What mnemonic helps remember extraocular muscle innervation?

LR6 SO4, all the rest 3. The Lateral Rectus is innervated by cranial nerve VI (abducens), the Superior Oblique by cranial nerve IV (trochlear), and all four remaining muscles (medial rectus, superior rectus, inferior rectus, inferior oblique) by cranial nerve III (oculomotor).

What are yoke muscles?

Yoke muscles are pairs of muscles, one from each eye, that work together to move both eyes in the same direction. For example, to look right, the right lateral rectus and the left medial rectus work as a yoke pair. Hering's Law states that equal innervation is sent to yoke muscles for coordinated binocular movement.

Why does the superior oblique depress the eye instead of elevating it?

The superior oblique's tendon passes through the trochlea (a pulley near the nose) and then inserts on the posterior-superior globe at an angle. This insertion angle means that when the muscle contracts and pulls, the back of the eye rotates upward, which tilts the front of the eye downward (depression). The naming refers to the muscle's origin position, not its action.

Extraocular Muscles: Eye Movement Control for ABO

Overview of Extraocular Muscles

The extraocular muscles (EOMs) are six muscles that control the movement of each eye. They work in coordinated pairs to produce smooth, precise eye movements in all directions of gaze. All six originate from the posterior orbit and insert onto the sclera.

The Six Muscles

Four Rectus Muscles

The rectus ("straight") muscles run relatively straight from their origin at the annulus of Zinn (a tendinous ring at the orbital apex) to their insertions on the sclera:

Muscle	Primary Action	Innervation
Medial rectus (MR)	Adduction (turns eye inward/nasally)	CN III (oculomotor)
Lateral rectus (LR)	Abduction (turns eye outward/temporally)	CN VI (abducens)
Superior rectus (SR)	Elevation (turns eye upward)	CN III (oculomotor)
Inferior rectus (IR)	Depression (turns eye downward)	CN III (oculomotor)

Two Oblique Muscles

The oblique muscles approach the globe at angles, giving them more complex actions:

Muscle	Primary Action	Secondary Actions	Innervation
Superior oblique (SO)	Intorsion (rotates top of eye nasally)	Depression, abduction	CN IV (trochlear)
Inferior oblique (IO)	Extorsion (rotates top of eye temporally)	Elevation, abduction	CN III (oculomotor)

The superior oblique is unique: it passes through a pulley-like structure called the trochlea (a cartilaginous ring attached to the frontal bone) before inserting on the posterior-superior globe.

A simple mnemonic for innervation: "LR6 SO4, all the rest 3." The Lateral Rectus is innervated by CN VI, the Superior Oblique by CN IV, and all remaining muscles (MR, SR, IR, IO) by CN III.

Muscle Actions in Different Gaze Positions

Each muscle has primary, secondary, and sometimes tertiary actions. The primary action is the muscle's main effect when the eye is in primary position (looking straight ahead). However, the effective action of each muscle changes depending on the eye's position.

For clinical testing, muscles are tested in their diagnostic action positions, where each muscle is the primary mover:

Gaze Position	Right Eye Muscle Tested	Left Eye Muscle Tested
Right and up	Superior rectus	Inferior oblique
Right	Lateral rectus	Medial rectus
Right and down	Inferior rectus	Superior oblique
Left and up	Inferior oblique	Superior rectus
Left	Medial rectus	Lateral rectus
Left and down	Superior oblique	Inferior rectus

Yoke Muscles and Hering's Law

Yoke muscles are pairs of muscles (one from each eye) that work together to move both eyes in the same direction. Hering's Law states that equal innervation is sent to yoke muscles, ensuring coordinated binocular movements.

Examples of yoke muscle pairs:

Right lateral rectus + Left medial rectus (both eyes look right)
Right superior rectus + Left inferior oblique (both eyes look right and up)
Right inferior rectus + Left superior oblique (both eyes look right and down)

Sherrington's Law

Sherrington's Law of Reciprocal Innervation states that when an agonist muscle contracts, its antagonist in the same eye simultaneously relaxes. For example, when the right lateral rectus contracts (abduction), the right medial rectus relaxes proportionally.

When a patient presents with a head tilt, consider a cranial nerve palsy. A head tilt away from the affected side is a classic compensatory strategy for a CN IV (superior oblique) palsy. The patient tilts their head to reduce the torsional diplopia caused by the weakened intorting muscle.

Clinical Testing

Eye movements are tested using several methods:

Versions: Binocular eye movements in the nine cardinal positions of gaze (the H-pattern)
Ductions: Monocular eye movements (testing one eye while the other is covered)
Cover test: Detects tropias (manifest deviations) and phorias (latent deviations)

Confusing which oblique muscle elevates and which depresses. The INFERIOR oblique ELEVATES the eye (seems backwards). The SUPERIOR oblique DEPRESSES the eye (also seems backwards). This is because they pull from opposite directions due to their angled insertions.

Key Takeaways

Six EOMs control each eye: four rectus (MR, LR, SR, IR) and two oblique (SO, IO)
Innervation: LR6, SO4, all the rest CN III
Yoke muscles (Hering's Law) ensure coordinated binocular movement
Antagonist muscles relax when agonists contract (Sherrington's Law)
The superior oblique passes through the trochlea and its primary action is intorsion