Eye Movements

Six Extraocular Muscles Move the Eye in the Orbit

Six small extraocular muscles ( Fig. 21.2 ) rotate each eye in its orbit, like a ball in a socket. Four rectus (“straight”) muscles ( medial, lateral, superior, and inferior ) originate from a common tendinous ring (the annulus of Zinn) in the back of the orbit and insert anteriorly in the sclera, 5 to 8 mm from the limbus. Two oblique muscles, as the name implies, pass obliquely over the surface of the eye and insert in the sclera of its posterior half. The superior oblique originates near the common tendinous ring, from the sphenoid bone at the back of the orbit, but it takes an unusual course in reaching the eye. Near the front of the orbit, the superior oblique tendon passes through a fibrous loop (the trochlea ^a

a Trochlea, from which the fourth cranial nerve derives its name, is Greek for “pulley.”

) attached to the frontal bone, turns laterally and posteriorly, and inserts in the sclera of the posterior half of the eye. The inferior oblique originates anteriorly and medially from the floor of the orbit, passes across the inferior surface of the eye, and inserts posteriorly.

Different patterns of contraction of the six extraocular muscles rotate the eye horizontally ( adduction = toward midline, abduction = away from midline), vertically ( elevation = up, depression = down), or around its anterior-posterior axis ( extorsion = rotate out, intorsion = rotate in) ( Fig. 21.3 ). As you recall from Chapter 12 , these muscles are innervated by three sets of cranial nerves. Cranial nerve III (oculomotor) innervates the majority of the muscles, including the superior, medial, and inferior rectus as well as the inferior oblique. The lateral rectus is innervated by cranial nerve VI (abducens), and the superior oblique is innervated by cranial nerve IV (trochlear).

The Medial and Lateral Recti Adduct and Abduct the Eye

The medial and lateral recti are situated in a horizontal plane, so contractions of these muscles rotate the eye around a vertical axis. This makes their actions straightforward ( Fig. 21.4 ): the medial rectus adducts and the lateral rectus abducts the eye. Both muscles pass through sleeves of connective tissue (referred to as pulleys ) as they travel from the common tendinous ring toward the eye, emerging a little posterior to their insertion points on the sclera. The pulleys, in turn, have connective tissue attachments to the walls of the orbit, so the positions of the muscles are constrained within the orbit as the eye moves. As a result, adduction and abduction continue to be the principal effects of the medial and lateral recti, even if the eye starts out in an elevated or depressed position ( Fig. 21.5 ). The superior and inferior recti and the inferior oblique have similar pulleys (in this sense, the superior oblique is not as different from the other extraocular muscles as it seems to be). The pulleys form important parts of an elaborate mechanical suspension system that helps control rotations of the eye; half the fibers of the recti and inferior oblique actually attach to the pulleys rather than the sclera, adjusting pulley positions as the eye moves.

The Superior and Inferior Recti and the Obliques Have More Complex Actions

The actions of the superior and inferior recti and the obliques are not quite so straightforward, because the anatomical axis of the orbit is deviated about 23 degrees laterally from the visual axis of the eye ( Fig. 21.6 ). The result, as indicated for the superior rectus and superior oblique in Fig. 21.7 but equally true for the inferior rectus and inferior oblique, is that when one of these muscles contracts, it looks as though the eye should rotate around an axis that is neither vertical nor transverse nor anterior-posterior. Consequently, each of these four muscles has one principal action and weaker additional actions ( Fig. 21.8 ). However, the pulley system constrains their directions of pull, and these secondary actions come into play only in eccentric gaze. Starting from straight-ahead gaze, the superior and inferior recti largely take care of elevation and depression, respectively; similarly, the superior and inferior obliques cause intorsion and extorsion, respectively.

To keep things reasonably simple in this chapter, torsional movements are not discussed, ^b

b We tend not to think much about torsional movements, but if you tilt your head to one side, both of your eyes counterrotate in partial compensation. Such tilting does not disturb images on the fovea too much, so torsional movements are less important for us than for more lateral-eyed animals.

and adduction, abduction, elevation, and depression are treated as the result of contraction of just one of the rectus muscles. However, eye movements actually involve coordinated changes in the activity of all six extraocular muscles ( Table 21.1 ), some contracting and some relaxing in response to changing levels of input from the oculomotor, trochlear, and abducens nuclei (see Fig. 12.4, Fig. 12.5, Fig. 12.6 ). For example, adduction involves simultaneous contraction of the medial, superior, and inferior recti and relaxation of the lateral rectus and the superior and inferior obliques; the intorsion and extorsion of the superior and inferior recti cancel each other.

TABLE 21.1

Extraocular Muscles Contributing to Movements Around Different Axes

Movement	Principal Muscle	Other Muscles Contributing
Adduction	Medial rectus	Superior rectus
		Inferior rectus
Abduction	Lateral rectus	Superior oblique
		Inferior oblique
Elevation	Superior rectus	Inferior oblique
Depression	Inferior rectus	Superior oblique
Extorsion	Inferior oblique	Inferior rectus
Intorsion	Superior oblique	Superior rectus

There Are Fast and Slow Conjugate Eye Movements

Conjugate movements serve one of two general purposes: to move an object's image onto the fovea or to keep it there (see Fig. 21.1 ). We use fast, steplike eye movements called saccades (from a French word meaning “a pull on the reins”) to redirect gaze so that a different image falls on the fovea. The same neural machinery comes into play whenever these brief, rapid movements are called for—to move our eyes voluntarily in any given direction, to glance over at something in the periphery, and in the fast phase of nystagmus. We use multiple kinds of smooth, slower eye movements to keep an image on the fovea, corresponding to the fact that an image can move on the retina if we move or if the object moves. The vestibuloocular reflex (VOR) (see Figs. 14.31 and 14.32 ) compensates for head movements, supplemented in this function by optokinetic movements (see Fig. 14.33A ); for large head movements, these smooth eye movements become the slow phase of nystagmus. Smooth pursuit movements are used to track a visual stimulus that is moving. Without special training, we are unable to move our eyes smoothly unless an image starts to leave the fovea ( Box 21.1 ).

A network of neural structures distributed through the brainstem, cerebellum, and cerebral hemispheres is involved in the initiation and coordination of conjugate eye movements ( Fig. 21.10 ). Basically it involves reflex connections and central pattern generators for these eye movements, located in the brainstem, together with cerebral and cerebellar centers that are able to trigger or modulate these brainstem mechanisms. The brainstem centers differ for different kinds of movement, but the cerebral and cerebellar centers overlap. No corticobulbar fibers reach the abducens, trochlear, or oculomotor nuclei directly (see Figs. 21.13 and 21.16 ).