Extraocular Muscles

epimysium

connective tissue around a muscle

perimysium

connective tissue around bundles or fascicles

endomysium

connective tissue around muscle fiber

sarcolemma

muscle cell plasma membrane

sarcoplasm

muscle cell cytoplasm

muscle

surrounded by epimysium; group of 10-100 fascicles

muscle fascicles

surrounded by perimysium, include many muscle fiber cells

muscle fiber (myocyte)

long cylindrical multinuclear cell containing many cytoplasmic myofibrils

muscle fibrils (myofibrils)

cytoskeletons of actin, myosin, and titin, and other proteins that hold them together

thick & thin

what are the 2 types of myofibrils?

thick myofibrils

composed of hundreds of myosin subunits, each subunit is a long, slender filament with 2 globular heads attached by arms at one end

thin myofibrils

formed by protein actin arranged in a double-helical filament, with a molecular complex of troponin and tropomyosin lying within grooves of the double helix

sarcomere

region b/t 2 Z lines; primary structural and functional unit of muscle tissue

calcium, troponin, tropomyosin

_________ released from sarcoplasmic reticulum can bind to __________ which changes the configuration of _______________ to expose actin binding sites

ATP, myosin, sliding

______ binds to the hinge region of the myosin to release ADP and phosphate to extend ______ head to reach up to the myosin binding site of the thin filaments, causing relative ____________ of actin and myosin filaments

6 (4 rectus, 2 oblique)

how many EOMs are there?

fewer

EOMs are striated muscle with ______ fibers per muscle unit than skeletal muscle

higher

density of motor innervation to EOMs is much _______ than typical striated muscle

fast movement, higher resolution

what does increased motor innervation to EOMs allow for?

fastest, most fatigue resistant

EOMs are among the _____ and ____________ muscles in the body

closer

fibers that are ______ to the surface have smaller diameters

deeper

fibers that are ______ in the muscle have larger diameters

pulleys

connective tissue sleeves that encircle each EOM and can affect muscle positioning

check ligaments

dense connective tissue septa b/t the EOM sheaths and b/t the sheaths and the orbital bones; contribute to framework supporting globe w/in orbit; restricts EOM movements

medial & lateral check ligaments

what is responsible for anchoring the horizontal rectus muscles to the periorbita at the anterior orbital walls

medial

which check ligament (medial or lateral) is better developed?

posterior

check ligaments are _________ to the orbital septum

common tendinous ring

anterior to fissure and optic canal; origin for 4 rectus muscles

spiral of Tillaux

imaginary line connecting the rectus muscle insertions

medial, inferior, lateral, superior

list the order of rectus muscles in the spiral from closest to furthest from the limbus

medial rectus

• largest EOM

• origin: common ring tendon and optic nerve sheath

• insertion: anterior globe

• length: 3.7mm

• primary action: adduction

• secondary action: none

lateral rectus

• origin: common ring tendon and greater wing of sphenoid

• insertion: anterior globe

• length: 8.8mm

• primary action: abduction

• secondary action: none

superior rectus

• origin: common ring tendon & optic nerve sheath

• insertion: superior, anterior globe

• length: 5.8mm

• primary action: elevation

• secondary action: adduction, intorsion

• 23 angle degree with sagittal axis

inferior rectus

• origin: common ring of tendon

• insertion: inferior, anterior globe

• length: 5.5mm

• primary action: depression

• secondary action: adduction, extorsion

• 23 degree angle with sagittal axis

superior oblique

• origin: lesser wing of sphenoid

• insertion: superior, posterior, lateral globe

• primary action: intorsion

• secondary action: depression, abduction

• passes through trochlea

• longest & thinnest EOM

trochlea

U-shaped piece of cartilage attached to orbital plate of frontal bone; physiologic/effective origin of the superior oblique muscle

inferior oblique

• origin: medial maxillary bone

• insertion: inferior, posterior, lateral globe

• primary action: extorsion

• secondary action: elevation, abduction

CNIII

what innervates the medial rectus?

CNVI

what innervates the lateral rectus?

CNIII

what innervates the superior rectus?

CNIII

what innervates the inferior rectus?

CNIV

what innervates the superior oblique?

CNIII

what innervates the inferior oblique?

version

when both eyes move in the same direction; aka binocular conjugate movements

X axis

horizontal/transverse axis; nasal → temporal; elevation and depression occur along this axis

Y axis

sagittal; anterior → posterior; torsional movements occur along this axis

Z axis

vertical/coronal; superior → inferior; abduction and adduction occur along this axis

locus

center of rotation of the eye; 13.5mm behind the cornea

ductions

monocular eye movements

adduction

movement of the eye nasally

abduction

temporal movement of the eye

intorsion (incycloduction)

nasal rotation of the vertical meridian

extorsion (excycloduction)

temporal rotation of the vertical meridian

abducted 23 degrees

when the eye is ___________ the vertical rectus muscles are parallel to the y-axis, contraction of superior rectus only causes elevation

adducted 67 degrees

when the eye is ______________ the vertical rectus muscles are perpendicular to the y-axis and contraction of superior rectus cannot cause elevation

adducted 55 degrees

when the eye is ___________ the oblique muscles are parallel to the y axis and contraction of the superior oblique only causes depression

abducted 35 degrees

when the eye is ____________, the oblique muscles are perpendicular to the y-axis and the contraction of superior oblique cannot cause depression

Sherrington’s law of reciprocal innervation

contraction of a muscle is accompanied by a simultaneous and proportional relaxation of the antagonist

yoke muscles

primary muscles in each eye that accomplish a given version; each EOM has one in the opposite eye to accomplish versions in each gaze

Hering’s law of equal innervation

states that innervation to the muscles of the 2 eyes is equal and simultaneous thus movements of 2 eyes are normally symmetric

vergence

when both eyes move in the opposite direction; aka binocular dijugate movements

convergence

both eyes toward the nasal region

divergence

both eyes move temporally

right lateral rectus, left medial rectus

what 2 muscles move the eyes in dextroversion?

left lateral rectus, right medial rectus

what 2 muscles move the eyes in levoversion?

right superior rectus, left inferior oblique

what 2 muscles move the eyes in dextroelevation?

left superior rectus, right inferior oblique

what 2 muscles move the eyes in levoelevation?

right inferior rectus, left superior oblique

what 2 muscles move the eyes in dextrodepression?

left inferior rectus, right superior oblique

what 2 muscles move the eyes in levodepression?

strabismus

congenital or acquired condition of uncoordinated movement b/t the 2 eyes and visual axes are not straight when the patient is asked to look in the primary position

suppresion

seen in congenital strabismus, adaptive response to prevent diplopia and must be overcome to retrain the muscles to achieve binocular vision

hyperthyroidism

condition that results in enlargement of the EOMs due to chronic inflammatory infiltration of the muscles with glycoprotein and mucopolysaccharide deposition and proptosis; restricts ocular motility

forced duction test

can be performed if fibrotic muscle is suspected; put pt under topical anesthesia and doctor grasps conjunctiva near the limbus, attempting to move eye in opposite direction from suspected restriction

fibrotic muscle

if the eye cannot be moved in a forced duction test, then what is the cause?

innervation problem

if the eye can be moved in a forced duction test, then what is the cause?

myasthenia gravis

chronic autoimmune neuromuscular disease; antibodies are formed that either block or destroy ACh receptors; muscle weakness and fatigue worsens through the day; ocular sx are limited to the eye and lid muscles resulting in diplopia and ptosis

Brown syndrome

congenital or acquired condition caused by a malfunction of the inferior oblique or more likely a limitation of the superior oblique muscle, causing the eye to have difficulty moving up when the inferior oblique contracts, particularly during adduction

Chronic progressive external ophthalmoplegia

bilateral, symmetric, painless, pupil-spared ptosis and ophthalmoplegia; oxidative phosphorylation (mitochondria) dysfunction