, the kind you would make to keep an eye level as you tilt your head to one side or the other. Intorsion rotates the top of the eye closer to the nose and extorsion rotates it away. Movements in these six directions are accomplished by six small extraocular muscles , but the correspondence between movements and individual muscles is not always direct ( Table 21.1 ).
TABLE 21.1
Extraocular Muscles, Eye Movements, and Cranial Nerves
| Movement | Principal Muscle | Other Contributors |
|---|---|---|
| Abduction | Lateral rectus (VI) | Inferior oblique (III) Superior oblique (IV) |
| Adduction | Medial rectus (III) | Inferior rectus (III) Superior rectus (III) |
| Depression | Inferior rectus (III) | Superior oblique (IV) |
| Elevation | Superior rectus (III) | Inferior oblique (III) |
| Extorsion | Inferior oblique (III) | Inferior rectus (III) |
| Intorsion | Superior oblique (IV) | Superior rectus (III) |
The Medial and Lateral Recti Adduct and Abduct the Eye
Adduction and abduction are the most straightforward. They are accomplished by contraction of the medial and lateral rectus , respectively, which originate in the back of the orbit and insert on the medial and lateral sides of the eye.
The Superior and Inferior Recti and the Obliques Have More Complex Actions
The four remaining muscles—the superior rectus , inferior rectus , superior oblique , and inferior oblique —do not lie entirely in the same plane as one of the directions of eye movement, so their actions are more complex. For example, the eye (when looking at something far away) points straight ahead in the orbit, but the axis of the orbit itself—the direction in which the superior and inferior recti pull—points not only backward but also toward the nose ( Fig. 21.1 ). The result is that contraction of the superior rectus mainly causes elevation, but also pulls the top of the eye toward the nose (i.e., intorsion and adduction). Similarly, the inferior rectus mainly causes depression, but also causes extorsion and adduction. The superior and inferior obliques mainly cause intorsion and extorsion, respectively. However, because they insert behind the middle of the eye and pull partially anteriorly, they too cause movement in additional directions (see Table 21.1 ).
We ordinarily use all six muscles in most eye movements, exciting some motor neurons and inhibiting others, contracting some muscles and relaxing others. For example, abduction involves simultaneous contraction of not only the lateral rectus but also both obliques, as well as relaxation of the other three muscles. To keep things manageable, however, this chapter only considers vertical and horizontal movements and pretends they are mediated solely by contractions of the four rectus muscles.
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