Lecture 5 - Extraocular Muscles

Ocular Anatomy

How many extraocular muscles are there, and what are they?

Six: superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, inferior oblique.

What do EOMs attach to and control?

They attach to the sclera and control globe movement.

What are the rectus muscles from longest to shortest?

Superior → medial → lateral → inferior.

“SMeLI”

What structural features make EOMs unique compared to other skeletal muscle?

• Dense blood supply

• Delicate connective tissue sheaths rich in elastic fibers

• Dense innervation for precise fine motor control

• Among fastest and most fatigue-resistant striated muscle

What are EOM pulleys and their function?

Rings of dense collagen (2 mm in length) with smooth muscle-connective tissue struts that anchor to periorbita.

• They act as functional origins, refining coordination of binocular movements and redirecting muscle pull.

What does orbital connective tissue provide?

A dense supportive framework, stabilizing muscle paths and limiting eye movement.

What is the common origin of the rectus muscles?

The common tendinous ring (annulus of Zinn) at the orbital apex, continuous with periorbita.

Which rectus muscles originate from both upper & lower parts of the ring?

Medial rectus and lateral rectus.

Which muscles originate from the upper limb only?

Superior rectus.

Which muscles originate from the lower limb only?

Inferior rectus.

Which recti are also attached to the dural sheath of the optic nerve?

Medial rectus and superior rectus.

Where do the four rectus muscles insert relative to the equator of the globe?

Anterior to the equator.

What line connects the rectus muscle insertions?

Spiral of Tillaux.

Order of recti insertions from closest to furthest from limbus?

Medial (5.5 mm) → Inferior (6.5 mm) → Lateral (6.6 mm) → Superior (7.7 mm).

Mnemonic: MILS.

Medial Rectus (MR):

• Largest EOM

• Origin: Upper & lower parts of tendinous ring + optic nerve sheath

• Insertion: ~5.5 mm from limbus

• Innervation: Inferior division of CN III

Lateral Rectus (LR):

• Origin: Upper & lower tendinous ring + spina recti lateralis (sphenoid)

• Insertion: ~6.6 mm from limbus

• Innervation: CN VI (abducens)

Superior Rectus (SR):

• Origin: Upper tendinous ring + optic nerve sheath

• Connections with levator sheath coordinate eyelid & globe movement

• Innervation: Superior division of CN III

Inferior Rectus (IR):

• Origin: Lower tendinous ring

• Connections with lower lid (tarsal plate) → lid lowering with down gaze

• Innervation: Inferior division of CN III

Superior Oblique (SO):

• Longest & thinnest EOM

• Origin: Lesser wing of sphenoid → passes through trochlea (pulley)

• Trochlea = functional origin

• Innervation: CN IV (trochlear)

Inferior Oblique (IO):

• Only EOM with anterior orbital origin (maxillary bone)

• Runs parallel to SO

• Innervation: Inferior division of CN III

What are Fick’s Axes for eye movement?

• X-axis (horizontal): center of cornea moves up/down

  • Runs nasal to temporal

  • (supraduction, infraduction)

• Y-axis (sagittal): torsion (intorsion, extorsion)

• Z-axis (vertical): cornea moves left/right (adduction, abduction

What are the types of eye movements?

• Ductions (one eye): adduction, abduction, elevation, depression, intorsion, extorsion

• Vergences (both eyes opposite): convergence (adduct), divergence (abduct)

• Versions (both eyes same): conjugate movements

What are the duction movements?

• Movements involving one eye

• Rotation around vertical z-axis

  • adduction

  • abduction

• Rotation around horizontal x-axis

  • supraduction

  • infraduction

• Rotation around sagittal y-axis

  • intorsion

  • extorsion

What are the vergences and versions movements?

Movements involving both eyes

• Version = eyes move in same direction

• Vergence = eyes move in opposite directions

  • Convergence = both eyes adduct

  • Divergence = both eyes abduct

What are the positions of gaze?

• Primary = eyes straight ahead with head erect

• Secondary = Rotation around vertical or horizontal axis

  • Eyes up/down/left/right

• Tertiary = Rotations around both vertical and horizontal axes

  • Eyes in oblique positions

What are the primary actions of the EOMs (from primary position)?

• MR: adduction

• LR: abduction

• SR: elevation (secondary: adduction, intorsion)

• IR: depression (secondary: adduction, extorsion)

• SO: intorsion (secondary: depression, abduction)

• IO: extorsion (secondary: elevation, abduction)

What are the movements from primary position of the medial rectus?

Adduction

What are the movements from primary position of the lateral rectus?

Abduction

What are the movements from primary position of the superior rectus?

• Primary action: elevation

• Secondary actions: adduction, intorsion

What are the movements from primary position of the inferior rectus?

• Primary action: depression

• Secondary actions: adduction, extorsion

What are the movements from primary position of the superior oblique?

• Primary action: Intorsion

• Secondary actions: Depression, abduction

What are the movements from primary position of the inferior oblique?

• Primary action: extorsion

• Secondary actions: elevation, abduction

Movements from secondary position of the vertical rectus muscles — eye is abducted 23 degrees from primary position

• Vertical rectus muscles parallel the y-axis

• Perpendicular to x-axis

• Only vertical movement occurs

  • Superior rectus will only elevate

  • Inferior rectus will only depress

Movements from secondary position of the vertical rectus muscles — eye is abducted 67 degrees from primary position

• Vertical rectus muscles parallel the x-axis

• Perpendicular to y-axis

• Only torsional movement occurs

  • Superior rectus will only intort

  • Inferior rectus will only extort

Movements from secondary position of the oblique muscles — eye is abducted 51-55 degrees from primary position

• Oblique muscles parallel to y-axis

• Perpendicular to x-axis

• Only vertical movement occurs

  • Superior oblique will only depress

  • Inferior oblique will only elevate

Movements from secondary position of the oblique muscles — eye is abducted 35-39 degrees from primary position

• Oblique muscles parallel to x-axis

• Perpendicular to y-axis

• Only vertical movement occurs

  • Superior oblique will only intort

  • Inferior oblique will only extort

What are the muscle actions in secondary positions?

• Vertical recti (SR, IR):

  • Eye abducted 23° → pure vertical (SR elevates, IR depresses)

  • Eye adducted 67° → pure torsion (SR intorts, IR extorts)

• Oblique muscles (SO, IO):

  • Eye adducted 51–55° → pure vertical (SO depresses, IO elevates)

  • Eye abducted 35–39° → pure torsion (SO intorts, IO extorts)

What is Sherrington’s Law?

• Contraction of a muscle → proportional relaxation of antagonist

• Examples:

  • Adduction: MR contracts, LR relaxes

  • Elevation: SR contracts, IR relaxes

What are the antagonist pairs?

• SR/IR

• MR/LR

• SO/IO

What are the agonist pairs (synergists)?

• Elevation = SR + IO

• Depression = IR + SO

• Intorsion = SR + SO

• Extorsion = IR + IO

What are yoke muscles?

Muscles in opposite eyes that act together for binocular movement, receiving equal and simultaneous innervation.

• Left gaze = Left LR + Right MR

• Right gaze = Right LR + Left MR

Summary of EOM innervation

• CN III (oculomotor):

  • Superior division: SR

  • Inferior division: MR, IR, IO + parasympathetic to ciliary ganglion

• CN IV (trochlear): SO

• CN VI (abducens): LR

LR6(SO4)3

What is the pathway of CN III (Oculomotor)?

• Nucleus in midbrain (motor + parasympathetic)

• Fibers pass between posterior cerebral artery & superior cerebellar artery

• Goes through cavernous sinus

• Divides into superior & inferior branches

• Superior orbital fissure (through CTR)

What is the pathway of CN IV (Trochlear)?

• Nucleus in midbrain below CN III

• Only cranial nerve exiting dorsally; decussates before orbit

• Travels with frontal nerve, enters orbit above CTR → innervates SO

What is the pathway of CN VI (Abducens)?

• Nucleus in pons

• Long tortuous course: exits pontomedullary junction → over petrous ridge → cavernous sinus (next to ICA) → enters orbit via superior orbital fissure within CTR → LR

What do the branches of the ophthalmic artery supply?

• Lateral muscular branch: SR, LR, SO

• Medial muscular branch: IR, MR, IO

What aging changes occur with EOM?

• MR & LR displaced inferiorly (MR > LR) → impaired elevation

• Greater muscle fiber size variability

• Increased connective tissue & fat

• General degenerative changes

What is Hering’s Law?

Synergist muscles in opposite eyes will receive equal and simultanious innervation for vergence movements.

Ex: right MR and left LR work together to produce right gaze.

“Schools of herings swim together”