|CHAPTER||16||The Efferent (Motor) System|
Nervous Control of Ocular Movements
The efferent ocular motor pathway includes the cortical centers, intermediate gaze centers, cranial nerve nuclei, cranial nerves, and extraocular muscles (EOMs) which produce eye movements.
Ocular muscles may be extrinsic or intrinsic (Flowchart 16.1).
The extraocular muscles (EOMs) and their central nervous control subserve the motility and coordination to the eyes.
■Anatomy of EOM
There are six EOMs. Five out of six extraocular muscles (except inferior oblique) originate at the orbital apex (Fig. 16.1). These EOMs are:
•Superior oblique (SO) arises near the apex of the orbit, superomedial to the optic foramen.
•Inferior oblique (IO) arises anteriorly from the inferonasal angle of the orbit, just behind the orbital rim and lateral to the lacrimal fossa.
SR and MR muscles are closely attached to the dural sheath of the optic nerve. That is why the pain occurs during elevation and adduction in patients with retrobulbar neuritis.
Recti muscles are inserted into sclera via tendons anterior to the equator of globe (Fig. 16.2).
Table 16.1 shows that the MR tendon inserts closest to the limbus. Insertions get further away from limbus and make a spiral pattern (spiral of Tillaux).
Table 16.1 Insertion of recti muscles
5.5 mm from nasal limbus
6.6 mm from inferior limbus
6.9 mm from temporal limbus
7.7 mm from superior limbus
Abbreviations: MR, medial rectus; IR, inferior rectus; LR, lateral rectus; SR, superior rectus.
Oblique muscles are inserted into sclera, posterior to the equator of globe (Fig. 16.3).
The SO tendon is inserted into the posterior upper temporal quadrant of the globe in a fan-shaped manner under the SR muscle. The insertion extends near the superotemporal vortex vein.
The IO muscle has almost no tendon at its insertion and inserts into the posterior lower temporal quadrant of the globe, close to the macula and inferotemporal vortex vein.
Nerve supply (Mnemonic-LR6 (SO4)3)
The third, fourth, and sixth cranial nerves supply the EOMs in the following manner:
•LR is supplied by abducens (6th) nerve.
•SO is supplied by trochlear (4th) nerve.
•SR is supplied by superior division of oculomotor (3rd) nerve.
•MR, IR, and IO are supplied by the inferior division of the oculomotor (3rd) nerve.
Course of EOM (from Origin to Insertion)
Visual axis: It is the line passing from the object of fixation to the fovea through the nodal point of the eye.
Orbital axis: Medial orbital walls are approximately parallel to each other. The lateral and medial walls of the orbit are at an angle of 45° with each other. The orbital axis therefore forms an angle of 22.5° (≈23°) with both the lateral and medial walls (Fig. 16.4a).
Primary position of eye: The primary position is defined as the position when the eye and head are both directed straight ahead. In the primary position, the visual axis forms an angle of 23° with the orbital axis (Fig. 16.4b).
Horizontal recti include MR and LR which arises from a common tendinous ring (the annulus of Zinn) and inserted into the sclera by a tendon. These rectus muscles move the eyes about the vertical axis (z-axis of Fick) (Fig. 16.5).
MR muscle: It courses anteriorly along the medial wall of the orbit.
•As MR muscle runs along the medial orbital wall, it can sustain damage during ethmoid sinus surgery.
•MR is the only rectus muscle that does not have a facial attachment to an oblique muscle. Thus, MR muscle is at a greatest risk of slippage during strabismus surgery.
LR muscle: It courses anteriorly along the lateral orbital wall.
Vertical recti (SR and IR muscles) run in line with the orbital axis, so they form an angle of 23° with the visual axis in primary positions (Fig. 16.6).
Obliques (SO and IO muscles) form an angle of 51° with the visual axis in primary position.
Actions of EOM
EOMs rotate the eye around the center of rotation situated about 13 mm behind the cornea. It lies, therefore, behind the nodal point of the eye on the visual axis. Ocular movements take place around three axes (termed as axes of Fick) passing through the center of rotation. These three axes are:
•Horizontal axis (x-axis of Fick): Movements around the horizontal axis are elevation or upward rotation and depression or downward rotation.
•Vertical axis (z-axis of Fick): Movements around the vertical axis are adduction or medial rotation and abduction or lateral rotation.
•Anteroposterior axis (y-axis of Fick): Movements around the anteroposterior axis are intorsion or incycloduction and extorsion or excycloduction (Fig. 16.7).
The actions of EOMs depend on the position of the globe at the time of muscle contraction and may be defined as primary or secondary. Primary action is the major effect of the EOM when the eye is in the primary position, while secondary actions are the additional actions which depend on the position of the eye.
These actions can be easily understood by the course of the respective EOMs and their relationship with the visual axis.
•When visual axis lies in the muscle plane, the direction of movement is only upward and downward.
•If the muscle plane (line of muscle pull) is at a greater angle to the visual axis, the EOM produces a much greater degree of torsion. In the primary position of the eyeball, obliques are at a greater angle (51°) with the visual axis than the vertical recti (23°). So, obliques produce a much greater degree of torsion than the vertical recti, that is, the primary action of obliques is torsion. The SO muscle is an intortor, while the IO muscle is an extortor (Table 16.2, Fig. 16.8).
Table 16.2 Actions of EOM
Subsidiary actions of EOM
Intorsion and adduction
Extorsion and adduction
Depression and abduction
Elevation and abduction
Abbreviations: EOM, extraocular muscle; IO, inferior oblique; IR, inferior rectus; LR, lateral rectus; MR, medial rectus; SO, superior oblique; SR, superior rectus.
The torsional movements are defined in terms of the direction of movement of the upper pole of cornea. When the upper pole of cornea rotates nasally, it is termed intorsion. When the upper pole of cornea rotates temporally, it is termed extorsion. Superiors (SO and SR) are intortors, while inferiors (IO and IR) are extortors.
Horizontal recti: As horizontal recti run along the orbital walls; therefore, the sole action of the MR muscle in the primary position is adduction and that of the LR muscle is abduction.
Vertical recti: As these muscles form an angle of 23° with the visual axis in the primary position, therefore:
•If globe is abducted 23°, visual axis and muscle axis align; SR acts only as an elevator and IR acts only as a depressor in this position.
•If the eye is adducted 67°, the angle between the visual and orbital axes would be 90°; SR acts only as an intortor and IR acts only as an extortor in this position (Fig. 16.9).