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Muscles of the Orbit

Contents

Overview

Muscles of the orbit are also referred to as the extrinsic (arising externally) or extraocular muscles. There are 6 of these extraocular muscles that control eye movement (cows only have 4 of these), and one muscle that controls eyelid elevation. The position of the eye at the time of muscle contraction is what determines how the 6 muscles of the orbit are engaged. 4 of the 6 extraocular muscles controls movement in the cardinal directions: north, east, south, west (or up, right, down, left). The other 2 of 6 extraocular muscles are responsible for counteracting head movements and adjusting eye movement accordingly. There is orbital fat that surrounds the sides and back of the eye which cushions it, allows it to move more freely, and functions to protect blood vessels and nerves as they pass through the rear of the orbit.

Muscles of the orbit
Recommended video: Muscles of the orbit
Overview of the muscles of the orbit and related structures.

Anatomy

The 4 extraocular muscles that control eye movement in the cardinal directions (along with their functions) are:

  1. Superior rectus: In the neutral position, this muscle is responsible for elevation, incyclotorsion and adduction (inward, rotational movement). During adduction, the superior rectus is responsible for intorsion, adduction and elevation. During abduction, this muscle is responsible for elevation. This muscle inserts on the anterior, superior surface of the eye. The origin is the Annulus of Zinn. Rectus muscles are “straight” muscles.
  2. Inferior rectus: In the neutral position, this muscle is responsible for depression, extorsion (outward, rotational movement) and adduction. During adduction, the inferior rectus is responsible for extorsion, adduction and depression. During abduction, this muscle is responsible for depression. This muscle inserts on the anterior, inferior surface of the eye. The origin is the Annulus of Zinn.
  3. Lateral rectus: In the neutral position, this muscle is responsible for abduction. This muscle inserts on the anterior, lateral surface of the eye. The origin is the Annulus of Zinn.
  4. Medial rectus: In the neutral position, this muscle is responsible for adduction. This muscle inserts on the posterior, superior, medial surface of the eye. The origin is the Annulus of Zinn.

The 2 other extraocular muscles controlling eye movement are:

  1. Superior oblique: In the neutral position, this muscle is responsible for intorsion, depression and abduction. During adduction, the superior oblique is responsible for depression, abduction and intorsion. During abduction, this muscle is responsible for intorsion, abduction and depression. This muscle inserts on the posterior, superior, lateral surface of the eye. The origin is the Annulus of Zinn (via the trochlea). The superior oblique travels along the medial wall of the orbit.
  2. Inferior oblique: In the neutral position, this muscle is responsible for extorsion, elevation and abduction. During adduction, the inferior oblique is responsible for elevation, abduction and extorsion. During abduction, this muscle is responsible for extorsion, abduction and elevation. This muscle inserts on the posterior, inferior, lateral surface of the eye. The origin is the maxillary bone. The inferior oblique travels from the medial wall of the orbit to the inferolateral aspect of the eye.

Finally, the muscle that controls eyelid elevation (which helps open your eyes) is:

  1. Levator palpebrae superioris: In the neutral position, this muscle is responsible for retracting and elevating the eyelid. This muscle inserts on the tarsal plate of the upper eyelid. The origin is the sphenoid bone.

Other notes: The superior, inferior, medial and lateral recti muscles originate from a shared tendinous ring on the posterior wall of the eye and insert on the anterior region of the eyeball, which is just beyond the visible sclera (“white of the eye”). Quoted from Saladin, “To visualize the function of the oblique muscles, suppose you turn your eyes to the right. The superior oblique muscle will slightly depress your right eye, while the inferior oblique slightly elevates the left eye. The opposite occurs when you look to the left. This is the primary function of the oblique muscles, but they also slightly rotate the eyes, turning the “twelve o’clock pole” of each eye slightly toward or away from the nose.”

Cranial nerve innervations

There are 3 cranial nerves (CN) that control the muscles of the orbit.

  1. Oculomotor nerve (CN III): This nerve controls the superior rectus (superior branch), inferior rectus (inferior branch), medial rectus (inferior branch), inferior oblique (inferior branch), and the levator palpebrae superior muscle.
  2. Trochlear nerve (CN IV): This nerve controls the superior oblique muscle.
  3. Abducens nerve (CN VI): This nerve controls the lateral rectus muscle and the retractor bulbi muscle.

Blood supply

The branches of the ophthalmic artery are the main suppliers of the extraocular muscles. Some branches of the ophthalmic artery include the ciliary arteries, which branches into the anterior ciliary arteries. Each of the recti muscles receives blood from 2 anterior ciliary arteries, except for the lateral rectus muscle, which receives blood from only one ciliary artery. However, between individuals, the exact number and arrangement of these cilary arteries can differ. The inferior rectus and inferior oblique muscles are supplied by branches of the infraorbital artery.

Mnemonic for adduction vs. abduction:

One way to remember the difference between aDduction and aBduction is to mentally picture the nose as the central or midline point between the eyes, and an UFO (or some other party) a-B-ducting the cells that dwell in the eye, and taking them away into outer space for experimental analysis. This means that abduction refers to looking away from the midline (nose) of the face, whereas adduction is looking toward the midline.

Mnemonic for simplified actions:

  1. Obliques Abduct whereas Rectii Adduct (except LR)
  2. Superiors Intort whereas Inferior Extort
  3. Rectii act according to their names whereas Obliques act opposite to their names.

Clinical Notes

The purpose of the clinical eye exam is to investigate whether or not the extraocular eye muscles are working and moving properly. All 6 of the muscles described above can be tested by drawing a large letter “H” in the air with a finger or pen in front of a patient’s face and having them follow the tip of the finger or pen with their eyes while keeping their head stationary.

When the eye is turned inwards (towards the nose) and horizontally, the function of the medial rectus muscle is being tested. When the eye is turned out (temporally, away from the nose) and horizontally, the function of the lateral rectus muscle is tested. When the eye is looking downwards and outwardly, the inferior rectus is contracting. The superior rectus is engaged when looking upwards and outwardly. Perhaps oddly, turning the eye upwards and inward uses the inferior oblique muscle, and turning it downward and inward uses the superior oblique.

Having the patient focus on the object (finger, pen, etc.) as it is moved in toward their face in the midline (towards their nose) tests for convergence, which is the eyes' ability to turn inward simultaneously to focus on a near object.

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Show references

References:

  • Kenneth Saladin, Anatomy & Physiology: The Unity of Forma and Function, 6th edition, McGraw-Hill Science/Engineering/Math, Chapter 16.

Author:

  • Alice Ferng, MD-PhD

Illustrators:         

  • Muscles of the orbit - anterior view - Yousun Koh 
  • Muscles of the orbit w/ neurovasculature - lateral view - Yousun Koh 
  • Muscles of the orbit - superior view - Yousun Koh 
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