Extraocular muscles

Seven small muscles located within the orbit serve to produce all of the movements of which the eyeball is capable, and also to elevate the upper eyelid. These muscles are collectively termed the extraocular muscles because they lie outside the eyeball (intraocular muscles, including the ciliary muscle and pupillary constrictor, lie inside the eyeball).

Other Names

Extrinsic eye muscles

Anatomy

Lateral view of the right orbit showing extraocular muscles. Source: Gray's Anatomy, [1]

One extraocular muscle originates from the lesser wing of the sphenoid and runs anteriorly through the orbit to insert on the tarsal plate of the upper eyelid. Because it serves to elevate the eyelid, this muscle is named levator palpebrae superioris. Whereas most of the fibers of this muscle represent voluntary, striated skeletal muscle, there are some smooth muscle fibers located along the inferior surface of the muscle. These fibers comprise a visceral component of the levator palpebrae superioris, and are considered a separate structure (superior tarsal muscle or Müller’s muscle) by some.

Four of the extraocular muscles originate from a ring-shaped ligament (the annulus tendineus) at the back of the orbit and run straight anteriorly to insert into the sclera just posterior to the cornea. Based on their relative positions in the orbit, these muscles are called the superior rectus, inferior rectus, medial rectus, and lateral rectus.

Two obliquely oriented extraocular muscles exist. The superior oblique originates from the lesser wing of the sphenoid just superior to the optic foramen. Its fibers travel anteriorly in the superomedial corner of the orbit, and pass through a fibrous pulley called the trochlea. After passing through the trochlea, the fibers of the superior oblique change direction, passing posterolaterally to insert into the sclera deep to superior rectus.

The inferior oblique originates from the medial orbital wall and passes posterolaterally beneath the eyeball to insert into the sclera deep to the lateral rectus.

Innervation of extraocular muscles

The superior oblique muscle is innervated by the trochlear nerve (CN IV). This can be easily remembered because it is the only extraocular muscle to pass through the trochlea.

The lateral rectus is innervated by the abducens nerve (CN VI). This can be easily remembered because the lateral rectus is the primary abductor of the eyeball.

The remaining rectus muscles, inferior oblique, and levator palpebrae superioris are innervated by the oculomotor nerve (CN III). This nerve, as its name suggests, is the primary motor nerve of the extraocular muscles. The nerve has two divisions: a superior division, which innervates superior rectus and levator palpebrae superioris, and an inferior division, which innervates inferior oblique, inferior rectus, and medial rectus. Postganglionic sympathetics from the superior cervical ganglion enter the skull via the carotid plexus, pass through the cavernous sinus, enter the orbit through the superior orbital fissure, and innervate the smooth muscle fibers of the superior tarsal muscle.

Function of extraocular muscles

The eyeball is capable of moving about three independent axes: a horizontal axis, a vertical axis, and an anteroposterior axis. The extraocular muscles serve to move the eyeball within the orbit about one or more of these axes. Movement about the horizontal axis causes elevation or depression. Movement about the vertical axis causes adduction or abduction. Movement about the anteroposterior axis causes intorsion or extorsion.

Whereas elevation, depression, adduction, and abduction can change the direction of one’s gaze, intorsion and extorsion do not. These movements cause the eye to rotate clockwise or counterclockwise, making the environment appear to spin. For this reason, torsion of the eyeball is generally undesirable, and to be avoided, and furthermore “no muscle will be recruited in isolation if its action will produce torsion of the eyeball.” ^[1] Two extraocular muscles create no torsion on the eyeball, and can be recruited individually: the medial rectus is a pure adductor of the eyeball, and the lateral rectus is a pure abductor. All other extraocular muscles create some torsion on the eyeball, and can never be recruited in isolation. The remaining muscles – the superior and inferior recti and the superior and inferior obliques, do cause torsion of the eyeball. Collectively these are termed the cyclovertical muscles, because each produces significant torsion as well as elevation or depression.

Torsion can be eliminated in one of two ways. First, the eye may be adducted or abducted (using medial or lateral rectus, respectively) to bring the eye into a position where the torsion effects of a cyclovertical muscle are minimal. For example, abduction of the eyeball (using lateral rectus) allows superior rectus to act as a pure elevator, and inferior rectus as a pure depressor. Similarly, adduction (using medial rectus) places the eyeball in a position where superior oblique is a nearly pure depressor, and inferior oblique a nearly pure elevator. The second way to eliminate torsion is to recruit muscles with opposite torsion effects. Elevation of the eyeball without first adducting or abducting can be accomplished by using superior rectus and inferior oblique, whose torsion effects cancel one another out. Similarly, simple depression of the eyeball recruits both inferior rectus and superior oblique, which have opposite torsion effects.

Clinical correlations

When an extraocular muscle is paralyzed (or the nerve that supplies it is damaged), the affected eyeball may point in a different direction than the unaffected eyeball. In other words, the optic axes diverge, a condition known as strabismus. Because light now falls upon non-corresponding portions of the retinas, the brain cannot correctly interpret the image, and diplopia (double vision) results. To test for weakness of an extraocular muscle, a physician can move a light in a large “H” pattern before the patient’s eyes. The horizontal motion of this pattern tests the medial and lateral recti; the two vertical motions test the cyclovertical muscles.

References

1. ↑ Stern, 1988. Essentials of Gross Anatomy

External Links

[2] Stern, 1988. Essentials of Gross Anatomy

[3] Eye Simulation Page - Interactive Shockwave animation of normal and pathological eye movements