Extraocular muscles

The extraocular muscles are the six muscles that control the movements of the (human) eye (there are four in bovines). The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.

List of muscles

Importance

Since only the fovea provides sharp distinct vision, the eye must move to follow a target. It must be precise and fast. This is seen in scenarios like reading, wherein the reader must shift gaze constantly, or following a small object like a golf ball, in which the extraocular muscles must lead the eye to follow the head movements. Although under voluntary control, most movement is done without thinking, such as those based on head or other body movement, or movement of objects in the area. Researchers still have some work in order to find the parallel nature of the environment-based (involuntary) and voluntary control.^[2]

orbiculris oris- facial nerve

Innervation

The nuclei or bodies of these nerves are found in the brain stem. The nuclei of the abducens and oculomotor nerves are connected. This is important in coordinating motion of the lateral rectus in one eye and the medial action on the other. In one eye, in two antagonistic muscles, like the lateral and medial recti, contraction of one leads to inhibition of the other. Muscles shows small degrees of activity even when resting, keeping the muscles taut. This "tonic" activity is brought on by discharges of the motor nerve to the muscle.^[2]

Actions^[3]

Note that intorsion and extorsion are not included in the following table; their actions are accounted for via summation of other actions.

• These motions are only for eye examinations. Note that they are different from the intrinsic motor functions of each muscle. This is done in an exam to separate out the muscle being tested specifically.

In an eye examination, the inability of the patient to move the eye in the specified direction can indicate a problem with the associated muscle, and the nerve associated with that muscle.

Coordination of Movement Between Both Eyes

Intermediate directions are controlled by simultaneous actions of multiple muscles. When one shifts the gaze horizontally, one eye will move laterally (toward the side) and the other will move medially (toward the midline). This may be neurally coordinated by the central nervous system, to make the eyes move together and almost involuntarily. This is a key factor in the study of squint, namely, the inability of the eyes to be directed to one point.

There are two main kinds of movement: conjugate movement (the eyes move in the same direction) and disjunctive (opposite directions). The former is typical when shifting gaze right or left, the latter is convergence of the two eyes on a near object. Disjunction can be performed voluntarily, but is usually triggered by the nearness of the target object. A "see-saw" movement, namely, one eye looking up and the other down, is possible, but not voluntarily; this effect is brought on by putting a prism in front of one eye, so the relevant image is apparently displaced. To avoid double vision from non-corresponding points, the eye with the prism must move up or down, following the image passing through the prism. Likewise conjugate torsion (rolling) on the anteroposterior axis (from the front to the back) can occur naturally, such as when one tips one's head to one shoulder; the torsion, in the opposite direction, keeps the image vertical.

The muscles show little inertia - a shutdown of one muscle is not due to checking of the antagonist, so the motion is not ballistic.^[2]

Paths

Five with paths from annulus of zinn

Five of the extraocular muscles have their origin in the back of the orbit in a fibrous ring called the annulus of Zinn.

Four of these then course forward through the orbit and insert onto the globe on its anterior half (i.e., in front of the eye's equator). These muscles are named after their straight paths, and are called the four rectus muscles, or four recti.^[2]

• superior rectus - inserts on the globe at 2
• inferior rectus - inserts on the globe at 3
• medial rectus - inserts on the globe at 4
• lateral rectus - inserts on the globe at 5

orbicularis oculi-facial nerve (Note that lateral and medial are relative to the subject, with lateral toward the side and medial toward the midline, thus the medial rectus is the muscle closest to the nose).

Two with more complex paths

The other two extraocular muscles follow more complicated paths.

• The superior oblique muscle originates at the back of the orbit (a little closer to the medial rectus, though medial to it, getting rounder as it ^[2] courses forward to a rigid, cartilaginous pulley, called the trochlea, on the upper, nasal wall of the orbit. The muscle becomes tendinous about 10mm before it passes through the pulley, turning sharply across the orbit, and inserts on the lateral, posterior part of the globe. Thus, the superior oblique travels posteriorly for the last part of its path, going over the top of the eye. Due to its unique path, the superior oblique, when activated, pulls the eye downward and medially.^[4]
• The last muscle is the inferior oblique, which originates at the lower front of the nasal orbital wall, and passes under the LR to insert on the lateral, posterior part of the globe. Thus, the inferior oblique pulls the eye upward and laterally .^[4][5][6]

Rolling

The superior and inferior recti are not strictly vertical. The oblique pull of the obliques causes a rolling opposite each other. Although bearing mutual strict antagonism, the superior and inferior rectus team up with the inferior and superior oblique to move the eye up or down, respectively. The extent of rolling in the recti is less than the oblique, and opposite from it.^[2]

Mnemonics

A good mnemonic to remember which muscles are innervated by what nerve is to paraphrase it as a molecular equation: LR₆SO₄R₃.^[7] or (LR₆SO₄)₃ i.e. "LR 6 SO 4 Whole 3."

• Lateral Rectus - Cranial Nerve VI
• Superior Oblique - Cranial Nerve IV
• the Rest of the muscles - Cranial Nerve III.

Another way to remember which nerves innervate which muscles is to understand the meaning behind all of the Latin words.

• The fourth cranial nerve, the trochlear, is so named because the muscle it innervates, the superior oblique, runs through a little fascial pulley that changes its direction of pull (the trochlea of superior oblique). This pulley exists in the superiomedial corner of each orbit, and "trochl-" is Latin for "pulley."
• The sixth cranial nerve, the abducens, is so named because it controls the lateral rectus, which abducts the eye (rotates it laterally) upon contraction.
• The third cranial nerve, the oculomotor, is so named because it is in charge of the movement (motor) of the eye (oculo-). It controls all of the other muscles.

See also

• Hering's law of equal innervation
• Sherrington's law of reciprocal innervation

Additional images

References

External links

• neuro/637 at eMedicine - "Extraocular Muscles, Actions"
• Atlas of anatomy at UMich eye_13