EXTRAOCULAR MUSCLES

Sanket Parajuli1st year Resident

Department of ophthalmology

Difference from skeletal muscles Fiber diameter 5-40um (sk musc 10-100) Nerve fiber to muscle fiber ratio 1:5 in EOM ( sk ms 1:100) Blood supply is very high in EOM (Rectus muscles have blood supply

greater than that of myocardium) EOM diffuse myoneural endings can be seen in some type of fibers(En

Grappe) EOM have high mitochondrial content Elastic tissue in EOM is very high T cells in EOM are CD8+ helper T cells( sk ms CD4+) Contraction time fast Acetylcholine sensitivity is high than skeletal muscles

Histologic Classification Earliest classification based on colour (Red and White fibers) Red colour 1. high vascularity2. Myoglobin high3. Rich in mitochondria4. Resist fatigue on repeated stimulation

White fibers1. Low mitochondria2. Glycolytic metabolism(high glucogen)3. Fatiguability on repeated stimulation

Some fibers were dark staining with many nuclei and abundant sarcoplasm (had regular myofibril arrangement—fibrillenstruktur)

• Innervated by a single ‘en plaque nerve endings’• Rapid twitch fibers

Others with clear and few nuclei and little cytoplasm (had irregular myofibril arrangement –felderstruktur)

• Innervated by diffuse “en grappe “ myoneural endings• Slow tonic fibers

Muscle fibers of EOM classified into orbital and Global zones1. Orbital zone : outer fibers facing the orbit , small in diameter(type 1

and type 2)2. Global zone: Facing the globe and are larger fibers (type 3 4 5 6)

Type 1 : orbital singly innervatedSmall fibers80% of orbital zoneMitochondria abundantRegular myofibrillar arrangementSingly innervatedFibrillen strukturFast twitch fibers

Type 2 : orbital multiply innervated20% of orbital zoneResemble skeletal fast twitch fibers(2c type)Felderstruktur

Type 3:global red singly innervated30% of global zoneHighly oxidative and glycolyticSimilar to skeletal 2a but has high mitochondria

Type 4: Global intermediate singly innervated25% of globalFast twitch(moderate levels of aerobic enzymes)

Type 5 : Global pale singly innervatedFast twitch( modest level of aerobic enzymes but high capacity of anaerobic)

Type 6 Global multiply innervated fibersSlow fibersFelder strucktur

Light Microscopy Muscle Muscle fibers Myofibrils

Endomysium

Perimysium

Epimysium Sarcolemma(plasma membrane) T-tubules

Electron Microscopy(Sarcomere is the basic contractile unit)

Motor innervation of EOM Motor nerve penetrates muscle @ middle and posterior thirds Singly innervated muscle fiber have motor end plate as in skeletal

muscles Multiple innervated muscle fibers have grape like motor end

plate(myelinated or non-myelinated)

Movement of the eye is controlled by Extraocular muscles. There are six extraocular muscles in each eye. 4 recti and 2 obliques.

4 recti are: superior rectus, Inferior rectus, lateral rectus, Medial rectus.

2 oblique muscles: Superior oblique muscle and Inferior oblique muscle.

Superior Rectus Arises: upper part of the tendinous ring superolateral to the

optic foramen. Passes forward & laterally (23 deg with globe AP axis) &

pierces facial sheath of the eyeball. Inserted into sclera 7.7mm posterior to the limbus by

means of a tendon 5.8mm. The muscle is 42mm in length & 9mm in width.

Relations: Superior- levator and frontal nerve which

separate from the roof of the orbit. Inferior- Orbital fat & ophthalmic artery &

nasociliary artery and nerves. Lateral-between superior rectus and lateral rectus

are lacrimal artery and nerve. Medial- ophthalmic artery & nasociliary nerve.

Nerve supply: the superior division of Oculomotor nerve.

Blood supply: The lateral muscular branch of the ophthalmic artery.

Inferior Rectus Arises: Tendinous ring below the optic

foramen. Passes forward and laterally and pierces the

fascial sheath of the eyeball. Inserted into sclera about 6.5mm from the

limbus by a tendon of 5.5mm long. It is 40mm in length and 9mm in width.

Relation

Superior: optic nerve and inferior division of the oculomotor nerve.

Inferior: floor of the orbit. Roofing the maxillary sinus, infraorbital vessels and nerve.

Laterally the nerve to the inferior oblique.

Nerve supply - Inferior division of the occulomotor nerve. Blood supply - the medial muscular branch of ophthalmic

artery.

Medial Rectus

It is the largest of the extraocular muscles. It arises from the medial portion of the tendinous ring and is attached

to the dural sheath of the optic nerve. Passes forward and inserted into the sclera about 5.5mm from the

limbus by the means of tendon 3.7mm long. It is 40mm long & 10.3mm wide, thicker than other EOM

Relation Superior: superior oblique, Ophthalmic artery & its branches, and

the nasociliary nerve. Inferior: the floor of the orbit. Medial: orbital fat, orbital plate of the ethmoid & ethmoidal sinuses. Laterally: orbital fat & optic nerve.

Nerve supply: Inferior division of the oculomotor nerve. Blood supply : medial muscular branch of ophthalmic artery.

Lateral rectus

Arises from the lateral portion of the tendinous ring A small head arises from the greater wing of the sphenoid bone. Inserted into the sclera about 6.9mm from the limbus by the mean

of the tendon 8.8mm long. It is 40.6mm long &9.2mm wide.

Relation Superior: lacrimal nerve and lacrimal artery Inferiorly: floor of the orbit. Medially: abducent nerve and the orbital fat. Nerve supply: Abducent nerve Blood supply: lacrimal artery.

Superior oblique muscle

It is the longest and thinnest EOM arises from the body of the sphenoid bone above and medial to

the optic canal just outside the tendinous ring. Runs forward between the roof and medial wall of the orbital

cavity & reaches the trochlea of SO muscle Trochlea is a thick fibrocartilagenous pulley that is present at the

trochlear fossa of the frontal bone

The muscle then bends downward, backward and laterally and pierces facial sheath of the eyeball, & passes inferior to the SR muscle and then tendon expands in a fan shaped manner and inserts into the sclera posterior to the equator of eyeball.

The muscle becomes tendinous about 10mm before reaching trochlea and remains so

It is 40mm long &10.8mm wide.

Relations Superiorly - roof of orbit Inferiorly-ophthalmic artery and its branches & nasociliary nerve. Nerve supply-trochlear nerve

Inferior oblique muscle It is the shortest muscle of all the EOM, 37mm long & 9.6mm

wide. Arises from the floor of the orbit, posterior to the orbital

margin and just lateral to the nasolacrimal canal. The muscle runs backward, upward and laterally passing

between the floor of the orbit and inferior rectus muscle. It is inserted by a short tendon in the posterior and external

aspect of the sclera.

Relation: superiorly-orbital fat, inferior rectus & the eyeball. Inferiorly- floor of the orbit. Nerve supply- inferior division of oculomotor nerve.

BLOOD SUPPLY OF EOM

Muscular artery, usually two –medial & lateral; branches of Ophthalmic artery. Medial muscular branch supplies MR, IR & IO Lateral muscular branch: LR, SR, Levator ms & SO A branch of Lacrimal artery : LR A branch of Infraorbital artery : IR, IO Veins from EOM correspond to artery & empty into superior & inferior

ophthalmic veins.

Levator palpebrae superioris and Superior palpebral Muscle Levator palpebrae superioris origin: lesser wing of sphenoid bone….passes forwards and below

the roof of orbit and superficial to SR and near to sup fornix ends in lev aponeurosis

Levator aponeurosis(surgical landmark in ptosis surgery : white line)

Insertion into lid: aponeurosis passes down to lids…..anterior to tarsal plate and directly apposed to palpebral portion of orbicularis (some believe the fibers pierce the orbicularis to insert into skin)

Superior palpebral Muscle aka mullers muscle

Arises from inferior aspect of LPS Conjunctiva is in contact with mullers muscle thus pharmacologic agents instilled in

conjunctiva have access to mullers muscle

Stimulate contraction (phenylephrine) inhibit contraction(guanethedine)

Attached to upper border of tarsal plate 3 mm of lid elevation

Nerve supply CN 3 superior division……supplies LPS Sympathetic fibers supplies unstraited superior palpebral muscle ) Axons located in the superior cervical ganglia of that side

Supplied by muscular branch of ophthalmic artery

Actions:LPS raises eyelid…..antagonist is palpebral portion of orbicularisSuperior palpebral muscle raises eyelid in surprise

Horners syndrome (ptosis of 3-4mm)CN3 palsy results in complete ptosis

SR…..7.7mmIR……6.5mmMR…..5.5mmLR……6.9mm

Scleral insertions via tendons fibers resemble scleral tissue (difference: size and longitudinal arrangement of tendon fibers ………….sclera dusky white while tendon glistening white)

Some fibers leave the main tendon and get inserted farther back…………missed in squint surgery

Actions of Recti and Obliques the actions here are WRONG although its from wolffe’s anatomy!!

MUSCLES Primary SecondaryMR AdductionLR AbductionSR Elevation(increases in abdution) Addution IntortionIR Depression(increases in abdution) Adduction ExtortionSO Depression(increases in adduction) Abdution Intortion IO Elevation(Increases in adduction) Abduction Extortion

Centre of rotation The centre of rotation is the fixed point within the globe around which all

other global points move The anterior pole is approximately the centre of the cornea The posterior pole is approximately the centre of macula An axis about which the eye moves extends between the anterior and posterior

poles through the centre of rotation

listing’s plane and Axes of Fick

This anteroposterior axis is referred to as the Y axis of Fick An equatorial plane – Listing’s plane – lying at right angles to either

pole an passing through the centre of rotation, divides the globe into anterior and posterior halves.

The vertical axis in Listing’s Plane is the Z axis of Fick, and the horizontal axis is the X axis of Fick.

Between these two principal axes, Listing’s Plane is composed of an infinite number of oblique axes about which oblique (combined vertical and horizontal ) eye movements occur

Axes of Fick Fick described 3 axes perpendicular to each other, to analyse all movements of

globe around hypothetical centre of rotation X: The globe moves up & down in horizontal X-axis. SR &IR are involved. Y: Torsional movement occur on the Y-axis which transverses the globe from

front to back. SO &IO are involved. Z: The globe rotates left & right on the vertical Z axis. MR & LR are involved.

General principles of EOM The lateral & medial orbital walls makes an angle of 45 degree to each

other. The orbital axis therefore forms an angle of 22.5 degree with both

lateral and medial wall. When the eye is looking straight ahead at a fixed point on the horizon

with the head erect ( primary position of gaze), the vertical axes forms an angle of 23 degree with the orbital axis.

The action of EOM depends on the position of the globe at the time of muscle contraction.

Horizontal Rectus Muscle

When the eye is in the primary position the horizontal recti are purely horizontal movers on the vertical z-axis and have only primary actions.

Medial rectus: sole action is adduction Lateral rectus: sole action is abduction

Vertical Rectus muscles

The vertical recti runs in the line with the orbital axis . They therefore forms an angle of 23 degree with the visual axis.

Superior rectus:

When the globe is abducted 23d, the visual and orbital axes coincide. In this position it can only act as an elevator.

So this is optimal position of globe to test action of SR muscle.

If the globe were adducted 67d, the angle between visual and orbital axis would be 90d and at this position SR can only act as an intortor.

Inferior rectus:

The primary action of the inferior rectus is depression. Secondary action are adduction and extorsion.

when the globe is abducted 23d, the inferior rectus acts as purely as a depressor. And this is the optimal position of the muscle to test this action.

If the globe were adducted 67d, the inferior rectus could only act as an extortor.

Oblique Muscles

The obliques are inserted forming an angle of 51d with the visual axis.

Superior oblique: primary action: intorsion Secondary action: depression and abduction. When the globe is adducted 51d, the visual axis coincides with the orbital

axis. In this position it can only acts as a depressor. When the eye is abducted 39d, the visual axis and the SO make an angle of

90d with each other. In this position the SO can only cause intorsion.

Inferior oblique: primary action- extorsion Secondary action: elevation & abduction When the globe is adducted 51d, the inferior oblique acts only as an elevator. When the eye is abducted 39d, its main action is extorsion.

Antagonist : are muscles of the same eye that move the eye in opposite direction. The

agonist is the primary muscle moving the eye in a given direction. The antagonist are the muscles having opposite actions in the same eye.

For eg, MR & LR, SR & IR, SO & IO are antagonists to each other in the same eye.

Synergists: are the muscles of the same eye that move the eye in the same direction. For eg, the rt SR & rt IO acts synergistically in elevation

Yoke muscles Are pairs of muscles one in each eye, that produce conjugate ocular

movements. For eg, rt MR lt LR

Sherrington’s law Sherrington law of reciprocal innervation states that increased innervation

to an EOM is accompained by a reciprocal decrease in innervation to its antagonist.

This means that when the medial rectus contracts the LR antomically relaxes & viceversa.

Sherrington law applies to both versions and vergences.

Hering’s law Hering law of equal innervation states that during any conjugate eye

movement, equal and simultaneous innervation flows from the brain to the pair of yoke muscles that contract simultaneously.

Eg. During dextroversion rt LR & lt MR muscle receive and equal & simultaneous flow of innervation

EYE movements Adduction Abduction Supraduction

combined contraction of SR and IO Combined relaxation of IR and SO

Infraduction Combined contraction of the IR and SO Incycloduction

Combined contraction of the SO and SR Combined relaxation of IO and IRExcycloduction Combined contraction of the IO and IR Combined relaxation of SO and SR

Binocular movements

Version Vergence

Version

Simultaneous movement of both eyes in the same direction The prime movers in each eye undergo equally graded contraction in accord

with Hering’s law equal innervation; and for each contracting muscle there is an opposite and equally graded antogonist ( Sherringtons’ law)

Version Horizontal versions

Dextroversion Levoversion

Vertical versions Supraversion Infraversion

Cycloversion Dextrocycloversion levocycloversion

Vergence Simultaneous and equal movement of the eyes in opposite directions Prime movers undergo equally graded contraction as per Hering’s law; and

have equally graded relaxing antagonists as per Sherington’s law Horizontal vergences

Convergence Divergence

Cyclovergence Incyclovergence excyclovergence

Anomalies Gracilis orbitis/comes oblique superioris from proximal dorsal surface

of SO Supplied by 4th CN

Accessory LR Homologous to nictating membrane Supplied by 6th

Tensor trochlea Medial border of LPS inserts into trochlea

Muscles Blood supply Nerve supplySR Ophthalmic artery 3rd CNIR Ophthalmic artery 3rd CNMR Ophthalmic artery 3rd CNLR Ophthalmic artery +

lacrimal artery6th CN

IO Infraorbital + Ophthalmic artery

3rd CN

SO Ophthalmic artery 4th CN

END