Presenter : Dr. Reshma Peter

Anatomy and Physiology ofExtra Ocular Muscles and Its Applied Aspects

Extraocular musles(EOM)

They are six in numberFour recti: Superior rectus Inferior rectus Medial rectus Lateral rectus

Two oblique muscles: Superior oblique Inferior oblique

SUPERIOR RECTUS MUSCLE

.

Origin Superior part of common annular tendon of ZinnCourse Passes anterolaterally beneath the levator At 23 degrees with the globe ‘s AP axis Pierces Tenon s capsuleInsertion into sclera by flat tendinous 10 mm broad insertion

7.7 mm behind sclero-corneal junction.

42 mm long 9 mm wide

Nerve supply Sup division of 3rd NBlood SupplyLateral Muscular br. of Ophthalmic A

APPLIED:SR loosely bound to LPS muscle.

• During SR resection- eyelid may be pulled forward nar-rowing 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

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

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

• 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

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 com-ponent

The elevating action of SR maximal in abducted positions of the eye.

APPLIED:SR- Only elevator in full abduction be-cause IO is ineffective .Thus when SR is paralysed , abducted eye cant be ele-vated

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.

INFERIOR RECTUSOrigin Inferior part of common tendon of Annulus of zinn below the optic foramenCoursePasses anterolaterally along the floor of the orbitAt an angle of 23 degreesInsertion obliquely in the sclera 6.5 mm behind sclero corneal junction by a 5.5mm long tendon

40 mm long 9 mm wide

Attached to lower lid by fascial sheath.

Nerve supplyInf. division 3rd NBlood 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 changesIR Recession –widens palpebral fissureIR Resection –narrows palpebral fissure

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

• 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

MEDIAL RECTUS

Origin-• Widely attached medial and inf to optic foramen by common tendon of annulus of zinn • from optic nerve sheathCourse :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 longLargest ocular muscle Thicker than the others

• 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

Nerve supplylower division of 3rd N the specific branch runs along the inside of the muscle cone on the lateral surface.Blood supplyMedial 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 exertsSlight elevator or depressor movements.

APPLIED :Such small movements Significant when displacing insertions of horizontal Recti for vertically incomitant squint

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

LATERAL RECTUSOriginAnnular 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 fatMore anteriorly , it passes medially and pierces tenon ‘s CapsuleInsertionon the sclera 6.9mm behind sclerocorneal junction by a tendon 8.8 mmNerve supplyabducent nerve which enters the muscle on the medial surface.Blood supplyLateral muscular branch of Ophthalmic A Lacrimal A

48 mm long 2/3rd CSA of MR

Relations• SUP: Lacrimal A and N Anteriorly , lacrimal gland • INF: Floor of the orbit IO Tendon • MED: 6TH NCiliary Ganglion Ophthalmic A Nerve to IO • LAT: Post: periorbitaAnt:perimuscular fatLacrimal gland

Apex of Orbit

Oculomotor foramen

ACTION - Primary abductor of eye.

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 perforationRisk minimized by • Spatulated needles• Clean dry blood free field• Loupe magnification• Head mounted fibreoptic light

SPIRAL OF TILLAUX

5.5 mm

6.5 mm6.9 mm

7.7 mm

Imaginary line joining the insertions of the 4 recti

SUPERIOR OBLIQUE

• Longest and thinnest EOMOrigin-body of sphenoid above and medial to optic canal.Attached superomedially to optic foramen by narrow tendon overlapping the levatorCoursePasses forwards b/w roof and medial wall of the orbit to its trochlea or pulley

RelationsBecomes a rounded tendon 1 cm posterior to trochlea ,turns posterolaterally at 55 degrees(Trochlear angle )Pierces tenon‘s capsule, descends inf to SRThis 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

ACTIONS

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

• 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

INFERIOR OBLIQUEOriginAnteromedial part of orbital floor, from a small depression on orbital plate of maxilla lateral to nasolacrimal groove.CourseInclined posterolaterally at an angle of 45 degrees with AP plane , almost parallel with tendon of SOInsertionposteroinferior surface of globe near the macula(2.2 mm from it)oblique line of attachment 9.4mm long

Actions• Primary action-extorsion• Subsidiary actions-elevations and abduction.• Only elevator in adducted position of eyeballNerve supplyinferior division of 3RD NBlood SupplyInfraorbital 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

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

Anomalous EOMGracilis 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

Two anomalous muscles may occasionally be associated with the LPS. Both muscles are supplied by the superior division of the ocu-lomotor 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, con-necting with the levator muscle en route.

OPHTHALMIC ARTERY

Medial Muscu-lar Artery

MR,IR,IO

Lateral Mus-cular Artery

LR,SR,SO,LPS

BLOOD SUPPLY

• 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 Re-

cession 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 si-multaneous surgery on 3 recti induce Anterior

segment ischaemia

• 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 re-gard to the anterior segment blood supply when disinserting more than two rectus muscle tendons during muscle surgery

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

• The muscle sheaths of 4 recti are connected by intermuscu-lar membrane, which closely relates these muscles to each other .

• Numerous 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.

The fascial sheath of the SR muscle closely adheres in its anterior external surface to the undersurface of the sheath of LPS of upper lidIn front of the equator it also sends a separate extension obliquely forward that widens and ends on the lower surface of the levator mus-cle.

The fascial sheath of the IR muscle divides anteriorly into two layers: an upper one, which becomes part of Tenon’s capsulea 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 co-operation of upper lid and globe in elevation of the eye, a fact that must be kept in mind during surgical proce-dures on the superior rectus muscle

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. Numerous fine fibrils that connect the inner surface of the sheath to the tendon

APPLIED :Potential 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

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 sup-ports the eyeballIncludes 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.

Fascial expansions of Extraocular muscles

Check Ligaments• well-developed fibrous membranes • extend from the outer aspect of the muscles to the corre-

sponding 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 as-

pect of the lateral palpebral ligament, and the lateral con-junctival fornix.

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 liga-ments

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 diffi-cult to determine accurately the width of the insertions.

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 be-come 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 mus-cles

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.

Developmental Anomalies of Extraocular Muscles and the Fascial System• Patients with congenital absence of a muscle present with

the clinical picture of complete paralysisThere 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 ap-proaches if a muscle cannot be located at the time of the opera-tion.

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 pro-duce the active movement may be quite normal anatomically and functionally.

• various forms of strabismus fixus and the SO tendon sheath Syndrome of Brown

• 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 posi-tions of the globe are designated as secondary positions.

• The oblique positions of the eye are termed tertiary po-sitions

Diagnostic positions of gaze:-9 1 Primary position of gaze:-assumed by eyes when fixating

a distant object with head erect.

4 secondary 1. Up2. Down 3. Right 4. Left

4 tertiary positions1. Dextroelevation2. Dextrodepression3. Levoelevation 4. Levodepression

6 cardinal positions :- to test 12 EOM in their main field of action

1. Dextroversion2. Laevo version3. Dextro elevation 4. Leavo elevation5. Dextro depression6. Laevo depression

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 mmIn 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

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 correspond-ing 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.

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 -> poste-rior

FICK’S AXESThese axes intersect at the center of rotation - a fixed point, defined as 13.5 mm behind cornea.

Basic KinematicsTranslatory 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 anteropos-

terior axis • the center would not shift its position• it would have zero velocity.

•  The posterior pole of the eye moves in an opposite direction, except in torsional movements.

• Despite their formalized terms, all movements are ro-tations, 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.

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

•  

• The arc of contact is the arc formed between the tangen-tial 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.

•

• 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.

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 muscleFor 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

3. The arc of contactDistance 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 gazeAdvancement of EOM has strengthening effect because ef increase in effective length as well as arc of contact

Types of Eye Movements

1. Uniocular Eye Movements Ductions

2.Binocular Eye Movements Version: (Binocular Conjugate Eye Movements) Vergence: (Binocular Disjugate eye movements)

Uniocular movements Ductions – only one eye is open, the other covered/closed

tested by asking the patient to follow a target in each direc-tion of gaze.

Types of ductions:-

Binocular movements

Versions:- Binocular ,simultaneous, conjugate movements in same di-

rection. 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.

Types of versions:-Dextroversion & laevo version Elevation & depressionDextro elevation & dextro depressionLaevo elevation & laevo depression

• Vergences:• binocular,simultaneous,disjugate/disjunctive movements

(opp.direction) Convergence– simultaneous adduction Divergence– outward movement from convergent position

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----elevatorsinf.rectus&sup.oblique-----depressors

• Yoke muscles :Muscles that cause the two eyes move in same direction

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

Laws of ocular motility 1.Hering’s law of equal innervation

During any conjugate movement equal & simultaneous inner-vation flows to yoke muscles to contract or relax• For movements of both eyes in the same direction, the corre-

sponding agonist muscles receive equal innervation

Isolated innervations to an ex-traocular 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.

Clinical application of Her-ings’law

APPLIED:• In patient with paralytic squint, Secondary Deviation > primary deviation

Primary dev- deviation of squinting eye, when patient fixates with normal eyeSec dev- deviation of normal eye under cover, when patient fixates with squinting eye

Excess innervation is required to the paralysed muscle to fix-ate, when patient fixates with squinting eye

Concomitant excess supply to yoke muscle causes excess con-traction leads to more secondary deviation

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 leftLess 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

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

Clinical application of Sherring-ton's law

APPLIED:

• Occurrence of strabismus following paralysis of EOM is explained by the law

• Reciprocal innervation must be kept in mind while per-forming surgery of extraocular muscles

• Exceptions • Duane’s retraction syndrome co-contrac-

tion of antagonistic muscles instead of relaxation antagonist muscle occurs. In duane s , it limits the amount of movement achievable

During fixation, saccades and smooth pursuit the eye ro-tates 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 po-sition and depends upon the amount of elevation or de-pression 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

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 or-bit 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

• Listing’s law implies that all eye movements from pri-mary position are true to the meridians and occurs without torsion with respect to the primary position.

• This law is obviously true for movements around hori-zontal and vertical axes in the equatorial plane.

• Listing’s law holds during fixation, saccades, smooth pursuit but not during sleep .

Extraocular muscles structure• EOM consists of cross striated fibres.• They show a high degree of differentiation• Perform functions of both white and red mus-

cles.• 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 tis-

sue Contain both slow and fast fibres Require and receive more O2

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 –Sarcolemma+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 stri-ated appearance in longitudinal section.

• 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

Each myofibril consists of linearly arranged thick and thin my-ofilaments, which form the chief element of fibre's repeating unit, the sarcomere THICK- MyosinTHIN- 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

Excitation Contraction Coupling

Rapid 'twitch' fibres Fibres of larger diameter With a 'fibrillenstruktur' having a regular distribution of my-

ofibrils and abundant sarcoplasm. Innervation is by single, 'en plaque' endings (i.e.motor end

plates) These fibres resemble somatic striated fibres else-where

Two types of striated twitch fibres are described

Slow or 'tonic' fibres so-called 'felderstruktur‘ ill-defined and myofibrillar arrange-

ments 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 extraoc-

ular muscle fibres .Although fibres are smaller in the orbital zones than in global zones, both contain mixtures in size of fi-bre.

•

Type I muscle fibres 'slow twitch‘'slow, oxidative and fatigue resistant'stain weakly for myosin ATPase at pH 9.4 but strongly at acid pHstrongly 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 4.3oxidative and glycolytic featuresresistant 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.

The extraocular muscles possess a resident population of im-munocompetent 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.

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,• In 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 maxi-mizes the possible change in length of the muscle in contraction, and contrasts with most skeletal muscles.

Types of eomSpencer 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• Individual fibres are ringed by capillaries and motor endplates• Abundant 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

.

Type 2: Orbital multiply innervated fibres a slow fibre comprising 20% of the orbital zone.• It stains strongly for myosin ATPase after acid preincu-

bation, 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 my-

ofibrillar arrangement by electron microscopy.

• Although they are multiply innervated, they show a twitch capability near their centre and a slow contractility proxi-mally 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 embryonid-neonatal myosin.

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.

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 re-semblance to skeletal type IIB

• fibre is granular and there are moderate levels of ox-idative 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 inner-vated fibres.

• Type 5: Global pale singly innervated • This fibre comprises 30% of the global layer. • a fast twitch fibre used infrequently because of low fa-

tigue resistance.• It resembles type lIB skeletal• Mitochondria are small and few and arranged singly be-

tween 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.

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 verylarge myofibrils, sparse membranous systems andoccasional mitochondria in single file. • multiply innervated, with numerous en-grappe end-

ings along its length.

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 3rd N ( SR, MR, IR, IO)

• LR and SO arises from its own adjacent tissue mass in Maxillomandibular mesoderm

• 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 SYNDROMECongenital absence of 6th NAbnormal innervation of Lateral Rectus by 3rd N

• Eom develop in at least 2 waves of Myogenesis, form-ing 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 originAPPLIED:APPLIED :in abnormal situations, pulleys may be heterotropic , may cause ocular motility problems

References • Wolff’s anatomy of eye -8th e• Clinical Anatomy of the eye –

SNELL• Von Noorden, A. Edward• A.k.Khurana Anatomy and physi-

ology of the eye- 5th e• Strabismus by Pradeep Sharma

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