Extraocular Muscle Outline
VOCABULARY:
1. Perimysium- connective tissue that surrounds parallel bundles of muscle fibers
2. Epimysium- connective tissue that surrounds a collection of muscle fiber bundles. It
is continuous with Tenon’s capsule. (Remember it covers muscles and globe.
3. Sarcolemma- membrane of the muscle fiber = cell plasma membrane
4. T-Tublule- portion of external cellular membrane that form tubes running
throughout the muscle fiber. (This helps with excitati0n & Calcium fluxes)
5. Sarcoplasmic Reticulum- membrane structures within muscle fibers that is a
reservoir of calcium that is used for initiation of muscle contraction.
6. Actin- the thin filament in muscle fiber
7. Myosin- the thick filament in muscle fibers
8. Motor Unit- the motor nerve axon and all the muscle fibers that it innervates
9. Muscle Fiber- units that make up the muscle. They are thin, elongated and
multinucleated.
TYPES OF EYE MOVEMENTS:
1. Voluntary- these are movements we are aware of or initiate.
a. Saccades
i. Quick, large movements used for looking around and scanning the
environment for particular images
ii. Magnitude <1º to 20º (average is 15º, if larger head movement
necessary)
iii. Occur within 50ms
iv. Speed, direction, and size occur equal and bilaterally
b. Smooth Pursuit
i. The eyes track small objects that is moving relative to a stationary
background
ii. It works similarly to the slow part of the optokinetic reflex, the
world is in motion, but the tracking reducing the retinal image
motion to zero
iii. This movement only occurs in organisms with a fovea or a
functional equivalent.
iv. Speed, direction, and size occur equal and bilaterally
2. Reflex- these are automatic movements that adjust the position of the eye to
stabilize the image on the eye. The stimulus can be head/body movement or
movement of the object of interest.
a. Vestibulo-ocular reflexes (VOR)
i. Eye movement generated by receptors of the inner ear that sense
changes in acceleration due to gravity
ii. Ex- when the head tilts the eyes can maintain their orientation due
to this reflex
iii. Speed, direction, and size occur equal and bilaterally
b. Optokinetic reflex
i. The stimulus is rapid motion of the world or the images on the
retina as the head rotates with the body remaining stationary
ii. The eyes track the slow rotation of images, but are interrupted by
quick movements in the opposite direction.
iii. Nystagmus is the term referring to the alteration between the
quick and slow movements of this reflex.
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UAB School of Optometry
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iv. Speed, direction, and size occur equal and bilaterally
c. Fixation
i. This is holding an image stationary on the retina
ii. The eyes seem motionless during this time, but they are actually
moving
iii. High-Frequency Tremor (Micronystagmus) is a constant, jittery
movement of the eye during fixation
iv. Minisaccades can occur also
v. There must be some motion on the retina or the image will fade
MOTION OF THE EYE:
1. Fick’s Axis
a. X axis- nasal to temporal
b. Y axis- anterior to posterior
c. Z axis- superior to inferior
d. Axes intersect at the center of rotation, a point 13.5 mm behind cornea
2. Versions
a. Movements of both eyes in the same direction
b. For versions to occur the eyes must be linked/yoked
c. Types of versions
i. Dextroversion- right movement
ii. Levoversion- left movement
iii. Supraversion- elevation
iv. Infraversion- depression
3. Duction
a. This is single eye movement
b. Rotates on Z axis- medially and laterally
c. Rotates on X axis – upward and downward
4. Torsion
a. Rotates on Y axis
b. Point at 12 o’clock on the superior limbus
i. Intorsion- rotation nasally
ii. Extorsion- rotation temporally
5. Vergence
a. Movements of both eyes in opposite directions
b. Motions are not yoked
c. Types of Vergences
i. Convergence- both eyes nasally
ii. Divergence- both eyes temporally
iii. Encyclovergence- intorsion
iv. Excyclovergence- extorsion
EXTRAOCULAR MUSCLES:
1. There are 6 extraocular muscles and are arranged in antagonistic pairs
2. Annulus of Zinn (common tendous ring)
a. A tendon in the back of the orbit from which all rectus muscles originate
b. Forms an oval ring of tissue
c. Continuous with periorbita
d. Anterior to optic foramen
e. Muscles attached to this ring create the Muscular Cone
3. Spiral of Tillaux
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UAB School of Optometry
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a. Refers to the insertion pattern of rectus muscles
b. Muscles pass tenon’s capsule and insert to the sclera
c. The spiral occurs from the distances the muscles insert from the cornea
i. Medial rectus- 5.5 mm
ii. Inferior rectus- 6.7 mm
iii. Lateral rectus- 6.9 mm
iv. Superior rectus- 7.3 mm
4. Muscles:
Muscle
Origin
Insertion
1ºAction
2ºAction
Innervation
Medial Rectus
Upper/Lower Limb
of Common Tendon
Ring & Optic Nerve
Sheath
Upper/Lower Limb
of Common Tendon
Ring & Greater
Wing of Sphenoid
Superior Limb of
Common Tendon
Ring & Optic Nerve
Sheath
Lower Limb of
Common Tendon
Ring
Lesser Wing of
Sphenoid
(Physiological
is
trochlea)
Maxillary
Bone
below Nasolacrimal
Fossa
Sclera, 5.5mm from
the cornea
Adduction
(toward the
midline)
none
Cranial Nerve
III
Sclera, 6.9mm from
the cornea
Abduction
(toward the
temples)
none
Cranial Nerve
VI
Sclera, 7.4mm from
the
cornea,
obliquely (23º)
Elevation
Adduction
&
Torsion
Cranial Nerve
III
Sclera, 6.7mm from
the
cornea,
obliquely (23º)
Just before the
trochlea,
lateral,
posterior quadrant
Depression
Adduction
&
Extorsion
Abduction
&
Depression
Cranial Nerve
III
Lateral, Posterior
side of Globe
Extorsion
Abduction
&
Elevation
Cranial Nerve
III
Lateral Rectus
Superior Rectus
Inferior Rectus
Superior Oblique
Inferior Oblique
Intorsion
Cranial Nerve
IV
5. Facts about Muscles:
a. The superior oblique is the longest, thinnest extraocular muscle
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UAB School of Optometry
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b. The inferior oblique is the only extraocular muscle that originates in the
anterior
c. Fascial expansion from the medial rectus and the lateral rectus for the
medial and lateral check ligaments, respectively.
d. Extraocular muscles are never totally relaxed; instead they exert tonic
contraction that always exerts some force on the eye.
e. The force of one set of muscles on the eye is opposed by the antagonist
group to keep the eye in a stationary position.
6. Force in Muscles
a. Occurs actively from the contraction of the muscle
b. Occurs passively due to the inherent elasticity of the muscle when it
stretches
i. This force is primarily responsible for maintaining eye position
7. Muscle Length
a. The length of the muscles shortens when it contracts (active force)
b. The muscle gets longer as it stretches (passive force)
8. Muscle Spindle & Golgi Tendon Organs
a. Muscle Spindle
i. is a sensory structure that reports muscle length
ii. Enclosed in capsule with in muscle
iii. Receives sensory nerve information in regards to stretching of
muscle
b. Golgi Tendon Organ
i. is a sensory structure that report muscle tension
ii. enclosed in capsule of tendons with coiled collagen strands
iii. under tension the collagen coils stretch and place mechanical
pressure on the nerve fiber branches
CONTRACTILE PROPERTIES:
1. Arrangement of fibers
c. The muscle fibers are striated and arranged parallel
d. The myofibril runs from z-line to z-line and contains actin and myosin
e. The actin and myosin create the striated appearance
2. Method of contraction
a. The sliding filament theory is generally accepted as the method for
contraction.
b. The ratchet model best explains it
i. The head of myosin molecules bends
ii. The myosin dissociates from the adjacent actin filament and
straightens
iii. The myosin attached to an actin filament in another location
c. The filaments move about 12 nm in about 5 ms
d. The ratchet method is a fast motion
3. Types of Fibers
a. Red Fast Twitch
i. High mitochondrial density
ii. High oxidative metabolism
iii. Contraction is very rapid and all or nothing
b. White Fast Twich
i. Lower mitochondrial density
ii. Lower oxidative metabolism
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UAB School of Optometry
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iii. Contraction is slower and all or thing
c. Red Slow Twitch
i. High mitochondrial density
ii. High oxidative metabolism
iii. Contraction is slower than red fast twitch and all or nothing
4. EOM differ from skeletal muscles because:
a. The extraocular muscles have fibers not found in skeletal muscles
b. Diameter is roughly 20m vs. 100m
c. Single innervated fibers
i. in both skeletal and extraocular muscles
ii. the motor end plate fits into a shallow depression on the muscle
fiber
iii. the neurotransmitter is released from the axon to the motor end
plate to activate the muscle
iv. there is one end plate per muscle fiber
v. have larger diameter
vi. called thick fibers
d. Multiply innervated fibers
i. not in skeletal muscles
ii. contacted in several placed by one nerve
iii. the nerve endings are called en gappe endings and are clusters of
round swellings at the end of axons
iv. have smaller diameter
v. called thin fibers
5. Properties of thin and thick fibers
a. Thick Fibers
i. Striated
ii. Single innervation- one nerve, no branching
iii. Motor end plate present (en plague transmission)
iv. Fast and Slow twitch fibers
v. All or Nothing Contration
b. Thin Fibers
i. Striated
ii. Multiple innervation- one nerve, with branching
iii. En Gappe End Plate (en gappe transmission)
iv. Slow twitch fibers (tonic)
v. Graded contraction- strength of contraction can vary
6. Segregation of Fibers
a. Rectus Muscles- The thin fibers are closer to the orbital portion of the eye
and the thick portion is closer to the globe
b. Oblique Muscles- the thick fibers are fully surrounded by the thin fibers
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UAB School of Optometry
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7. Motor Units
a. The motor units in extraocular muscles are smaller then in other muscles
b. The motor nerve contacts 5-18 muscle fibers
c. Chart of fibers
Muscles
# of muscle fibers
Size of motor unit
Medial Rectus
29000
7.7
Lateral Rectus
35000
14-17.5
Superior Rectus
20000
5.3
Inferior Rectus
26000
6.9
Superior Oblique
15000
12-13.6
Inferior Oblique
18000
4.8
8. Types of Contraction
a. Tension is generated by a single nerve impulse causing an action potential
b. Tension sums overtime with multiple action potentials occur
c. Repetitive firing creates sustained contraction
d. Tetnus occurs from unresolved individual twitches
e. Recruitment occurs in the motor nerve in a sequential pattern; the
smaller fibers fire first, followed by the larger fibers
9. Aceytlcholine and excitation
a. Aceytlcholine is the neurotransmitter used in all extraocular muscles to
deliver axon impulses. It binds to the receptors in the sarcolemma
b. The aceytlcholine serves as the ligand to regulate the sodium flow into the
sarcolemma
c. Depolarization occurs and propagates an impulse until termination
occurs
BLOOD FLOW AND DEVELOPEMT
1. Five muscular arteries are supplied by the ophthalmic art. 4 branch off of the
ophth. art. Directly, one (to L.R.) is a branch off of the lacrimal artery.
2. ALL EOM ARE DERIVED FROM MESENCHYME, appear before bone and nerves.
MUSCULAR PROBLEMS:
1. Myasthenia Gravis
a. Autoimmune disease where the immune system makes antibodies that
destroys aceytlcholine receptors
b. Symptoms
i. Ptosis
ii. Strabismus
iii. Diplopia
iv. Abnormal eye movement
2. Strabismus
a. The deviation of an eye in a direction other then forward
i. Correctable with surgery or botox
3. Brown’s Syndrome
a. Fascial sheath around superior oblique tendon is too short and eye does
no depress properly during adduction
4. Duane’s Syndrome
a. One or more muscles is partially or completely formed from connective
tissue instead of muscle tissue.
b. Often occurs in the lateral rectus and abduction does not occur properly
VS 112 Ocular Anatomy
UAB School of Optometry
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