What are the muscles of the eye, their functions, and anatomical relations, including those adjacent to the frontal nerve?

Extraocular Muscles: Functions and Anatomical Relations for EMQ Preparation

The Six Extraocular Muscles

Rectus Muscles (Four)

The four rectus muscles originate from the annulus of Zinn at the orbital apex and insert anteriorly on the globe 1:

• Medial Rectus (MR)

  • Primary action: Adduction 1
  • Innervation: CN III (inferior division) 2
  • Clinical note: Most commonly involved in thyroid eye disease, causing esotropia and hypotropia 1
  • Anatomical relations: Flanks the optic nerve medially; frequently injured during retrobulbar/peribulbar anesthesia 3, 1

• Lateral Rectus (LR)

  • Primary action: Abduction 1
  • Innervation: CN VI 2
  • Anatomical relations: Connected to superior rectus by the SR-LR connective tissue band, which maintains fixed distance between them posteriorly 3, 1; flanks optic nerve laterally 1
  • Clinical note: Age-related degeneration of SR-LR band causes sagging eye syndrome with divergence insufficiency 3

• Superior Rectus (SR)

  • Primary action: Elevation 1
  • Secondary actions: Adduction and intorsion 1
  • Innervation: CN III (superior division) 2
  • Anatomical relations: Lies directly above the optic nerve 1; connected to lateral rectus by SR-LR band 3, 1; most commonly injured during retrobulbar/peribulbar anesthesia 3, 1

• Inferior Rectus (IR)

  • Primary action: Depression 1
  • Secondary actions: Adduction and extorsion 1
  • Innervation: CN III (inferior division) 2
  • Anatomical relations: Most commonly injured muscle during retrobulbar or peribulbar blocks 3, 1; frequently damaged in lower lid blepharoplasty 3

Oblique Muscles (Two)

• Superior Oblique (SO)

  • Primary action: Intorsion 1
  • Secondary actions: Depression and abduction 1
  • Innervation: CN IV 4
  • Anatomical relations: Originates from orbital apex, passes through the trochlea (pulley) in anteromedial orbit, then inserts posterolaterally on globe 1
  • Clinical note: Trochlear damage can occur after upper lid blepharoplasty 3; entanglement of superior oblique tendon can occur after scleral buckling 3

• Inferior Oblique (IO)

  • Primary action: Extorsion 1
  • Secondary actions: Elevation and abduction 1
  • Innervation: CN III (inferior division) 2
  • Anatomical relations: Originates from anteromedial orbital floor (unique—does not originate from annulus of Zinn) 1
  • Clinical note: Overaction commonly seen in V-pattern esotropia 3

Eyelid Muscles (Critical for EMQs)

• Levator Palpebrae Superioris (LPS)

  • Primary action: Elevation of upper eyelid 5
  • Innervation: CN III (superior division) 5
  • Anatomical relations: Adjacent to the frontal nerve (as you noted in your question) 5; functions in conjunction with superior and inferior rectus muscles in coordinated eye/lid movements 5
  • Clinical note: Motoneurons distributed bilaterally throughout caudal central division of oculomotor nucleus 5; ptosis occurs with CN III palsy 4

• Orbicularis Oculi

  • Primary action: Eyelid closure 5
  • Innervation: CN VII (facial nerve), dorsal subdivision 5
  • Anatomical relations: Antagonistic to levator palpebrae superioris 5

Critical Anatomical Relationships for EMQs

The SR-LR connective tissue band is essential for maintaining normal muscle position 3, 1:

• Failure of this band in elderly or high myopes causes medial shift of superior rectus and inferior shift of lateral rectus 3
• Results in sagging eye syndrome with divergence insufficiency and mild ptosis 3

Relationship to optic nerve 1:

• Superior rectus: directly above optic nerve
• Medial and lateral rectus: flank optic nerve medially and laterally

Muscle injury patterns during anesthesia 3, 1:

• Superior and inferior rectus most vulnerable during retrobulbar/peribulbar blocks 3, 1
• Any extraocular muscle including obliques can be involved 3

Testing Individual Muscle Function (EMQ Scenarios)

Use these specific deficits to identify which muscle is affected 2:

• Inability to adduct: Medial rectus or CN III (inferior division) dysfunction 2
• Inability to elevate from abduction: Superior rectus or CN III (superior division) dysfunction 2
• Inability to depress from abduction: Inferior rectus or CN III (inferior division) dysfunction 2
• Inability to elevate from adduction: Inferior oblique or CN III (inferior division) dysfunction 2
• Inability to depress from adduction: Superior oblique or CN IV dysfunction 2
• Inability to abduct: Lateral rectus or CN VI dysfunction 2

Common Clinical Pitfalls for EMQs

Never assume normal anatomical muscle positions in thyroid eye disease—muscle enlargement (particularly inferior and medial rectus) causes mechanical restriction mimicking weakness 2, 1:

Partial CN III palsies with incomplete ptosis or partial extraocular muscle involvement cannot be assumed to be microvascular—neuroimaging with MRI/gadolinium and MRA or CTA is required to exclude compressive lesions 4, 2:

Pupil-involving third nerve palsy requires immediate imaging within hours to exclude posterior communicating artery aneurysm 4:

Oculocephalic rotations (doll's-head maneuver) are particularly valuable in infants and young children to reveal normal ductions that may not be apparent on voluntary testing 3, 2: