Extraocular muscles: anatomy, physiology and ocular motility

1. Anatomy of EOMs & Ocular Motility
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By: Dr Henok Samuel (R1)
Moderator: Prof Abebe Bejiga (professor of Ophthalmology, pediatrics and
strabismus surgeon)

2. Contents
• Introduction
• Recap on Embryology & microscopic anatomy
• Gross Anatomy of EOM
• Origin, Nerve and blood supply
• Action of EOM
• Basic Kinematics, Mechanics
• Ocular Movements
• Fundamental Laws governing ocular motility
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3. Why do we move our eyes?
A. To acquire objects for central viewing
Saccadic eye movements
B. To maintain objects in foveal view
Pursuit eye movements
C. To stabilize the world on the retina
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4. Extraocular Muscles
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 EOMS are:
 Striated voluntary muscles
 Among muscles with fastest but also
most sustained contraction
 Get high blood flow exceeded only by
myocardium
 Have got high innervation ratio

5. Extraocular Muscles
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 Play a vital role in
 Stereopsis
 Conjugate eye movements
 Maintenance of primary gaze position
 Motor fusion – maintaining corresponding visual elements
within the binocular field on corresponding retinal loci.
 Following of moving objects (smooth pursuit)
 Accomplish rapid changes in fixation (saccades).

6. Embryology
 The EOMs are derived from three primordial condensations - paraxial and
prechordal mesoderm.
 A pair of premandibular mesodermal condensation – 26th day
 Those muscles innervated by CNIII
 MR, SR, IR & IO .
 Two maxilomandibular condensations – 27th
 Superior oblique .
 Lateral rectus .
 Associated periorbital CT and smooth muscle are derived from the neural crest.
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7. Embryology
 Their development begins at 3–4 weeks gestation.
 All of the extraocular muscles and their surrounding tissues are present and in
their final anatomic positions by 6 months gestation.
 The tendon insertions initially extend from the limbus to the equator.
 Continue to change until 18 months to 2 years after birth - 2–3.0 mm narrower in
infants
 At about 1 month, the nerves to the extraocular muscles reach their respective
destinations in the sequence oculomotor, abducens, and trochlear.
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8. Microscopic Anatomy of EOMs
 Different from other skeletal muscles by their
 Diameter of its fibers is small
 Contain both slow and fast fibers
 denser connective tissue & blood supply
 Contain an enormous amount of fibroelastic tissue
 connective tissue sheaths are more delicate & richer in elastic fibers
 Have a large nerve to muscle fiber ratio; about 1:5 to 1:10 compared to 1:100
or more in other skeletal muscles
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9. Microscopic Anatomy of EOMs
 Components of a muscle fiber
 Sarcolemma: the plasma cell membrane
surrounding each muscle fiber.
 Transverse tubules (T tubules): a series of
invaginations of the sarcolemma into the cell.
 Sarcoplasm: is the cell cytoplasm & contains
normal cellular structures and special muscle
fibers, THE MYOFIBRILS.
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10. Microscopic Anatomy of EOMs
 Components of a MYOFIBRIL; two
types
1. Thick myofilaments
2. Thin myofilaments

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11. Microscopic Anatomy of EOMs
Twitch single innervated fibers ( thick ):-
 contain little mitochondria ( i.e. anaerobic metabolism ) → rapid
response to the stimulus with rapid contraction of high amplitude &
short duration.
 responsible for saccadic eye movement & help fixation & pursuit
movement.
Tonic multiple innervated fibers ( thin ):-
 contain numerous mitochondria ( i.e. aerobic metabolism ) → slow
response to the stimulus with slow contraction of low amplitude &
long duration.
 responsible for gaze in all positions including 1ry position.
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12. Bony orbit
•
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13. Extraocular Muscles (EOMs)
 Vertical rectus
- Superior rectus
- Inferior rectus
 Horizontal rectus
- Medial rectus
- Lateral rectus
 Obliques
- Superior oblique
- Inferior oblique
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14. Origin of EOMs
 The four recti muscles have their origin on the
common tendinous ring (annulus of Zinn).
 The area enclosed by the tendinous ring is called
the oculomotor foramen
 Superior Oblique originates from the lesser wing
of sphenoid
 Inferior Oblique originates from the maxillary
bone
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15. Several blood vessels & nerves pass through
oculomotor foramen
1. The optic nerve
2. Ophthalmic artery
3. Abducens nerve
4. Oculomotor nerve Upper and lower
divisions
5. Nasociliary branch of the ophthalmic 15

16. Insertion
 Scleral insertions are by tendons
whose fibres are parallel to the
long axis of the muscle.
 The tendon fibres enter the
superficial sclera and quickly
merge into it.
 The EOMs penetrate it ≈10 mm
posterior to their insertions
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MUSCLE LENGTH OF
TENDON IN mm
MEDIAL
RECTUS
3.7
LATERAL
RECTUS
8.8
SUPERIOR
RECTUS
5.8
INFERIOR
RECTUS
5.5
SUPERIOR
OBLIQUE
25
INFERIOR
OBLIQUE
1

17. Arterial supply
 Three main sources
1. Muscular branches of ophthalmic artery
Upper (lateral) branch
Lower (medial) branch
1. Infraorbital artery from ECA
2. Lacrimal artery
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18. Arterial supply
 The muscular branches give rise to the anterior ciliary
arteries accompanying the rectus muscles; each
rectus muscle has 1–4 anterior ciliary arteries.
 Anterior segment circulation is most dependent on
arteries from vertical rectus muscles and least
dependent on arteries from the lateral rectus muscle
 Surgical manipulation of the rectus muscles
permanently disrupts the anterior ciliary arteries.
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19. Venous drainage
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 Follow the arterial path
 EOMs drain into two
veins:
1) Superior ophthalmic
vein and
2) Inferior ophthalmic
vein

20. Nerve
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 Without neural activity, the
visual axes are usually mildly to
moderately divergent.
 The major tonic input to ocular
motility is supplied CNs III, IV,
and VI

21. Superior rectus
• Origin
• Insertion
• Nerve supply
• Blood supply
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22. Inferior rectus
• Origin
• Insertion
• Nerve supply
• Blood supply
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23. Medial rectus
• Origin
• Insertion
• Nerve supply
• Blood supply
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24. Lateral rectus
• Origin
• Insertion
• Nerve supply
• Blood supply
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25. Spiral of tillaux
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 Imaginary line joining the insertions of the 4 recti and
is an important anatomical landmark when
performing surgery.

26. Superior Oblique
• Origin
• Insertion
• Nerve supply
• Blood supply
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27. Inferior Oblique
• Origin
• Insertion
• Nerve supply
• Blood supply
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28. Introduction
• Obliques are inserted behind equator & form an angle of 51°
with visual axis.
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29. Orbital Connective Tissue
 Tenon's capsule (fascia bulbi) a layer of delicate
connective tissue that completely envelops the globe -
EOMs penetrate it ≈10 mm posterior to their insertions
 The fascial sheath of the SR muscle closely adheres to the
sheath of LPS of upper lid in front of the equator which
accounts for the cooperation in elevation of the eye.
 The sheath of the IO muscle fuses with the sheath of the
IR and forms the suspensory ligament of lockwood.
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30. Pulleys of the EOMs
 Are about 2mm in length & located near the
equator
 Function as a mechanical origins of the EOMs
 Reduce sideslip of the extraocular muscles
during globe rotation and help to determine
the effective direction of pull
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31. Pulleys of the EOMs
 They are Composed of
 Elastic fibers
 Collagenous
 The smooth muscles
 The most prominent smooth muscle is the
inframedial orbital muscle extending between
the MR and IR pulleys.
 Anteriorly, these sleeves thin to form slings
known as the intermuscular septum.
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32. Muscle Capsule
 Each rectus muscle has a surrounding fascial capsule
 Extends with the muscle from its origin to its insertion
 Its smooth avascular surface allows the muscles to slide
smoothly over the globe.
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33. Action of extraocular muscles
Types of Eye Movements
1. Ductions refer to monocular movements of each eye.
2. Versions refer to binocular conjugate movements of both eyes.
3. Vergences refer to binocular disjunctive movements.
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34. Uniocular movements
 Ductions – only one eye is open, the other
covered/closed tested by asking the pt. to
follow a target in each direction of gaze.
 Types of ductions:-
1. Adduction
2. Abduction
3. Supraduction
4. Infraduction
5. Incycloduction
6. excycloduction
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35.  Three Axes of Eye Rotations
 Fick’s Axis
 Listing Plane
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Uniocular movements

36. Binocular movements
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 Versions:-both eyes open, attempting to fixate a target
& moving in same direction.
 Binocular, simultaneous, conjugate movements in
same direction.
 Abduction of one eye accompanied by adduction of
other eye is called conjugate movements.

37. Introduction
 Types of versions:-
Dextroversion & laevo version
Supraversion & Infraversion
Dextro elevation & dextro depression
Laevo elevation & laevo depression
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38. Introduction
• Torsional movements/righting reflexes:-
When you tilt head to maintain upright image.
• Vergences:-
binocular,simultaneous,disjugate/disjunctive movements (opp
direction)
Convergence– simultaneous adduction
Divergence– outward movement from convergent position
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39. Actions of EOMs
• The anterior pole of the globe is the reference point used in the description of any eye
movement
• Eye movements are described based on the movement of the muscle insertion towards its
origin.
• The primary action of a muscle is its major effect when the eye is in the primary position.
• Subsidiary/secondary actions are the additional effects & depend on the position of the eye
• The point at which the center of the muscle or its tendon first touches the globe is called the
tangential point and indicates the direction of the pull.
• Muscles exert force in proportion to their cross-sectional area and length.
• For normal amplitude of rotation (45-50 degree ) 10mm change in muscle length is required
in each direction 39

40. Positions of Gaze
 Primary position: defined as position of the eye with
 Both head & body erect
 Both eyes are looking straight ahead
 Object of regard is at infinity
 The eye located at the intersection of the sagittal plane of the
head and the horizontal plane passing through the centers of
rotation of bot eyes
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41. Positions of Gaze
 Secondary position are rotations around either the vertical axis
or the horizontal axis.
 ADDUCTION
 ABDUCTION
 ELEVATION
 DEPRESSION
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42. Positions of Gaze
 Teritiary position: are rotations around both the vertical &
horizontal axis
 DEXTROELEVATION
 DEXTRODEPRESSION
 LEVOELEVATION
 LEVODEPRESSION
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43. Positions of Gaze
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44. Medial Rectus
 Lies parallel to the sagittal axis & perpendicular to
the vertical axis
 As a result has only one action, which is rotation
around the vertical axis medially => ADDUCTION
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45. Lateral Rectus
 Lies parallel to the sagittal axis & perpendicular to
the vertical axis
 Contraction causes rotation in a temporal direction
ABDUCTION
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46. Superior Rectus
 Run in line with the orbital axis and are
inserted in front of the equator.
 Forms an angle of 23° with the visual axis.
 With the insertion above the origin and on
the anterior globe, movement around the
horizontal axis causes ELEVATION –
PRIMARY ACTION.
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47.  The muscle insertion is lateral to the origin,
so movement around the vertical axis causes
ADDUCTION.
 Its oblique insertion with the nasal side closer
to the limbus than the temporal side on the
superior surface of the globe causes
INTORSION on contraction.
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Superior Rectus

48. Inferior Rectus
 Its insertion is below its origin and on the
anterior globe, so movement around the
horizontal axis causes DEPRESSION
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49. Inferior Rectus
 The muscle insertion is lateral to the
origin, so movement around the vertical
axis causes ADDUCTION.
 Its oblique insertion on the inferior
surface of the globe causes EXTORSION
on contraction.
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50. Superior Oblique
 The oblique insertion on the
posterosuperior lateral aspect of the
globe causes rotation of the eye
around the sagittal axis causing
INTORSION.
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51. Superior Oblique
 The insertion is posterior and
inferior to the physiologic origin;
contraction of the muscle pulls
the back of the eye up, and the
anterior pole moves down-
DEPRESSION.

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52. Superior Oblique
 Because the insertion is lateral to the
trochlea, contraction pulls the back of
the globe medially, thus moving the
anterior pole laterally – ABDUCTION.

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53. Inferior Oblique
 Because the muscle wraps around the lower portion of the
globe and the insertion is superior and lateral to the origin
contraction causes EXTORSION.

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54. Inferior Oblique
 Because the insertion is on the
posterior eye and above the
origin, contraction pulls the back
of the eye down, elevating the
front – ELEVATION.

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55. Inferior Oblique
 because the insertion on the
back of the eye is pulled toward
the medial side; thus the
anterior pole is moved laterally
in ABDUCTION

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56. Actions of the EOMs from secondary position
 HORIZONTAL RECTUS MUSCLES: LR & MR
 If the eye is elevated, contraction of the LR & MR no longer
causes strictly adduction or abduction but also causes a
slight elevation
 if the eye is depressed, contraction causes further
depression
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57. VERTICAL RECTUS MUSCLES : SR & IR
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 When the globe is abducted 23°, the
visual and orbital axes coincide.
 In this position they have no secondary
actions
 The SR can act only as an elevator & IR
only as a depressor

58. VERTICAL RECTUS MUSCLES : SR & IR
 When the eye is adducted 670 the plane of
the vertical rectus muscles is at a right angle
to the sagittal axis and thus parallel to the
horizontal axis.
 The superior rectus could only act as an
intortor & the inferior rectus could act only as
an extortor.
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59. THE OBLIQUE MUSCLES : SO & IO
 The obliques form an angle of 51° with the
visual axis
 So if the eye is adducted 510, the plane of
the oblique muscles becomes parallel to
the sagittal axis and perpendicular to the
horizontal axis.
 Thus contraction of the SO will cause only
depression, and the IO will cause only
elevation
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60. THE OBLIQUE MUSCLES : SO & IO
 When the eye is abducted 390, the plane
of the oblique muscles makes a right angle
with the sagittal axis & parallels the
horizontal axis,
 The superior oblique can cause only
intorsion & the IO extorsion
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61. Actions of EOMs
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62. Fundamental Laws governing Ocular Motility
 Synergists:- refers to muscles having same primary action
in same eye.
Ex:- sup.rectus & inf.oblique----elevators
inf.rectus & sup.oblique-----depressors
 Antagonists:- muscles having opposite action in same eye
Ex:- sup. & inf. Recti
sup. & inf.oblique
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63. Fundamental Laws governing Ocular Motility
 Yoke muscle (contralateral synergists):-
Ref. to pair of muscles (one from each eye) which contract
simultaneously during version movements.
Ex :- In dextroversion RLR &LMR
 Contralateral antagonist:- pair of muscle (one from each eye)
having an opposite action.

Ex:-In dextroversion RLR & LLR
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64. Fundamental Laws governing Ocular Motility
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 DONDER’S LAW
 To each positon of line of sight belongs a definite orientation of the vertical and
horizontal retinal meridians relative to the coordinates of space.
 The orientation of the retinal meridians pertaining to a particular position of
globe is achieved irrespective of the path the eye has taken to reach that
position.
 In short, it implies that there is one and only one orientation of the retinal
meridians with each position of the eyes.

65. Fundamental Laws governing Ocular Motility
 Hering’s Law of Equal Innervation
 Also known as Hering’s law of motor correspondence
 States ‘ equal and simultanous innervation flows from the
brain to a pair of muscles of both eyes (yoke muscles) which
contract simultaneously in different binocular movements.’
 Eg: Right Lateral R and Left Medial rectus: Dextroversion
Both Medial Rectus : Convergence
Right IR and Left Superior Oblique: Dextrodepression
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66. Fundamental Laws governing Ocular Motility
 Sherrington’s Law of Reciprocal
Innervation
 ‘During ocular motility, an increased flow of innervation to the
contracting agonist muscle is accompanied by a decreased flow
of innervation to the relaxing antagonist muscle’.
 Eg. During Dextroversion,
Increased innervation – Right LR and Left MR
Decreased innervation – Left LR and Right MR
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67. References
 Wolf anatomy of the eye and orbit
 Clinical anatomy and physiology of the visual system
 Duane’s Ophthalmology 2007 edition parts 1 and 2
 Kanski clinical ophthalmology 7thedition chapter18
 BCSC 2019, section 2 and 7
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68. 68
Thank You!