What does the mnemonic LR6 SO4 3 mean?

LR6 means the Lateral Rectus is innervated by Cranial Nerve VI (Abducens). SO4 means the Superior Oblique is innervated by Cranial Nerve IV (Trochlear). The 3 means all remaining muscles (Medial Rectus, Superior Rectus, Inferior Rectus, and Inferior Oblique) are innervated by Cranial Nerve III (Oculomotor).

What are yoke muscles?

Yoke muscles are the pair of muscles, one from each eye, that contract together to produce conjugate (both eyes moving in the same direction) eye movements. For example, when looking to the right, the right lateral rectus and left medial rectus are yoke muscles. Hering's Law states that equal neural impulses are sent to yoke muscles to ensure coordinated binocular movement.

What is the presentation of a CN III palsy?

A complete CN III palsy affects the medial rectus, superior rectus, inferior rectus, and inferior oblique muscles, plus the levator palpebrae and pupil sphincter. The affected eye is positioned down and out because the intact lateral rectus (CN VI) and superior oblique (CN IV) are unopposed. There is ptosis from levator weakness and potentially a fixed, dilated pupil from parasympathetic fiber involvement.

Extraocular Muscles and Their Cranial Nerve Innervation

The Six Extraocular Muscles

Each eye is controlled by six extraocular muscles (EOMs) that work in coordinated pairs to produce smooth, accurate eye movements. Understanding which muscles move the eye in which direction and which cranial nerve controls each muscle is fundamental knowledge for the COA exam and daily clinical work.

Muscles and Their Innervation

The classic mnemonic LR6(SO4)3 tells you everything you need to know about innervation:

LR6: The Lateral Rectus is innervated by Cranial Nerve VI (Abducens)
SO4: The Superior Oblique is innervated by Cranial Nerve IV (Trochlear)
3: All remaining four muscles are innervated by Cranial Nerve III (Oculomotor)

The four muscles innervated by CN III are:

Medial Rectus (MR): Moves the eye inward (adduction)
Superior Rectus (SR): Moves the eye upward (elevation)
Inferior Rectus (IR): Moves the eye downward (depression)
Inferior Oblique (IO): Moves the eye upward and outward

CN III also innervates the levator palpebrae superioris (upper lid elevation) and carries parasympathetic fibers to the pupil sphincter (pupil constriction). This is why a complete CN III palsy produces ptosis, a fixed dilated pupil, and limitation of most eye movements.

Primary Actions and Cardinal Positions

Each muscle has a primary action (its main movement) along with secondary and tertiary actions. The cardinal positions of gaze are six directions that isolate the primary action of each muscle:

Gaze Direction	Right Eye Muscle	Left Eye Muscle
Right	Lateral Rectus (CN VI)	Medial Rectus (CN III)
Left	Medial Rectus (CN III)	Lateral Rectus (CN VI)
Up and Right	Superior Rectus (CN III)	Inferior Oblique (CN III)
Down and Right	Inferior Rectus (CN III)	Superior Oblique (CN IV)
Up and Left	Inferior Oblique (CN III)	Superior Rectus (CN III)
Down and Left	Superior Oblique (CN IV)	Inferior Rectus (CN III)

Yoke Muscles

Yoke muscles are the pair of muscles (one from each eye) that work together to produce conjugate eye movements in a given direction. When you look right, your right lateral rectus and left medial rectus fire together as yoke muscles. Hering's Law states that equal innervation is sent to yoke muscles, ensuring both eyes move together smoothly.

Knowing the yoke muscle pairs and their innervation allows you to predict which muscle and nerve are affected when a patient has limited movement in a particular gaze direction. If a patient cannot look up and to the right with the right eye, the right superior rectus (CN III) is likely involved.

Cranial Nerve Palsies

CN III (Oculomotor) Palsy

Affects four EOMs plus the lid and pupil. Signs include ptosis, a "down and out" eye position (because the intact lateral rectus and superior oblique pull unopposed), and possibly a fixed dilated pupil.

CN IV (Trochlear) Palsy

Affects only the superior oblique. The patient has difficulty looking down and inward with the affected eye. They often compensate with a head tilt away from the affected side to reduce diplopia.

CN VI (Abducens) Palsy

Affects only the lateral rectus. The patient cannot abduct the affected eye (look outward), causing an esotropia that worsens in the direction of the palsied muscle.

CN VI palsy is the most common isolated cranial nerve palsy affecting eye movement. If a patient presents with sudden onset of horizontal double vision and an inability to look outward with one eye, think CN VI palsy first.

Key Takeaways

Six extraocular muscles control each eye, innervated by three cranial nerves: CN III (four muscles), CN IV (superior oblique), and CN VI (lateral rectus)
The mnemonic LR6(SO4)3 is the essential memory tool for innervation
Cardinal positions of gaze isolate individual muscle function for clinical testing
Yoke muscles (one from each eye) work together for conjugate movements per Hering's Law
Cranial nerve palsies produce predictable patterns of limited movement and diplopia