Anatomy and physiology of extraocular muscles and applied aspects

Anatomy and phusiology of eom

1. Presenter : Dr . Reshma Peter Anatomy and Physiology of Extra Ocular Muscles and Its Applied Aspects

2. Extraocular musles (EOM) They are six in number Four recti: Superior rectus Inferior rectus Medial rectus Lateral rectus Two oblique muscles: Superior oblique Inferior oblique

3. SUPERIOR RECTUS MUSCLE . Origin Superior part of common annular tendon of Z inn Course Passes anterolaterally beneath the levator At 23 degrees with the globe ‘s AP axis Pierces Tenon s capsule Insertion into sclera by flat tendinous 10 mm broad insertion 7.7 mm behind sclero -corneal junction. 42 mm long 9 mm wide

4. Nerve supply Sup division of 3 rd N Blood Supply Lateral Muscular br. of Ophthalmic A APPLIED : SR loosely bound to LPS muscle. During SR resection- eyelid may be pulled forward  narrowing palpebral fissure In hypotropia  pseudoptosis may be present Origin of SR and MR are closely attached to the dural sheath of the optic nerve pain during upward & inward movements of the globe in RETROBULBAR NEURITIS

5. RELATIONS SUP :Frontal nerve INF :Nasociliary nerve ophthalmic artery tendon of SO muscle LAT :Lacrimal A and Nerve MED :Ophthalmic A Nasociliary N

6. Action Primary action - Elevation (Superior Insertion ) Secondary action – Adduction (Lateral Insertion ) Tertiary action - Intorsion (Oblique Insertion)

7. In primary position , SR muscle plane forms an angle of 23 degrees with the y-axis (the median plane of the eye) and therefore does not coincide. Thus, in primary position, SR not only elevates the globe but also adducts it and rotates it around AP Y -axis , causing incycloduction

8. If the globe is abducted its axis of rotation approaches the y -axis more and more when it is abducted 23 degrees Axis of rotation and Y axis coincide. SR becomes a pure elevator ,no longer has a cycloducting component The elevating action of SR maximal in abducted positions of the eye. APPLIED: SR- Only elevator in full abduction because IO is ineffective .Thus when SR is paralysed , abducted eye cant be elevated

9. The opposite effect applies to incycloduction The more the globe is adducted the greater the incycloduction effect. If the globe could be adducted 67 degrees SR would produce pure incycloduction . Since the globe cannot adduct that far, there is some elevating component to the action of the SR, even in adduction.

10. INFERIOR RECTUS Origin Inferior part of common tendon of Annulus of zinn below the optic foramen Course Passes anterolaterally along the floor of the orbit At an angle of 23 degrees Insertion obliquely in the sclera 6.5 mm behind sclero corneal junction by a 5.5mm long tendon 40 mm long 9 mm wide

11. Attached to lower lid by fascial sheath . Nerve supply Inf. division 3 rd N Blood Supply Med. muscular br. of Ophthalmic A APPLIED: In Thyroid orbitopathy , MR and IR thicken. especially near the orbital apex - compression of the optic nerve as it enters the optic canal adjacent to the body of the sphenoid bone. APPLIED: Alteration of IR – ass with palpebral fissure changes IR Recession –widens palpebral fissure IR Resection –narrows palpebral fissure

12. RELATIONS SUP :Optic N Inf. Div of 3 rd N INF : Floor of the orbit roofing the maxillary sinus Orbital Palatine process Infraorbital vessels and nerves IO LAT :Nerve to IO

13. Fascial attachments below attached to inferior lid coordinate depression and lid opening. Actions Primary depressor. Subsidiary actions are adduction and extorsion . Depression increases in abduction,becomes nil in full adduction. Subsidiary actions increase with adduction

14. MEDIAL RECTUS Origin - Widely attached medial and inf to optic foramen by common tendon of annulus of zinn from optic nerve sheath Course : Passes along medial wall of the orbit Insertion- in sclera 5.5mm behind sclero -corneal junction by a tendon 3.7 mm in length 40 mm long Largest ocular muscle Thicker than the others

15. APPLIED: In Thyroid orbitopathy , MR and IR thicken; Visibility of Muscle insertion through conjunctiva allows swelling to be detected in Endocrine Exophthalmos Pain in Retrobulbar neuritis- Origin close to dural sheath of Optic Nerve Medial rectus inserts closest to the limbus and is therefore susceptible to injury during ant. segment surgery. Inadvertent removal of the MR is a well known complication of Pterygium removal

16. Nerve supply lower division of 3 rd N the specific branch runs along the inside of the muscle cone on the lateral surface . Blood supply Medial muscular branch of Ophthalmic A Action Primary adductor of the eye in Primary position If axis elevated or depressed by other muscles , horizontal recti no longer exert purely around vertical axis, also exerts Slight elevator or depressor movements. APPLIED :Such small movements Significant when displacing insertions of horizontal Recti for vertically incomitant squint

17. RELATIONS SUP : Separated from SO by Ophthalmic A Ethmoidal N Infratrochlear N INF : Floor of the orbit MED: Peripheral fat Orbital plate of ethmoid Ethmoidal sinuses LAT : Central orbital fat Optic N

18. LATERAL RECTUS Origin Annular Tendon of zinn where it crosses Sup Orbital Fissure,continuous with spina recti lateralis on greater wing of sphenoid Course At first adjoins lateral orbital wall separated by fat More anteriorly , it passes medially and pierces tenon ‘s Capsule Insertion o n the sclera 6.9mm behind sclerocorneal junction by a tendon 8.8 mm Nerve supply abducent nerve which enters the muscle on the medial surface. Blood supply Lateral muscular branch of Ophthalmic A Lacrimal A 48 mm long 2/3 rd CSA of MR

19. Relations SUP : Lacrimal A and N Anteriorly , lacrimal gland INF : Floor of the orbit IO Tendon MED: 6 TH N Ciliary Ganglion Ophthalmic A Nerve to IO LAT : Post: periorbita Ant:perimuscular fat Lacrimal gland

20. Apex of Orbit Oculomotor foramen

21. ACTION - Primary abductor of eye.

22. INSERTION OF THE RECTI Sclera is thinnest (0.3mm) just posterior to 4 recti muscle insertions APPLIED: Site for most procedures, specially Recession  Risk of Scleral perforation Risk minimized by Spatulated needles Clean dry blood free field Loupe magnification Head mounted fibreoptic light

23. SPIRAL OF TILLAUX 5.5 mm 6.5 mm 6.9 mm 7.7 mm Imaginary line joining the insertions of the 4 recti

24. SUPERIOR OBLIQUE Longest and thinnest EOM Origin- body of sphenoid above and medial to optic canal. Attached superomedially to optic foramen by narrow tendon overlapping the levator Course Passes forwards b/w roof and medial wall of the orbit to its trochlea or pulley

25. Relations Becomes a rounded tendon 1 cm posterior to trochlea , turns posterolaterally at 55 degrees(Trochlear angle ) Pierces tenon‘s capsule, descends inf to SR This is the only extraocular muscle with a rich vascular tunic . Insertion- Posterosuperior quadrant of sclera behind equator of eyeball . Line of insertion :10.7 mm long Nerve supply- trochlear nerve Blood supply- Sup. Muscular branch of Ophthalmic A

26. ACTIONS Primary action- intorsion . Subsidiary actions-abduction and depression. It is the only Adductor in depression .

27. When the globe is adducted to 51 ͦ, the visual axis coincides with the line of pull of the muscle, the SO acts as a depressor When the globe is abducted to 39 ͦ, the visual axis and the SO make an angle of 90 ͦ, the SO causes only intorsion

28. INFERIOR OBLIQUE Origin Anteromedial part of orbital floor, from a small depression on orbital plate of maxilla lateral to nasolacrimal groove. Course Inclined posterolaterally at an angle of 45 degrees with AP plane , almost parallel with tendon of SO Insertion posteroinferior surface of globe near the macula(2.2 mm from it)oblique line of attachment 9.4mm long

29. Actions Primary action- extorsion Subsidiary actions-elevations and abduction. Only elevator in adducted position of eyeball Nerve supply inferior division of 3 RD N Blood Supply Infraorbital and medial muscular br. of Ophthalmic A. . APPLIED : Parasympathetic supply to Sphincter pupillae and ciliary muscle accompanies N. to IO , pupillary abnormalities from surgery in this area N. To IO enters lateral portion of muscle where the muscle crosses IR- chance of damage in this area

30. Actions of EOM ACTION PRIMARY SECONDARY TERTIARY MR ADDUCTION ------ --------- LR ABDUCTION ------ --------- SR ELEVATION INTORSION ADDUCTION IR DEPRESSION EXTORSION ADDUCTION SO INTORSION DEPRESSION ABDUCTION IO EXTORSION ELEVATION ABDUCTION

31. Anomalous EOM Gracilis orbitis or comes obliqui superioris originates from the proximal dorsal surface of the SO and inserts on the trochlea or its surrounding connective tissue. It is supplied by the trochlear nerve Accessory lateral rectus muscle is a single slip . sometimes found in the monkey homologous to the nictating membrane. It is supplied by the abducent nerve  

32. Two anomalous muscles may occasionally be associated with the LPS. Both muscles are supplied by the superior division of the oculomotor nerve Tensor trochleae arises from the medial border of the levator muscle inserts into the trochlea or its environs . Transversus orbitus attaches between the medial and lateral walls of the orbit, connecting with the levator muscle en route.

33. BLOOD SUPPLY

34. The arteries to the four rectus muscles give rise to the anterior ciliary arteries. IO and IR also receive a branch from the infraorbital artery , and the medial rectus muscle receives a branch from the lacrimal artery. The veins from the extraocular muscles correspond to the arteries and empty into the superior and inferior orbital veins , respectively. APPLIED : Accident risk of severing of Vortex veins during IR and SR Recession or Resection , IO muscle weakening and SO muscle tendon exposure Blood supply to EOM supplies most of anterior segment ,Part of nasal half supplied by Long Posterior ciliary artery…thus simultaneous surgery on 3 recti induce Anterior segment ischaemia

35. The anterior ciliary arteries pass to the episclera , give branches to the sclera, limbus, and conjunctiva, and pierce the sclera not far from the corneoscleral limbus. These perforating branches cross the suprachoroidal space to terminate in the anterior part of the ciliary body . Here they anastomose with the lateral and medial long ciliary arteries to form the major arterial circle of the iris . APPLIED : Variations in the number of anterior ciliary arteries supplied by each muscle become clinically relevant with regard to the anterior segment blood supply when disinserting more than two rectus muscle tendons during muscle surgery

36. Muscle Sheaths and Their Extensions EOM pierce Tenon’s capsule, enter the subcapsular space, and insert into the sclera. In their extracapsular portions, muscles are enveloped by a muscle sheath - reflection of Tenon’s capsule and runs backward for 10 to 12 mm. APPLIED :During Strabismus sx , Buckling for RD , Periocular trauma ,care must be taken to avoid penetration of Tenon ‘s Capsule. If integrity lost 10 mm posterior to limbus, Fatty tissue prolapse forms restrictive adhesion  limit ocular motility

37. The muscle sheaths of 4 recti are connected by intermuscular membrane , which closely relates these muscles to each other . N umerous extensions from all the sheaths of EOM, form an intricate system of fibrous attachments interconnecting the muscles, attaching them to the orbit,supporting the globe, and checking the ocular movements.

38. The fascial sheath of the SR muscle closely adheres in its anterior external surface to the undersurface of the sheath of LPS of upper lid In front of the equator it also sends a separate extension obliquely forward that widens and ends on the lower surface of the levator muscle. The fascial sheath of the IR muscle divides anteriorly into two layers: an upper one, which becomes part of Tenon’s capsule a lower one, which is about 12 mm long and ends in the fibrous tissue between the tarsus of the lower lid and the orbicularis muscle This lower portion forms part of Lockwood’s ligament. APPLIED : The fusion of SR and LPS accounts for the cooperation of upper lid and globe in elevation of the eye , a fact that must be kept in mind during surgical procedures on the superior rectus muscle

39. The fascial sheath of the reflected tendon of SO muscle consists of two layers of strong connective tissue . The two layers are 2 to 3 mm thick, so the tendon and its sheath have a diameter of about 5 to 6 mm . Many attachments extend from the sheath of SO to the sheath of the levator muscle to the sheath of the SR muscle to the conjoined sheath of these two muscles to Tenon’s capsule, behind,above , and laterally . N umerous fine fibrils that connect the inner surface of the sheath to the tendon APPLIED : P otential space between the sheath and the tendon is continuous with the episcleral space. Material injected into Tenon’s space therefore may penetrate into this space

40. The fascial sheath of the inferior oblique muscle covers the entire muscle. It is rather thin at the origin but thickens as the muscle continues laterally and develops into a rather dense membrane where it passes under IR At this point, the sheath of the IO muscle fuses with the sheath of IR muscle  suspensory ligament of Lockwood . This fusion may be quite firm and complete or so loose that the two muscles may be relatively independent of each other . The extensions that go from there upward on each side to the sheaths of the MR and LR muscles form a suspending hammock, which supports the eyeball Includes extensions of fibrous bands to the tarsal plate of the lower lid, the orbital septum, and the periosteum of the floor of the orbit. Near the insertion of the muscle the sheath of the inferior oblique muscle also sends extensions to the sheath of LR muscle and to the sheath of the optic nerve.

41. Fascial expansions of Extraocular muscles

42. Check Ligaments well-developed fibrous membranes extend from the outer aspect of the muscles to the corresponding orbital wall The check ligament of LR appears in horizontal sections as a triangle apex is at the point where muscle sheath pierces Tenon’s capsule. goes forward and slightly laterally fans out to attach to the zygomatic tubercle, the posterior aspect of the lateral palpebral ligament, and the lateral conjunctival fornix.

43. The check ligament of the MR extends from the sheath of the muscle attaches to the lacrimal bone behind the posterior lacrimal crest and to the orbital septum behind. triangular and unites at its superior border with a strong extension from the sheath of LPS a weaker extension from the sheath of SR Inf border is fused to extensions from the IO and IR muscle sheaths. The other EOM do not have clearly distinct check ligaments

44. Intracapsular Portion of the Muscle The muscles move freely through the openings in Tenon’s capsule In the intracapsular portion, they have no sheath but are covered by episcleral tissue fused with the perimysium. This tissue expands laterally, going along the muscle on each side, from the entry of the muscle into the subcapsular space to the insertion. Posteriorly,this tissue attaches to the capsule and laterally to the sclera. At the tendon this tissue becomes rather dense and appears to serve to fixate the tendon, forming the falciform folds of Gue´rin or adminicula of Merkel . Merkel and Kallius remarked that these structures make it difficult to determine accurately the width of the insertions .

45. Thus , the position of the center of rotation of the eyeball remains fairly constant in relation to the orbital pyramid Due to the action of the check ligaments, the eye movements become smooth and dampened . As the muscles contract, their action is graduated by the elasticity of their check systems, which limits the action of the contracting muscle and reduces the effect of relaxation of the opposing muscles This ensures smooth rotations and lessens the shaking up of the contents of the globe when the eyes suddenly stop or change the direction of their movement. Functional Role of the Fascial System Serve as a cavity within which the eyeball may move Connect the globe with the orbit Supporting and protecting the globe In the control of the eye movements. It prevents or reduces retractions of the globe, as well as movements in the direction of action of the muscle pull.

46. Developmental Anomalies of Extraocular Muscles and the Fascial System Patients with congenital absence of a muscle present with the clinical picture of complete paralysis There may be no preoperative clues to alert the surgeon that the apparently paralyzed muscle is absent. Consequently, the surgeon must be prepared to use alternative surgical approaches if a muscle cannot be located at the time of the operation . Anomalies of the fascial system more common act as a check to active and passive movements of the globe in certain directions, although the muscles that should produce the active movement may be quite normal anatomically and functionally. various forms of strabismus fixus and the SO tendon sheath Syndrome of Brown

47. The primary position is assumed by the eye in binocular vision when one is looking straight ahead with body and head erect object of regard is at infinity lies at intersection of sagittal plane of head and horizontal plane passing through centres of rotation of 2 eyeballs The adducted, abducted, elevated, or depressed positions of the globe are designated as secondary positions . The oblique positions of the eye are termed tertiary positions

48. Diagnostic positions of gaze :- 9 1 Primary position of gaze:-assumed by eyes when fixating a distant object with head erect. 4 secondary Up Down Right Left 4 tertiary positions Dextroelevation Dextrodepression Levoelevation Levodepression

49. 6 cardinal positions :- to test 12 EOM in their main field of action Dextroversion Laevo version Dextro elevation Leavo elevation Dextro depression Laevo depression

50. Centre of Rotation The eye performs rotary movements around a center of rotation within the globe Centre of rotation moves in a semicircle in the plane of rotation- Space centroid In primary position the center of rotation is located 13.5 mm behind the apex on the cornea on the line of sight 1.3 mm behind the equatorial plane (in myopes --- posterior -14.5 mm In hyperopes --anterior ) For practical purposes, one may assume that a line connecting the middle of the lateral orbital margins goes through the center of rotation of the two eyes if they are emmetropic

51. Action of an individual muscle is controlled by the direction of its pull to 3 axes around which the globe rotates . Ocular movements take place round a centre corresponding approximately to that of the eye, which is therefore not significantly displaced. The movements defined relative to 3 primary axes which pass through the centre of movement at right-angles to each other.  

52. Elevation & Depression – Around the transverse axis (X) nasal -> temporal Adduction & Abduction – Around the vertical axis ( Z ) superior -> inferior Intortion & Extortion – Around the AP axis (Y) anterior -> posterior FICK’S AXES These axes intersect at the center of rotation - a fixed point, defined as 13.5 mm behind cornea.

53. Basic Kinematics Translatory movements The body can move sideways, up or down, and forward or backward the center of the body moves with it Rotary movements it can rotate around a vertical, horizontal, or anteroposterior axis the center would not shift its position it would have zero velocity.

54.   The posterior pole of the eye moves in an opposite direction , except in torsional movements. Despite their formalized terms, all movements are rotations, and are not necessarily confined to the above arbitrary axes, the movements of which are sometimes called cardinal . Because of the geometric relations between the orbital and global attachments of each muscle, each acts to greatest effect in one plane, and this is known as its primary action .

55. The point at which the center of the muscle or of its tendon first touches the globe is the tangential point . indicates the direction of pull of that muscle. The position of this point changes when the muscle contracts or relaxes and the globe rotates  

56. The arc of contact is the arc formed between the tangential point and the center of the insertion of the muscle on the sclera. Since the position of the tangential point is variable, the arc of contact changes in length as the muscle contracts. It is longest when the muscle is relaxed and its antagonist contracted and shortest when the muscle is contracted and its antagonist relaxed.  

57. The muscle plane is determined by the tangent to the globe at the tangential point and the center of rotation. It is the plane determined by the centers of origin and insertion and the center of rotation The muscle plane describes the direction of pull of the muscle and determines the axis around which the eye would rotate if the particular individual muscle were to make an isolated contraction Axis of rotation , which is perpendicular to the muscle plane erected in the center of rotation, corresponds to each muscle plane .

58. Factors involved in mechanics of EOM action 1. Cross sectional area of the muscle Muscles exert force in proportion to their crosssectional area 2.Length of the muscle For normal amplitude of rotation 45-50 degres  10mm change in muscle length is required in each direction APPLIED : Antagonists such as medial and lateral recti are similar in size –balancing opposing forces APPLIED : Sacrifice of muscle length during resections reduces the amplitude of eye rotations

59. 3 . The arc of contact Distance between the anatomic and physiologic insertion The power of the muscle is proportionate to its length and arc of contact, APPLIED : Recession weakens muscle action by shortening its effective length and its arc of contact in various positions of gaze Advancement of EOM has strengthening effect because ef increase in effective length as well as arc of contact

60. Types of Eye Movements Uniocular Eye Movements Ductions 2.Binocular Eye Movements Version: (Binocular Conjugate Eye Movements) Vergence : ( Binocular Disjugate eye movements)

61. Uniocular movements Ductions – only one eye is open, the other covered/closed tested by asking the patient to follow a target in each direction of gaze. Types of ductions :-

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

63. Types of versions :- Dextroversion & laevo version Elevation & depression Dextro elevation & dextro depression Laevo elevation & laevo depression

64. Vergences : binocular,simultaneous,disjugate /disjunctive movements ( opp.direction ) Convergence– simultaneous adduction Divergence– outward movement from convergent position

65. Agonist,Antagonist,synergists and yoke muscles Agonist :a muscle producing movement on contraction Antagonist muscles : A muscle producing a movement in the direction opposite produced by agonist. EG -sup .&inf. Recti ,sup.& inf.oblique Synergists muscles :Two muscles moving an eye in the same direction are synergists. Ex:- sup.rectus & inf.oblique ----elevators inf.rectus&sup.oblique ----- depressors Yoke muscles :Muscles that cause the two eyes move in same direction

66. Yoke muscle(contralateral synergists ) Ref. to muscles which are primary muscles (one from each eye) that accomplish (contract) a given version.

67. Laws of ocular motility 1. Hering’s law of equal innervation D uring any conjugate movement equal & simultaneous innervation flows to yoke muscles to contract or relax For movements of both eyes in the same direction, the corresponding agonist muscles receive equal innervation Isolated innervations to an extraocular muscle of the eye do not occur nor can the muscles from the one eye alone innervated , to perform an eye movement, impulses are always integrated.

68. Clinical application of Herings’law

69. APPLIED: In patient with paralytic squint, Secondary Deviation > primary deviation Primary dev- deviation of squinting eye, when patient fixates with normal eye Sec dev- deviation of normal eye under cover, when patient fixates with squinting eye Excess innervation is required to the paralysed muscle to fixate, when patient fixates with squinting eye Concomitant excess supply to yoke muscle causes excess contraction leads to more secondary deviation

70. APPLIED: Inhibitional palsy of contralateral antagonist muscle in paralytic squint is also based on Hering’s law Eg – In RSO paresis, fixating with Right eye on an object located up and to the left Less innervation of its antagonist RIO is required less innervation of LSR Inhibitional Palsy of the antagonist of the yoke muscle of paretic muscle LSR LIR RSO

71. 2. Sherrington law of reciprocal innervation Increased innervation to an EOM is accompanied by reciprocal decrease in innervation to its antagonist. The antagonist relaxes as the agonist contracts

72. Clinical application of Sherrington's law

73. APPLIED: Occurrence of strabismus following paralysis of EOM is explained by the law Reciprocal innervation must be kept in mind while performing surgery of extraocular muscles Exceptions Duane’s retraction syndrome co-contraction of antagonistic muscles instead of relaxation antagonist muscle occurs. In duane s , it limits the amount of movement achievable

74. During fixation, saccades and smooth pursuit the eye rotates freely in horizontal and vertical dimensions but torsion is constrained. This restriction on ocular torsion is described by donder’s law and listing’s law. 3.Donder’s law Donder stated that each position of line of sight belongs to the definite orientation of vertical and horizontal retinal meridian relative to the coordinate of the space. Orientation of retinal meridian is always same irrespective irrespective of the path the eye has taken to reach that position and depends upon the amount of elevation or depression and lateral rotation of the globe, after returning to the initial position the retinal meridian is oriented exactly as it was before the movement was initiated

75. 4.LISTING’S LAW Listing ‘s law states that each movement of the eye from the primary position to any other position involves a rotation around a single axis lying in the equatorial plane ,also called as listing’s plane. This plane was defined earlier as being fixed in the orbit and passing center of rotation of the eye and its equator, when the eye is primary position Any position of the eye can described by specifying the orientation of the axis of rotation in listing’s plane and magnitude of rotation from primary position

76. Listing’s law implies that all eye movements from primary position are true to the meridians and occurs without torsion with respect to the primary position. This law is obviously true for movements around horizontal and vertical axes in the equatorial plane. Listing’s law holds during fixation, saccades, smooth pursuit but not during sleep .

77. Extraocular muscles structure EOM consists of cross striated fibres . They show a high degree of differentiation Perform functions of both white and red muscles. The motor units are small, with only from 5 to 18 muscle fibers contact by each motor nerve Differ from other skeletal muscles in terms of Diameter of these fibres is small Richly supplied by nerves and vessels Contains enormous amount of fibroelastic tissue Contain both slow and fast fibres Require and receive more O2

78. Each muscle is made up of large no: of muscle fibres . Each muscle fibre is a long cylindrical multinucleated cell , surrounded by a cell membrane –Sarcolemma Sarcotubular system – S arcolemma+Sarcoplasm Each fibre has a diameter of 5-40 um (c/f 10-100 um in skeletal muscles. punctiform appearance in transverse section and a striated appearance in longitudinal section.

79. Vessels and nerves enter each muscle belly at its hilum. The blood supply of the recti is greater than that of the myocardium, primarily due to richness of the closed type capillary network in the orbital layer. This blood supply is required by the larger numbers of fast twitch fibres in the orbital layer, which have a highly aerobic metabolism. Blood flow is thought to be highest in the MR, although that in SR may be higher

80. Each myofibril consists of linearly arranged thick and thin myofilaments , which form the chief element of fibre's repeating unit, the sarcomere THICK- Myosin THIN- Actin. Tropomyosin, Troponin T, I , C Between myofibrils are two membranous systems involved in excitation and contraction : the transverse tubular system and the sarcoplasmic reticulum

81. Excitation Contraction Coupling

82. Rapid 'twitch' fibres Fibres of larger diameter With a ' fibrillenstruktur ' having a regular distribution of myofibrils and abundant sarcoplasm. Innervation is by single, ' en plaque' endings ( i.e.motor end plates) These fibres resemble somatic striated fibres elsewhere Two types of striated twitch fibres are described S low or 'tonic' fibres so-called ' felderstruktur ‘ ill-defined and myofibrillar arrangements and little sarcoplasm. Their respiratory metabolism is chiefly aerobic innervated by diffuse (' en grappe ') myoneural endings. Mitochondria are in general fewer in skeletal than in extraocular muscle fibres .Although fibres are smaller in the orbital zones than in global zones, both contain mixtures in size of fibre .

83. Type I muscle fibres 'slow twitch‘ 'slow, oxidative and fatigue resistant ' stain weakly for myosin ATPase at pH 9.4 but strongly at acid pH strongly for oxidative enzymes but weakly for glycolytic enzymes. ' Type II muscle fibres 'fast twitch' stain strongly for ATPasc at pH 9.4. IIA fibres stain poorlv on preincubation at pH 4.6 and pH 4.3 oxidative and glycolytic features resistant to fatigue on repeated stimulation IIB fibres stain poorly at only pH 4.3. glycolytic features are fatiguable IIC fibres found chiefly in infancy and differs from that shown in adult muscle.

84. The extraocular muscles possess a resident population of immunocompetent cells including numerous macrophages and a smaller number of HLA-DR positive cells and T cells; B cells are absent. The majority of the T cells are CD8 (suppressor/cytotoxic) positive, whereas in skeletal muscle, CD4-positive (helper) cells predominate The medial and inferior recti contain about twice as many macrophages as the lateral rectus and superior oblique muscles. APPLIED :Of importance in certain orbital immune disorders such as endocrine ophthalmopathy .

85. ORBITAL AND GLOBAL ZONES at birth fibre size - generally uniform later an outer shell of smaller-diameter fibres is distinguished from a core of larger fibres this pattern is retained into adult life These zones are referred to as orbital (outer ; facing the orbit) global (inner ; facing the globe and contents of the muscle cone) Orbital fibre diameter - 5 and 15 IJ.m , global fibres diameter - 10 and 40 IJ.m . The global layer of SO is totally enclosed by the orbital layer, I n the recti orbital zones are deficient on their internal surfaces, so that the global layer is exposed to adjacent adipose tissue around the optic nerve at a 'hilum '. The recti are strap-like, with maximum width at their global insertion the global fibres are longer than the orbital and only the global tonic fibres appear to run the full length of the muscle belly which maximizes the possible change in length of the muscle in contraction, and contrasts with most skeletal muscles.

86. Types of eom Spencer and Porter nomenclature Type 1: Orbital singly innervated Type 1 fibres small and make up 80% of the orbital layer accounts for most of the sustained force generated by the muscle. Mitochondria occur in abundant clusters I ndividual fibres are ringed by capillaries and motor endplates A bundant and well-delineated SR and T-system and a regular myofibrillar arrangement . They correspond to skeletal type II differs from skeletal IIA by its high fatigue resistance and unique myosin profile. coarse , fast twitch fibres rich in oxidative enzymes (e.g. SOH) but also capable of anaerobic metabolism. singly innervated .

87. Type 2: Orbital multiply innervated fibres a slow fibre comprising 20 % of the orbital zone. It stains strongly for myosin ATPase after acid preincubation , but variably with alkaline ATPase associated with a structural variation along their length The staining properties not uniform along the length of the muscle fibre ; thus acid-stable myosin ATPase is found only in the proximal and distal thirds of the fibre . moderate oxidative activity. has sparse membranous systems and an irregular myofibrillar arrangement by electron microscopy.

88. Although they are multiply innervated, they show a twitch capability near their centre and a slow contractility proximally and distally Centrally the fibres resemble skeletal fast twitch fibres (IIC) (but with a lower oxidative capacity), At either end they show the ultrastructural features and slow ATPase of slow contracting fibres and contain embryonidneonatal myosin.

89. Type 3: Global red singly innervated This makes up 30 % of the global layer. It stains coarsely resembles the orbital singly innervated fibre . It is highly oxidative and glycolytic regarded as fast-twitch and fatigue-resistant fibre . does not show longitudinal structural variation Contains no coexpressed fast or embryonidneonatal myosins fibre does express myosin IIA isoform along its length, but differs from skeletal IIA by its high mitochondrial content.

90. Type 4: Global intermediate singly innervated This fibre makes up 25% of the global layer. Ultrastructure and ATPase content suggest that it is a fast-twitch fibre and myosin reactivity suggests a resemblance to skeletal type IIB fibre is granular and there are moderate levels of oxidative and aerobic enzymes. There are numerous small mitochondria, singly or in clusters. Myofibril size and sarcoplasmic reticulum content are intermediate between that of the other 2 singly innervated fibres .

91. Type 5: Global pale singly innervated This fibre comprises 30 % of the global layer. a fast twitch fibre used infrequently because of low fatigue resistance . It resembles type lIB skeletal Mitochondria are small and few and arranged singly between myofibrils . Fibre diameter increases from types 3 to 5. All show a regular myofibrillar arrangement on electron microscopy with well-developed sarcoplasmic reticulum and T systems in types 3 and 4 and slightly less so in type 5 They are singly innervated .

92. Type 6: Global multiply innervated fibres makes up 10% of the global layer a slow fibre with strong acid-stable ATPase features and weak oxidative properties. Ultrastructurally , it shows a felderstruktur with very large myofibrils, sparse membranous systems and occasional mitochondria in single file. multiply innervated , with numerous en-grappe endings along its length.

93. Embryological development All EOM develop from 3 distinct masses of Primordial cranial mesoderm 3 masses correspond to Rhombomeres and 3 cranial nerves innervate them accordingly Premandibular condensation gives rise to eye musces innervated by 3 rd N ( SR, MR, IR, IO ) LR and SO arises from its own adjacent tissue mass in Maxillomandibular mesoderm

94. LR and SO lie as B/l masses close to stalk at 13.5 mm stage( 6 weeks ) 4 Recti differentiate at 20 mm stage ( 7 weeks ) LPS differentiates from SR in its medial part at 8 weeks Later it grows laterally on a higher plane than SR at 3 months . Critical development occurs at 6-8 weeks ( maybe upto 12 weeks APPLIED : Close proximity of analgens may facilitate development of anomalous innervation of eye muscles DUANES RETRACTION SYNDROME Congenital absence of 6 th N Abnormal innervation of Lateral Rectus by 3 rd N

95. Eom develop in at least 2 waves of Myogenesis , forming primary and secondary generation fibres . Global multiply innervated fibres are phylogenetically old and formed first while orbital layers mature last. EOM Pulleys – sleeves of Collagen, elastin , smooth muscle encircle EOM and are attached to orbital wall and adjacent connective tissues Muscle with sheath passes through these pulleys Located near the equator of globe Seem to deflect anterior part of the muscle in gazes other than primary gaze Act as functional origin APPLIED : APPLIED : in abnormal situations, pulleys may be heterotropic , may cause ocular motility problems

96. References Wolff’s anatomy of eye -8 th e Clinical Anatomy of the eye –SNELL Von Noorden , A. Edward A.k.Khurana Anatomy and physiology of the eye- 5 th e Strabismus by Pradeep Sharma

97. THANK YOU