Extraocular muscles
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The extraocular muscles are the six muscles that control the movements of the eye. The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction.
Contents |
[edit] List of muscles
| Muscle | Innervation | Origin | Insertion | Primary function | Secondary function | Tertiary function |
|---|---|---|---|---|---|---|
| Superior rectus | Oculomotor nerve | Annulus of Zinn | eye (anterior, superior surface) | Elevation | Intorsion | Adduction |
| Inferior rectus | Oculomotor nerve | Annulus of Zinn | eye (anterior, inferior surface) | Depression | Extorsion | Adduction |
| Lateral rectus | Abducens nerve | Annulus of Zinn | eye (anterior, lateral surface) | Abduction | ||
| Medial rectus | Oculomotor nerve | Annulus of Zinn | eye(anterior, medial surface) | Adduction | ||
| Superior oblique | Trochlear nerve | Annulus of Zinn | eye (posterior, superior, lateral surface) | Intorsion | Depression | Abduction |
| Inferior oblique | Oculomotor nerve | Lacrimal bone | eye (posterior, inferior, lateral surface) | Extorsion | Elevation | Abduction |
[edit] Paths
[edit] 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. They insert on the globe at 12, 3, 6, and 9 o'clock, and are called the superior, lateral, inferior and medial rectus muscles. (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).
[edit] 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 and courses forward to a rigid pulley, called the trochlea, on the upper, nasal wall of the orbit. The muscle passes through the pulley, turning sharply across the orbit, and inserts on the lateral, posterior part of the globe. Thus, the superior oblique goes backward for the last part of its path, and because it goes over the top of the eye, it pulls it downward and lateralward.
- 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 lateralward.<ref>Roger H.S. Carpenter (1988); Movements of the Eyes (2nd ed.). Pion Ltd, London. ISBN 0-85086-109-8.</ref>
<ref>Westheimer Gerald, McKee Suzanne P (1975); "Visual acuity in the presence of retinal-image motion". Journal of the Optical Society of America 65(7), 847-50.</ref>
[edit] Yoked muscles vs. antagonistic muscles
[edit] Mnemonic
A good mnemonic to remember which muscles are innervated by what nerve is to paraphrase it as a molecular equation: LR6SO4R3.
- 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. 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.
- All of the other muscles are controlled by the third cranial nerve, the oculomotor, which is so named because it is in charge of the movement (motor) of the eye (oculo-).
[edit] References
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