Figure 5-2 Cardinal positions and yoke muscles. RSR, right superior rectus; LIO, left inferior oblique; LSR, left superior rectus; RIO, right inferior oblique; RLR, right lateral rectus; LMR, left medial rectus; LLR, left lateral rectus; RMR, right medial rectus; RIR, right inferior rectus; LSO, left superior oblique; LIR, left inferior rectus; RSO, right superior oblique.
See Chapter 7 for additional discussion of positions of gaze.
Extraocular Muscle Action
The 4 rectus muscles are traditionally thought of as fixed straight strings running directly from the orbital apex to the muscle insertions. The oblique muscles were historically thought to simply attach obliquely to the globe. In light of ongoing discoveries that lend support to the active pulley hypothesis (discussed in Chapter 3), some of these older concepts, as well as descriptions of extraocular muscles (EOMs) and their actions, have undergone revision.
Arc of contact
The point of effective, or physiologic, insertion is the tangential point where the muscle first contacts the globe. The action of the eye muscle may be considered a vector of force that acts at this tangential point to rotate the eye. The length of muscle actually in contact with the globe constitutes the arc of contact.
The traditional concepts of arc of contact and muscle plane are based on straight-line, 2- dimensional models of orbital anatomy and do not take into account muscle pulleys and their effect on the linearity of muscle paths. Magnetic resonance imaging scans have shown that the rectus muscles may not follow the shortest or straightest paths from the orbital apex to the scleral insertions. In the active pulley model, the direction of pull of a muscle is partially determined by the relationship between the muscle’s pulley and its scleral insertion. This view has been challenged by some authors, who have shown magnetic resonance evidence that the traditional shortest-path model may still have validity. See Chapter 3 for further discussion of these models.
Eye Movements
Motor Units
An individual motor nerve fiber and its several muscle fibers constitute a motor unit. The electrical activity of motor units can be recorded by electromyography. An electromyogram (EMG) is a useful research tool in the investigation of normal and abnormal innervation of eye muscles. A portable EMG device connected to an insulated needle is often used during injection of botulinum toxin into eye muscles, helping the surgeon localize the appropriate muscle within the orbit, especially when the muscle has been operated on previously.
Recruitment during fixation or following movement
As the eye moves farther into abduction, more and more lateral rectus motor units are activated and brought into play by the brain to help pull the eye. This process is called recruitment. In addition, as the eye fixates farther into abduction, the frequency of activity of each motor unit increases until it reaches a peak number of contractions per second (several hundred, for some motor units).
Monocular Eye Movements
Ductions
Ductions are monocular rotations of the eye. Adduction is movement of the eye nasally; abduction is movement of the eye temporally. Elevation (supraduction or sursumduction) is an upward rotation of the eye; depression (infraduction or deorsumduction) is a downward rotation of the eye. Intorsion (incycloduction) is defined as a nasal rotation of the superior portion of the vertical corneal meridian. Extorsion (excycloduction) is a temporal rotation of the superior portion of the vertical corneal meridian.
The following are important terms relating to the muscles used in monocular eye movements:
agonist: the primary muscle moving the eye in a given direction
synergist: the muscle in the same eye as the agonist that acts with the agonist to produce a given movement (eg, the inferior oblique muscle is a synergist with the agonist superior rectus muscle for elevation of the eye)
antagonist: the muscle in the same eye as the agonist that acts in the direction opposite to that of the agonist (eg, the medial rectus and lateral rectus muscles are antagonists)
Sherrington’s law of reciprocal innervation states that increased innervation and contraction force of a given EOM are accompanied by a reciprocal decrease in innervation and contraction force of its antagonist. For example, as the right eye abducts, innervation of the right lateral rectus muscle is increased, generating increased force; simultaneously, innervation of the right medial rectus is reduced, creating a matching reduction in this muscle’s force.
Field of action
The term field of action refers to the gaze position (one of the cardinal positions) in which the effect of the muscle is most readily observed. For the lateral rectus muscle, the direction of rotation and the position of gaze are both abduction; for the medial rectus, they are both adduction. However, the direction of rotation and the gaze position are not the same for the vertical muscles. For example, the inferior oblique muscle, acting alone, is an abductor and elevator, pulling the eye up and out—but its elevation action is best observed in adduction. Similarly, the superior oblique muscle, acting alone, is an abductor and depressor, pulling the eye down and out—but its depression action is best observed in adduction.
The clinical significance of fields of action is that a deviation (strabismus) which increases with gaze in some directions may result from weakness of the muscle normally pulling the eye in that direction, from restriction of its action by its antagonist muscle, or from a combination of these 2
factors.
Primary, secondary, and tertiary action
With the eye in primary position, the horizontal rectus muscles are purely horizontal movers around the vertical axis and therefore have a primary horizontal action. Recent anatomical studies have shown compartmentalization of the innervation to the horizontal rectus muscles in some patients; this may explain the finding of small vertical actions of these muscles in these cases (see Chapter 3). The vertical rectus muscles have a direction of pull that is mostly vertical as their primary action, but the angle of pull from origin to insertion is inclined 23° to the visual axis (or midplane of the eye), giving rise also to torsion, which is defined as any rotation of the vertical corneal meridians. Intorsion is the secondary action of the superior rectus; extorsion is the secondary action of the inferior rectus; and adduction is the tertiary action of both muscles. Because the oblique muscles are inclined 51° to the visual axis (or midplane of the eye), torsion is their primary action. Vertical rotation (depression/elevation) is their secondary action, and abduction is their tertiary action). The levator palpebrae superioris is also an EOM, and its sole action is elevation of the upper eyelid. See Table 5-1 for a summary of the EOM actions.
Table 5-1
Changing muscle action with different gaze positions
The gaze position determines the effect of EOM contractions on the rotation of the eye. There are 7 gaze positions: primary position and the 6 cardinal positions (see Fig 5-2). In each of the cardinal positions, each of the 6 oculorotatory EOMs has different effects on the eye’s rotation based on the relationship between the visual axis of the eye and the orientation of the muscle plane to the visual axis. In each cardinal position, the angle between the visual axis and the direction of pull of the muscle being tested is minimized, thus maximizing the horizontal effect of the medial or lateral rectus or the vertical effect of the superior rectus, inferior rectus, superior oblique, or inferior oblique. By having the patient move the eyes to the 6 cardinal positions, the clinician can isolate and evaluate the ability of each of the 6 oculorotatory EOMs to move the eye. See also Binocular Eye Movements later in this chapter.
With the eye in primary position, the horizontal rectus muscles share a common horizontal plane that contains the visual axis (Fig 5-3). The clinician can assess the relative strength of the horizontal rectus muscles by observing the horizontal excursion of the eye as it moves medially from primary position to test the medial rectus and laterally to test the lateral rectus.
Figure 5-3 The right horizontal rectus muscles. A, Right medial rectus muscle. B, Right lateral rectus muscle. (Modified with
permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:3.)
The muscle actions of the vertical rectus muscles and the oblique muscles are more complex because, in primary position, the muscle axes are not parallel with the visual axis (see Figs 5-4 through 5-7).
Figure 5-4 The right superior rectus muscle, viewed from above. (Modified with permission from von Noorden GK. Atlas of Strabismus.
4th ed. St Louis: Mosby; 1983:3.)
Figure 5-5 The right inferior rectus muscle, viewed from below. (Modified with permission from von Noorden GK. Atlas of Strabismus. 4th
ed. St Louis: Mosby; 1983:5.)
Figure 5-6 The right superior oblique muscle, viewed from above. (Modified with permission from von Noorden GK. Atlas of Strabismus.
4th ed. St Louis: Mosby; 1983:7.)
Figure 5-7 The right inferior oblique muscle, viewed from below. (Modified with permission from von Noorden GK. Atlas of Strabismus. 4th
ed. St Louis: Mosby; 1983:9.)
In primary position, the superior and inferior rectus muscle planes form an angle of 23° with the visual axis (y-axis) and insert slightly anterior to the z-axis (Figs 5-4, 5-5). Therefore, from primary position, the contraction of the superior rectus has 3 effects: primary elevation around the x-axis,