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Extraocular Muscles
(EOM) & Eye movement
Muscle Fibers - cellular & molecular nuts & bolts
Motor Units - how is tension developed?
Eye movements - definitions of stimulus & action
Dr. Busatini has an entire course on this subject, so we can
skip pages 143 -152, and 176-185.
VS 112 Ocular Anatomy
I. Just what sort of movements
do your eyes make?

Those which are you aware of, those that are
voluntary (or at least the initiation of them is
voluntary).
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Reflex eye movements - automatic adjustment
of the eye position to stabilize an image on the
eye.
Eye movements
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Smooth tracking (pursuit)
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Saccades – quick jumps, saccadic eye
movements (< 50 msec) (up to 20°) voluntary
initiation
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Combined head and eye tracking
Mini saccades (see fixation).
Fixation – holding a steady gaze (which we
never actually do – see figure 4.1).
Reflex eye movements.
The stimulus can be head/body movement
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i)
VOR vestibular ocular reflexes – if you
move your head the eyes can maintain fixation
upon the target.
ii)
Optokinetic reflex : stimulus = rapid
motion of the world, such as motion of a stream, or
motion of world outside of the side car window.

Nystagmus (alternating slow & quick phases of
movement)
Reflex eye movements.
The stimulus can be movement, motion of an
object of interest.
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iii)
Smooth pursuit - one object is targeted
and remains clear on the retina the rest of the
visual world is in motion – the standard stimulus
for the optokinetic reflex,(only seen in foveate
animals)
iv)
Fixation - holding a steady gaze on a
target of interest. NOT just a special form of
pursuit.
Fixation – holding a steady gaze
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The eyes are in constant motion
Slow drift
Minisaccades
Tremor (high frequency jitter in position)
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aka micronystagmus.
In fact, if you could perfectly stabilize an image on
the retina it would fade and disappear. Some retinal
motion is necessary, that is some movement of the
image across the retina must be present or the image
fades.
Eye Movements:

See figure 4.1 over head
Motion of an Eye
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To describe eye motions we need a set of
defined axes (Fick’s Axes - draw on board)
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X axis : nasal -> temporal
Y axis: anterior -> posterior
Z axis: superior -> inferior
These axes intersect at the center of rotation - a
fixed point, defined as 13.5 mm behind cornea.
Ductions (single eye movements)
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Rotation about the Z axis (Z axis runs vertically
superior to inferior)
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Medial Rotation - adduction toward midline
Lateral Rotation - abduction away from midline
Rotation about the X axis (X axis runs
horizontally, from nasal to temporal)
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Upward, elevation (supraduction)
Downward, depression (infraduction)
Torsion - cyclorotations
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Rotation about the Y axis (Y axis runs
horizontally, from anterior to posterior)
These are described with respect to a point at
12 o‘clock on the superior limbus
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Intorsion (incyclorotation) rotation nasally

Extorsion (excyclorotation) rotation of the 12
o’clock position temporally.
Counteracting head tilt (up to 7-9°)
Version & Vergences
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Some eye movements are paired, that is both
eyes do the same thing. . . . Versions
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Sometimes eyes move in the opposite
directions simultaneously. . . Vergences
Vergences
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Disjunctive eye movements (opposite left- right
movments). Non-yolked motion
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Convergence (simultaneous movement nasally)
Divergence (simultaneous temporal movement)
Encyclovergence (intorsion)
Excyclovergence (extorsion)
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Versions (conjugate eye movement)
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Dextroversion - rightward gaze (demo)
Levoversion - leftward gaze
Supraversion - elevation
Infraversion - depression
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Also up and right, up and left
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Down and right, down and left
ALL BEHAVIOR IS THAT OF YOLKED EYES
Extraocular Muscles
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4 rectus muscles - origin is in the common
tendous ring (annulus of Zinn)
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Oval ring of connective tissue
Continuous with periorbita
Anterior to optic foramen
Medial and lateral rectus attached to both the
upper and lower tendon limbs
The muscles traveling from the this tendon
ring to the insertions create muscle cone.
The Muscle Cone
Spiral of Tillaux
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The rectus muscle pass through tenon’s
capsule and insert into the sclera.
The muscles insert at different distances from
the cornea.
The insertion pattern is a spiral with the medial
rectus closest to the cornea (5.5 mm) and the
superior rectus the furthest away from the
cornea (7.4 mm).
Common Tendon of Zinn
Medial Rectus
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Originates on both the upper and lower limb of
the common tendous ring and the optic nerve
sheath.
Inserts along a vertical line 5.5 mm from the
cornea. The horizontal plane of eye bisects the
insertion.
Fascial expansion from muscle sheath forms
the medial check ligament and attach to medial
wall of orbit.
Medial Rectus cont.
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Innervation is via cranial nerve III, the
oculomotor nerve, and the specific branch runs
along the inside of the muscle cone, on the
lateral surface.
The superior oblique, ophthalmic artery and
nasociliary nerve all lie above the medial
rectus.
Spiral of Tillaux
7.4 mm
5.5 mm
6.9 mm
6.7 mm
Lateral rectus
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Originates on both the upper and lower limb of
the common tendous ring. . .AND a process of
the greater wing of the sphenoid bone.
Inserts parallel to medial rectus 6.9 mm from
the cornea. (Tendon 9.2 mm wide, 8.8 long).
Fascial expansion from muscle sheath forms
the lateral check ligament and attach to lateral
wall of orbit at Whitnalls tubercle.
Lateral Rectus cont.
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Innervated by the abducens nerve, Cranial n VI
which enters the muscle on the medial
surface.
The lacrimal artery and nerve run along the
superior border.
The abducens n., ophthalmic artery and ciliary
ganglion lie medial to the lateral rectus and
between it and the optic nerve.
Superior Rectus
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Originate on superior limb of the tendonous
ring, and optic nerve sheath.
Muscle passes forward underneath the levator,
but the two sheaths are connected resulting in
coordinated movements.
Insertion 7.4 mm from limbus, and obliquely.
The angle from the origin to the insertion is
23° beyond the sagital axis. (see overhead)
Superior Rectus cont.
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Frontal nerve runs above the s. rectus & levat.
The nasociliary nerve and ophthalmic artery
run below.
The tendon for insertion of the superior
oblique muscle runs below the anterior part of
the superior rectus.
Innervationis via superior division of CN III,
from the inferior surface; additional branches
make their way to the levator.
Action of Superior Rectus
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Primary action is elevation . . But since the
insertion on the globe is lateral as well as
superior, contraction will produce rotation
about the vertical axis toward midline
Thus secondary action is adduction
Finally, because the insertion is oblique,
contraction produces torsion nasally Intorsion.
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(overhead figure 10-13A)
Superior view of Sup. Rectus
23°
Because the muscle runs at an angle to the Fick’s axes, contraction
is not confined to one axis
Inferior rectus
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Originates on lower limb of common
tendonous ring.
Inserts 6.7 mm from limbus, insertion is an arc
It is parallel to superior rectus, making a 23°
angle beyond the sagittal axis.
Innervated by inferior division of CN III which
runs above it (within the muscle cone).
Below is the floor of the orbit and inf. oblique
Inferior Rectus cont.
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Fascial attachments below attached to inferior
lid coordinate depression and lid opening.
Fascia below Inf. Rectus and Inf. Oblique
contribute to the suspensory ligament of
lockwood.
Primary Action downward gaze depression
2° Adduction, as is the case for sup. Rectus
Also extorsion due to oblique arc of insertion.
Vectors of Sup & Inf Recti
Superior Oblique
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Anatomical origin is on the lesser wing of the
sphenoid bone. The physiological origin is the
trochlea, a cartilagenous “U” on the superior
medial wall of the orbit.
Longest thinnest EOM, the muscle ends before
the trochlea, tendon is 2.5 cm, smooth
movement through trochlea.
Innervation by CN IV, the trochlear nerve
posterior in the orbit.
Action of Sup. Oblique
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Primary action is intorsion _ rotation of 12
o’clock position toward midline.
Because the insertion of the oblique muscle is
in the lateral, posterior quadrant the secondary
actions are
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Rotating the back half of the globe from lateral to
medial (the anterior pole will move away)
ABDUCTION
Also depression (posterior superior quadrant of the
globe being pulled upward).
Inferior Oblique
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Originates on the maxillary bone inferior to the
nasolacrimal fossa. The ONLY EOM
originating in the anterior orbit.
Inserts on the posterior lateral aspect of globe
mostly inferior, below the ant.-post. horizontal
plane.
Innervation from inferior division of CN III
inserts on the upper surface (within muscle
cone.)
Origins/Insertions of Oblique muscles
Action of Inf. Oblique
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Primary is extorsion
2° is due to posterior, lateral, inferior insertion
being pulled around, underneath globe and
toward the anterior inferior insertion medially.
Rotation about the Z axis will be nasal to
temporal (abduction).
Rotation about the X axis will be elevation
(see overhead figure 10.14)
And from here it’s complicated
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There is a balance of tension in the pairs of muscle to
start with. . . .
Actions of the muscle can and do depend on the
starting position.
For example elevation of the eye in the straightahead position and lateral position is accomplished by
the sup. rectus.
 But when the eye is medial rotated elevation is
accomplished by the inferior oblique muscle
action. (see fig 10-17 over head).
Defining Muscle groups
What is a muscle, how does it
work?
Thick & thin filaments
Elements within, between the Z-lines make up a contractile unit
Ratchet Model
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Myosin head binds to actin
filament.
The ratchet motion moves the
two filament about 12 nm
with respect to each other.
It takes only 5 ms
Because of the large number
of z-line segments or
contractile units along a fiber,
a fast motion is attained.
Thick and thin filament
specializations with EOM
Each motor fiber is innervated by only one nerve
Extraocular muscles are special
A motor nerve can and does contact more than one fiber usually 100’s
The motor units are small, with only from 5 to
18 muscle fibers contact by each motor nerve
EOM’s are special
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THICK FIBERS
Striated muscle
Singly innervated
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1 nerve, 1 branch
Motor end plate
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Terminaison en plaque
Includes both fast and
slow twitch fibers
All or none contraction
THIN FIBERS
Striated musle
Multiply innervated
En grappe end plate
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Many branches 1 nerve
Terminaisons en grappe
Thought to be slow
sustained (tonic)
Graded contractions
Thick and thin filament
specializations with EOM
Each motor fiber is innervated by only one nerve
Fiber types segregate in the muscle
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A single nerve impulse
Generates a muscle AP and
contraction - tension.
Multiple nerve AP’s
Tensions sum over time
Repetitive firing results in a
sustained contraction.
Tetanus (unresolved
individual twitches).
3 Basic types of motor units
Slow
Fast-fatigue resistant
Fast- fatigable
Cat gastroc muscle
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ATPase activity at neurtral pH
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ATPase activity at acidic pH
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NADH dehydrogenase stain
Recruitment within the motor
nerve
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Groups of cell bodies innervating the same
muscle make up a MOTOR NUCLEUS
. . . .and give rise to a motor nerve.
Nerves within a motor nerve are activated in a
characteristic sequence - the size principle.
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Smaller fibers, fire first, and larger later
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There is a characteristic pattern of recruitment, tension
is added and removed in a repeated pattern.
Muscle Spindle & Golgi Tendon
Organs
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Specialized sensory organs
within the muscle provide feed
back to the brain.
How much tension is in the
muscle?
Is there any stretch imposed?
What is muscle length?
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EOM’s do not have the typical
stretch reflex