Lecture 5: Extraocular Muscles (EOMs)

What are the functions of the EOMs?

• Attach to sclera

• Control movement of globe

What type of muscles are EOMs?

• Voluntary extrinsic muscles

• Striated skeletal muscle

What is an epimysium?

Connective tissue network surrounding entire muscle

What is perimysium?

Connective tissue network organizing myofibers into bundles (fascicles)

What is endomysium?

Connective tissue sheath surrounding individual muscle fibers.

What is a sarcolemma?

Cell membrane that surround striated skeletal muscle.

What are transverse (T-) tubules?

Extensions of sarcolemma into muscle fiber

Each myofiber is made of many ______.

Myofibrils

Each myofibril is made of _______.

Myofilaments

What are two types of cytoskeletal proteins that are found in striated skeletal muscle?

Actin (thin filament)

Myosin (thick filament)

What is a sacroplasmic reticulum?

A specialized smooth endoplasmic reticulum that store and release calcium ions (Ca2+).

How do EOMs differ from typical skeletal muscle?

• Denser blood supply

• More delicate connective tissue sheaths

• Richer in elastic fibers

• More precise fine motor control (dense innervation and fewer muscle fibers)

• More fatigue resistant

What EOMs do medial and lateral check ligament connects?

Medial check: medial rectus → medial orbital wall

Lateral check: lateral rectus → lateral orbital wall

Each EOM has a ______, which is a soft tissue that encircles the muscle.

Pulley

Where do pulleys for the EOMs connect?

Connects to the Tenon’s capsule

Where are pulleys attached to?

Orbital wall

What are the functions of pulleys?

• Control position and movement

• Stabilize muscle path

• Determine direction of pull

• Prevent EOM displacement

What is the common tendinous ring (Annulus of Zinn)?

An oval band of connective tissue continuous with periorbita

Where is the common tendinous ring located?

Located at the apex of orbit anterior → optic canal

What are some nerves and vessels that pass through the oculomotor foramen?

• Nasociliary nerve

• Oculomotor nerve (superior and inferior regions)

• Abducens nerve

• Optic nerve

• Ophthalmic artery

What is the oculomotor foramen?

An area enclosed by the common tendinous ring where nerves and vessels pass through.

Where is the origin of the rectus muscles?

Both medial and superior recti muscle sheaths are attached to the dural sheath of the optic nerve.

Where do all rectus muscles insert?

They insert into the globe anterior to the equator.

List the EOMs from the closest to furthest from the limbus.

Medial → Inferior → Lateral → Superior

“MILS”

What does the insertion of the rectus muscles create?

Spiral of Tillaux

What are some intraconal structures?

• Optic nerve

• Ophthalmic artery

• Central retinal artery and vein

• Adipose tissue

What are some extraconal structures?

• Lacrimal gland

• Adipose tissue

• Nerves

What is the origin of the superior rectus?

Common tendinous ring and optic nerve sheath

Where is the superior rectus inserted?

Sclera

What innervates the superior rectus?

Superior division of oculomotor nerve (CN III)

What is the function of the superior rectus?

Coordinate eye movement with eyelid position. (due to sheath connecting to levator muscle sheath)

What’s the longest EOM?

Superior Rectus

What is the origin, insertion, and innervation of the inferior rectus?

Origin: Common tendinous ring

Insertion: Sclera

Innervation: Inferior division of oculomotor nerve (CN III)

What is the shortest rectus muscle?

Inferior rectus

What is the function of the inferior rectus?

Coordinate eye movement with eyelid position (due to sheath connecting to inferior tarsal plate)

What is the origin, insertion, and innervation of the medial rectus?

Origin: Common tendinous ring and optic nerve sheath

Insertion: Sclear

Innervation: Inferior division of oculomotor nerve (CN III)

What is the largest EOM?

Medial Rectus

The fascia from the medial rectus muscle sheath connect to ________ and forms ________.

Orbital wall; Medial check ligament

What is the origin, insertion, and innervation of the lateral rectus?

Origin: Common tendinous ring

Insertion: Sclera

Innvervation: Abducens nerve (CN VI)

What is the origin, insertion, and innervation of the superior oblique?

Origin: Lesser wing of sphenoid and Trochlea

Insertion: Superior lateral globe (behind equator)

Innervation: Trochlear nerve (CN IV)

What’s the longest and thinnest EOM?

Superior oblique

What is the trochlea?

U-shaped piece of cartilage attached to orbital plate of frontal bone

What is the origin, insertion, and innervation of the inferior oblique?

Origin: maxillary bone

Insertion: inferior lateral globe (behind equator)

Innervation: inferior division of oculomotor nerve (CN III)

What is the only extraocular muscle that has an anterior origin?

Inferior oblique

What are the three main axes of eye movements?

X axis, Y axis, and Z axis

What are some rotations around the x-axis?

Elevation: eye moves up

Depression: eye moves down

What are some rotations around the y-axis?

Intorsion: eye rotates toward nose.

Extorsion: eye rotates away from nose.

What are some rotations around the z-axis?

Adduction: eye moves towards midline

Abduction: eye moves away from midline

Define convergence.

Eyes both move medially / toward midline

Define divergence.

Eyes both move laterally / away from midline

What is primary position?

Head straight and eyes focused at infinity.

What is secondary position?

Rotations around vertical (Z) or horizontal (X) axis

What is tertiary position?

Rotations around vertical (Z) and horizontal (X) axis

Where do medial and lateral rectus muscles lie?

• Parallel to y-axis

• Perpendicular to x- and z- axis

What action does the medial and lateral rectus do?

Rotation around the z-axis

What is the primary action of the medial rectus?

Adduction

What is the primary action of the lateral rectus?

Abduction

What angle does the superior and inferior rectus muscles insert?

Inserts at 23 degrees → Y axis

What is the primary action of the superior rectus?

Elevation (around X axis)

What are the secondary actions of the superior rectus muscle?

Adduction (Z axis) and Intorsion (Y axis)

What is the primary action of the inferior rectus muscle?

Depression

What are the secondary actions of the inferior rectus muscle?

Adduction (Z axis) and extorsion (Y axis)

What angle does the superior and inferior oblique muscles insert at?

~51 degree angle → Y axis

What angle does the superior oblique insert at?

55 degree angle (Y axis)

What is the primary action of the superior oblique muscle?

Intorsion (Y axis)

What are some secondary actions of the superior oblique muscle?

Depression (X axis) and Abduction (Z axis)

What angle does the inferior oblique insert?

51 degree angle (Y axis)

What primary action does the inferior oblique do?

Extorsion (Y axis)

What are the secondary actions of the inferior oblique muscles?

Elevation (X axis) and abduction (Z axis)

Why does only vertical movement occur when the superior and inferior rectus are in secondary position?

Because the eye abducts at 23 degrees and vertical rectus muscles parallel the y axis and perpendicular to the x axis.

(SR → only elevates, IR → only depresses)

Why does only vertical movement occur when the superior and inferior oblique are in secondary position?

Because the eye abducts at 23 degrees and vertical oblique muscles are perpendicular to y axis.

(SO → intorts, IO → extorts)

When the eye is adducted 51-55 degrees, what vertical movement can SO and IO do?

SO: only depresses

IO; only elevates

When the eye is adducted 51-55 degrees, what vertical movement can SR and IR do?

SR: intorts

IR: extorts

What are cardinal positions?

6 positions of gaze used to isolate field of action for each EOM

What mainly supplies the EOMs?

Ophthalmic artery

What EOMs are supplied by the lateral muscular artery?

• Superior rectus

• Lateral rectus

• Superior oblique

What EOMs are supplied by the medial muscular artery?

• Inferior rectus

• Medial rectus

• Inferior oblique