Extraocular muscles: anatomy, actions and nerve supply
Quick answer
Six extraocular muscles move each eye: the medial, lateral, superior and inferior recti, and the superior and inferior obliques. The medial and lateral recti move the eye horizontally; the superior and inferior recti mainly elevate and depress it; the obliques mainly rotate it (torsion) and also move it vertically. The lateral rectus is supplied by the sixth cranial nerve, the superior oblique by the fourth, and all the others by the third — remembered as "LR6 SO4 R3".
Open the simulatorOrigins and insertions
| Muscle | Origin | Insertion |
|---|---|---|
| Medial rectus | Annulus of Zinn | Medial sclera, about 5.5 mm from the limbus |
| Inferior rectus | Annulus of Zinn | Inferior sclera, about 6.5 mm from the limbus |
| Lateral rectus | Annulus of Zinn | Lateral sclera, about 6.9 mm from the limbus |
| Superior rectus | Annulus of Zinn | Superior sclera, about 7.7 mm from the limbus |
| Superior oblique | Body of the sphenoid, just above and medial to the annulus; the tendon turns through the trochlea | Posterior superotemporal sclera, behind the equator |
| Inferior oblique | Anterior orbital floor, just lateral to the nasolacrimal canal | Posterior inferotemporal sclera, near the macula |
A line joining the four rectus insertions forms the spiral of Tillaux: the insertions lie progressively further from the limbus in the order medial, inferior, lateral and superior rectus.
Actions in primary position
| Muscle | Primary | Secondary | Tertiary |
|---|---|---|---|
| Medial rectus | Adduction | — | — |
| Lateral rectus | Abduction | — | — |
| Superior rectus | Elevation | Intorsion | Adduction |
| Inferior rectus | Depression | Extorsion | Adduction |
| Superior oblique | Intorsion | Depression | Abduction |
| Inferior oblique | Extorsion | Elevation | Abduction |
Memory aid: "RAD SIN" — the vertical Recti ADduct, and the Superior muscles INtort. The obliques abduct.
Why the actions change with the direction of gaze
- The superior and inferior recti run at about 23 degrees to the visual axis. With the eye abducted by about 23 degrees their pull lines up with the visual axis and they act as pure elevator and depressor — which is why the vertical recti are tested in abduction.
- The oblique muscles pull at about 51 degrees to the visual axis. With the eye adducted by about 51 degrees the inferior oblique acts mainly as an elevator and the superior oblique as a depressor — which is why the obliques are tested in adduction.
- In the opposite positions the same muscles act mainly as rotators, which explains their torsional actions.
Nerve supply
| Nerve | Supplies | Palsy in one line |
|---|---|---|
| Oculomotor (III) | Medial, superior and inferior recti, inferior oblique and levator palpebrae; parasympathetic fibers to the pupil sphincter and ciliary muscle | Ptosis, eye down and out, possibly a dilated pupil |
| Trochlear (IV) | Superior oblique | Hypertropia worse on opposite gaze and same-side head tilt |
| Abducens (VI) | Lateral rectus | Esotropia with limited abduction |
The superior division of the third nerve supplies the superior rectus and levator; the inferior division supplies the medial rectus, inferior rectus and inferior oblique and carries the parasympathetic fibers to the ciliary ganglion. The fourth nerve is the only cranial nerve that leaves the back of the brainstem, and its fibers cross, so each trochlear nucleus supplies the superior oblique of the opposite eye.
Yoke muscles and the laws of innervation
Yoke muscles are the pair — one in each eye — that move the eyes in the same direction, such as the right lateral rectus and the left medial rectus in right gaze. Hering's law gives yoke muscles equal innervation, and Sherrington's law relaxes each muscle's antagonist as it contracts. The ocular motility guide shows how these laws are used when examining eye movements.
Practice it on the virtual patient
- Open the 3D anatomy view and select one muscle at a time.
- Change the contraction and watch the globe move, then follow the agonist and antagonist bars (Sherrington's law).
- Use the cardinal gaze diagram to see which muscle of each eye leads in each of the nine positions.
- Then apply it to a patient with a fourth nerve palsy or Brown syndrome.
Frequently asked questions
What does LR6 SO4 R3 mean?
It is a memory aid for the nerve supply of the extraocular muscles: the lateral rectus is supplied by cranial nerve VI, the superior oblique by cranial nerve IV, and the rest — medial, superior and inferior recti and inferior oblique — by cranial nerve III.
What is the spiral of Tillaux?
The spiral of Tillaux is the imaginary line joining the insertions of the four rectus muscles, which lie progressively further from the limbus: medial rectus about 5.5 mm, inferior rectus 6.5 mm, lateral rectus 6.9 mm and superior rectus 7.7 mm.
Why is the superior oblique tested in adduction?
In adduction the line of pull of the superior oblique lines up with the visual axis, so its depressing action is strongest. Weakness shows as reduced depression of the adducted eye.
Which extraocular muscle does not start at the orbital apex?
The inferior oblique, which arises from the front of the orbital floor. The other five muscles arise at the orbital apex: the four recti from the annulus of Zinn and the superior oblique from the sphenoid bone just outside it.
Related guides
- Ocular motility and the H-test: ductions, versions and the nine positions of gaze Trace the H, know which muscle each gaze position tests and interpret limited movements.
- Third, fourth and sixth cranial nerve palsies and the three-step test Recognize each ocular motor palsy, run the three-step test and spot the red flags.
- Hirschberg and Krimsky tests: measuring strabismus with the corneal light reflex Read the corneal light reflex, convert millimeters to prism dioptres and neutralize with prisms.