Seven striated, voluntary muscles. Four recti + two obliques + levator palpebrae superioris.

Common Tendinous Ring (Annulus of Zinn)

• Four recti arise from common annular tendon attached to middle part of superior orbital fissure, surrounding optic canal
• Lateral rectus has additional tendinous head from greater wing of sphenoid lateral to ring; abducent nerve passes through gap between two lateral rectus heads

Origins

Course and Insertion

Recti: Forward to sclera just posterior to limbus. Average distances from limbus: superior rectus 7.7 mm; lateral rectus 6.9 mm; inferior rectus 6.5 mm; medial rectus 5.5 mm (nearest)

Superior oblique: Forward along medial orbital wall → tendon turns through fibrocartilaginous trochlea (in trochlear fossa of frontal bone) → passes laterally, downward, backward beneath superior rectus → inserts into sclera behind equator, between superior rectus + lateral rectus

Inferior oblique: Fleshy throughout; passes laterally, upward, backward beneath inferior rectus → deep to lateral rectus → inserts into sclera close to superior oblique insertion

LPS: Flat tendon splits into:

• Superior lamella (voluntary, striated) → anterior surface of superior tarsus + skin of upper eyelid
• Inferior lamella (involuntary, smooth = superior tarsal/Müller’s muscle) → upper margin of superior tarsus + superior conjunctival fornix

Nerve Supply

CN III divisions: superior division → superior rectus + LPS; inferior division → medial rectus, inferior rectus, inferior oblique (+ parasympathetic root of ciliary ganglion via nerve to inferior oblique)

Mnemonic: LR6 SO4, rest 3 — Lateral Rectus = CN VI; Superior Oblique = CN IV; rest = CN III

Actions of Individual Muscles

Pure (Combined) Movements

Applied Anatomy

Strabismus (Squint) — unilateral muscle paralysis → deviation of eye + diplopia (image on macula normal eye, peripheral retina of paralysed eye)

LPS paralysis:

• Complete ptosis = CN III palsy (striated LPS loss)
• Partial ptosis only = Horner’s syndrome (only smooth muscle/Müller’s muscle lost; sympathetic supply lost)

Mnemonic applied clinically: LR6 SO4 rest 3 → isolated lateral rectus weakness = CN VI; isolated superior oblique weakness = CN IV; any combination of remaining 5 muscles (± ptosis ± pupil) = CN III

The extraocular (extrinsic) muscles of the eyeball are seven striated, voluntary “miniature ribbon muscles” with short tendons of origin and long tendons of insertion. They comprise four recti (superior, inferior, medial, lateral), two obliques (superior, inferior), and the levator palpebrae superioris.

Common Tendinous Ring (Annulus of Zinn)

The four recti arise from a common annular tendon, the common tendinous ring (annulus of Zinn), which is attached to the middle part of the superior orbital fissure, surrounding the optic canal. The lateral rectus has an additional small tendinous head arising from the orbital surface of the greater wing of the sphenoid, lateral to the tendinous ring; the abducent nerve passes through the gap between the two heads of the lateral rectus.

Origins

• Superior rectus, inferior rectus, medial rectus, lateral rectus — arise from the common tendinous ring (annulus of Zinn).
• Superior oblique — arises from the undersurface of the lesser wing of the sphenoid, superomedial to the optic canal.
• Inferior oblique — arises from the orbital surface of the maxilla, lateral to the lacrimal groove, near the anterior margin of the orbit.
• Levator palpebrae superioris — arises from the orbital surface of the lesser wing of the sphenoid, anterosuperior to the optic canal and to the origin of the superior rectus.

Course and Insertion

Recti: The four recti pass forward to insert into the sclera a little posterior to the limbus (corneoscleral junction). Average distances of insertion from the limbus are: superior rectus 7.7 mm, lateral rectus 6.9 mm, inferior rectus 6.5 mm, medial rectus 5.5 mm.

Superior oblique: Passes forward along the medial wall of the orbit and its tendon turns through a fibrocartilaginous pulley (trochlea) attached to the trochlear fossa of the frontal bone. The tendon then passes laterally, downward, and backward beneath the superior rectus, to insert into the sclera behind the equator of the eyeball, between the superior rectus and lateral rectus.

Inferior oblique: Fleshy throughout its course. Passes laterally, upward, and backward beneath the inferior rectus and then deep to the lateral rectus, inserting into the sclera close to the insertion of the superior oblique, a little below and posterior to it.

Levator palpebrae superioris: Its flat tendon splits into a superior (voluntary, striated) lamella and an inferior (involuntary, smooth muscle — superior tarsal/Müller’s muscle) lamella.

• The superior lamella inserts into the anterior surface of the superior tarsus and into the skin of the upper eyelid.
• The inferior lamella inserts into the upper margin of the superior tarsus and into the superior conjunctival fornix.

Nerve Supply

• Superior oblique — trochlear nerve (CN IV).
• Lateral rectus — abducent nerve (CN VI).
• All remaining five muscles — superior rectus, medial rectus, inferior rectus, inferior oblique, and levator palpebrae superioris (striated/voluntary part) — supplied by the oculomotor nerve (CN III).
  • The superior division of the oculomotor nerve supplies the superior rectus and levator palpebrae superioris.
  • The inferior division of the oculomotor nerve supplies the medial rectus, inferior rectus, and inferior oblique; the nerve to the inferior oblique also gives off the parasympathetic root of the ciliary ganglion.
• The smooth muscle (involuntary/superior tarsal) part of levator palpebrae superioris is supplied by postganglionic sympathetic fibres from the superior cervical ganglion.

Mnemonic: LR6 SO4, rest 3 — Lateral Rectus by CN VI, Superior Oblique by CN IV, the rest (superior rectus, medial rectus, inferior rectus, inferior oblique, levator palpebrae superioris) by CN III.

Axes and Movements of the Eyeball

Movements occur around three axes, defined in terms of movement of the centre of the pupil (rotations) or the upper margin of the pupil at the 12 o’clock position (torsions):

• Transverse axis: Elevation (upward rotation, ~33°) and depression (downward rotation, ~33°).
• Vertical axis: Adduction (medial rotation, ~50°) and abduction (lateral rotation, ~50°).
• Anteroposterior axis: Intorsion and extorsion.

Actions of Individual Muscles

Single (Pure) Movements

Pure movements result from combined actions of muscles — similar actions summate, opposing actions cancel:

• Elevation: Superior rectus + inferior oblique.
• Depression: Inferior rectus + superior oblique.
• Adduction: Medial rectus + superior rectus + inferior rectus.
• Abduction: Lateral rectus + superior oblique + inferior oblique.
• Intorsion: Superior oblique + superior rectus.
• Extorsion: Inferior oblique + inferior rectus.

Conjugate Movements

Normally, movements of the two eyes are coordinated such that the visual axes remain parallel — these are called conjugate ocular movements. For example, looking to one side involves the lateral rectus of one eye and the medial rectus of the other eye acting together. Direct upward and downward gaze requires the superior/inferior recti to act in concert with the corresponding oblique muscles of both eyes, since the recti’s pull is not purely vertical given the orbital axis orientation.

Clinical Correlation

Strabismus (Squint): Unilateral paralysis of an individual extraocular muscle, due to involvement of its nerve, produces strabismus (deviation of the eye) and may result in diplopia (double vision), where the image falls on the macula of the normal eye but on the peripheral retina of the paralyzed eye.

Levator Palpebrae Superioris Paralysis: Paralysis of the striated (voluntary) part of levator palpebrae superioris due to oculomotor nerve involvement causes ptosis (drooping of the upper eyelid). Since the smooth muscle (superior tarsal/Müller’s) part is supplied by sympathetic fibres, loss of sympathetic supply (as in Horner’s syndrome) causes partial ptosis only, whereas complete oculomotor nerve palsy causes complete ptosis.

LR6SO4(rest 3) Mnemonic Applied Clinically: This mnemonic is the quickest way to localize a cranial nerve lesion from the pattern of extraocular muscle weakness — isolated lateral rectus weakness implicates CN VI, isolated superior oblique weakness implicates CN IV, and any combination of the remaining five muscles (with or without ptosis and pupillary involvement) implicates CN III.

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