eom physiology archana

8/2/2019 Eom Physiology Archana

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Dr. Archana Preethi

Physiology of Extraocularmuscles


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Extra ocular

musclesStriated,voluntary muscles.

6 in number in each eye.Exhibit greatest structural

diversity.

Charecterised as beingweak,fast,fatigue resistant.

sarcomere-fuctional unit


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Physiological

propertiesUnder isometric

conditions,they have shortcontraction(time req. toreach peak twitch

force)and halfrelaxation(time from peak

to half peak twitch


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Physiological

propertiesMax. forces obtained arejust in fraction to those

obtained from limbmuscles.

Shallow contractionsLow twitch/tetanus ratio

High oxygen demand


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Differences from otherskeletal muscle

Small diameter

Rich nerve and blood supply

Enormous fibroelastic tissue

Faster ca+2 transients,due to abundantSR.

Presence of both slow&fast fibers.

Diff.in kinetics ofactomyosin,dueto

expression of EOM specific myosin.


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Layered organisation ofE.O.M

1)ORBITAL 2)GLOBAL

ORBITALOuter,adjacent to the

periorbital&orbital bone

2 types of smallfibers;OSIF(80%)&OMIF(20%)

At either end, myomyous


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Layered organisation ofE.O.M

GLOBAL

Inner adjacent to optic nerve&eye

One MIMF&3 SIMF.

Extends to full musclelength&inserts through a well

defined tendon into sclera to


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Motor units of EOM

Small,1:3/4 in small mf&1:10/more inlarge muscle fibers,in skeletal mf-1:125

This allows precise targeting of eyemovement&fine adjustment to preventdiplopia


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Innervation of EOM

As,globe is a small fixed loadcompensatory stretch reflexes are not

needed. AFFERENTS:

1)MUSCLE SPINDLE:small,3-l0 intrafusal

mf,thin capsule,no nuclear bagregion,distributed in prox.&distal 1/3 ofmuscle.

2)GOLGI TENDON ORGAN


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3)SPIRAL ENDING

4)TEMP. SENSITIVE RECEPTORS

5)RECEPTORS FOROCULOCARDIAC&OCULORESPIRATORYREFLEXES

EFFERENTS

1)GAMMA MOTOR NEURONS:innervatesslow fibers as multiple n.

endings(engrappe endings),cond. Vel. 6-


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Basic kinematics

POSITIONS OF GAZE

PRIMARY : defined by scobee,with head

erect,object of regard is at infinity &liesat intersection of the sagital plane ofhead&a horiz. Plane passing throughcenters of rotation of two eyeballs.

SECONDARY :supraversion,infraversion,dextroversion,levoversion.

TERTIARY : comb. Of vertical &horiz.


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Center of rotation

Hypothetical point around which eyeballperfoms rotatory movements.

Center of rotation moves in a semicirclein the plane of rotation,this locus iscalled SPACECENTROID.

It is considered as fixed point. In pri. Position,C.R lies 13.5 mm behind

apex,when measured on line of sight.

In reality,a little behind the actual


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Cardinal positions of gaze

Allow ex. Of each of 12 EOM.

6,dextroversion,

levoversion,

Detroelevation,

levoelevation,

dextrodepression&

levodepression.


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Ficks axes&Listings plane

3 AXES,perpendicular to eachother,intersect at C.R of the eye.

X(hori) elevation/depresionY(AP) - extorsion/intorsion

Z(vertical)-adduction/abduction

X&Z axes are in same planeLISTINGSPLANE


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Traslatory&rotatorymovements

TRANSLATORY - movements of eyeballas a whole in the orbit,with eyeremaining in pri. Position of gaze.

Include upwards/downwards,ant/post&sideways.

ROTATORY around 3 ficks axes POSITION OF REST:exotropia of 2.25

degrees in each eye &inc. with age.


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Mechanics of action of EOM

1)CROSS SECTIONAL AREA OF MUSCLE

Force is directly proportional to

crossectional areaHori.recti have max. c.s.a

Vertical recti have 75%&obliques have

50% of the size of horiz. Muscles.

In general,hori. Muscles MR&LR havesimilar size,there by balancing

opposing forces.


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Mechanics of action of EOM

2)LENGTH OF MUSCLE

For normal amp. Of rotation(45-50

deg.each away from pri.Position)approx. 10 mm(25% of n.resting length)change is required ineach direction.

As muscle contracts, site of insertionmoves closer to the origin site.

resection usually reduce amplitude ofrotation


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3)ARC OF CONTACT

Distal portion of each EOM,lies in flush

against globe for variable dist. Before itblends into sclera (ANATOMICINSERTION)

The point where muscle /tendon 1sttouches globe isTANGENTIALPOINT/PHYSIOLOGIC/EFFECTIVEINSERTIONof muscle

Tan ent drawn to this oint indicates


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Arc of contact is the dist. On slceralcircumference between T & A on sclera

LR -15mmMR-6

SR-8.4

IR-9

SO-5

IO-17


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So that TORQUE(force ofrotation)remains same.

Arc of contact is zero,where T coincideswith A

RESCESSION WEAKENS ACTION DUE

DEC. IN EFFECTIVELENGTH&REDUCTION OF ARC OFCONTACT,ADVANCEMENT OF MUSCLEHAS A STRENGTHING EFFECT.


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4)MUSCLE PLANE

Imaginary plane passing through O,A

,T,C&long axis of muscle. Angle formed by muscle plane with

visual direction /line of fixation depends

on position of globe. Paired muscles MR&LR(muscle plane

coincide with horiz. Plane of the globe)

For SR&IR(MP makes an angle of 23


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5)MUSCLE AXIS OF ROTATION

Perpendicular to muscle plane erected

in center of rotation. ACTIONS OF EOM

PRI.ACTION : major action when eye is

in pri.position.The additional actions(other than

pri.action) in the pri.position are called

subsidary actions(sec.&ter.actions)


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HORIZANTALRECTI

Axis of rotation coincides with z axis of

globe. In pri.position,act as purely horiz.

Movers,LR-ABDUCTION&MR-ADDUCTION

VERTICALRECTI SR&IR-common muscle plane,coincide

with orbital axis making 23 deg.from

optical axes.


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OBLIQUEMUSCLES

Inserted behind the equator,makes 51

deg. Angle with optical axis. SO

Adducted 51 deg.-depression

Abducted 39 deg.-only intortion

IO

Adducted 51 deg.-elevation


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AGONIST

Any particular EOM,producing specific

ocular movement SYNERGISTS

Two muscles moving eye in same

direction ANTAGONISTS

Having opp.action in same eye.


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C/LANTAGONISTS(ANTAGONISTOFYOKEMUSCLES)

Pair of muscles (from each eye)havingopp.action eg.rt. LR&lt. LR


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LAWS OF OCULAR MOTILITY

DONDERSLAW:states that for eachtertiary position, there is one and onlyone orientation of the vertical andhorizontal meridians of retina. Thisorientation depends solely upon theamount of elevation( Z-axis) and

horizontal(X-axis) movement and isindependent of the path by which thisposition was arrived. There is norotation around the anteriopaoterior(Y-

axis) i.e, no torsion or twist occurs.


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Listings Law

When we have coordinate systems, onefixed (orbital ) and one movable (globe),there is an angle produced when themoved system is compared to the fixedsystem. Such motion can occur withoutany true torsional movement having

actually been made across Y-axis.


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Herings law of equalinnervation

Also called HERINGS LAW OF MOTORCORRESPONDENCE.

Equal&simultaneous innervation flowsfrom brain to a pair of yokemuscles,which contractsimultaneouslyin diff. binocular vision

Eg.RLR&LMR

CLINICALIMP.

SEC.DEVIATION-in paralytic squint, of


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SHERRINGTONS LAW OFRECIPROCAL INNERVATION

During ocular motility,an inc. flow ofinnervation to contracting agonist isassociated with dec.flow of innervationto relaxing,antagonist.eg.during DV,incflow to RLR&LMR,dec. flow to RMR&LLR.

CLINICAL IMP.

Certain pathological states like DUANESRETRACTIONSYND. &RETRACTIONNYSTAGMUS,contraction of the

antagonist occur instead of relaxation.


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OCULAR MOVEMENTS

1)MONOOCULAR(DUCTIONS)

Adduction

Abduction

Supraduction(sursumduction)

Infraduction(deosursumduction)

Incycloduction(intorsion)

Excyclodction(extorsion)


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2)BINOCULAR

A. VERSIONS B.VERGENCES

VERSIONS conjugate movements

Simultaneous,synchronus,symmetricmotion of both eyes in the same

direction. Voluntary/involuntary

Involuntary-due to

optical,acoustic&other stimuli


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Dextroversion

Levoversion

Supraversion

Infraversion

Dextroelevation

Dextrodepression

Levoelevation

Levodepression


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Depending on specific pattern of eyemovements

1)SACCADES : rapid conjugatemovements,to bring image quickly onto the fovea.

Vol./invol. Eg.:-command random movements

Vol. refixation saccades

Sensory evoked


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Have a pre programed velocity,nochange occurs during movement

Long delay of 200msec from stimulus toexecution.

Vel. Is 100 deg.to 700 deg./sec

2)SMOOTHPURSUITS/FOLLOWINGMade when tracking a moving object

Func. Is to match eye&target vel.


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3)POSITIONMAINTENANCEMOVEMENTS

Help to maintain a specific gaze by

rapid micromovements called FLICKS &slow micromovements called DRIFTS.

4)STABILIZATIONMOVEMENTS

DYNAMIC due to vestibular system,ifhead is turned rt,eyes turn lt.

TONIC gravity&other lin. Accelerations

also influence eye position by


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VERGENCES

Disjugate,synchronous,symmetric of

both eyes in opp. Dir.Tonic movements,much slower than

versions(80-250 deg/sec)

May be a vol./optomotor reflexTYPES

CONVERGENCE


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CONVERGENCE

Disjugate,where both eyes rotate

inward Allow bifoveal single vision to be

maintained at any fixation dist.

Remains same throughout life Power can be inc. by exercises

ANGLE OF CONVERGENCE


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NEARPOINTOFCONVERGENCE

Point at which foveal lines of sight

intersect,when max. convergece isattained

It is the closet point at which object is

seen singly. Always closer than NPA

TYPES

VOLUNTARY at will,not a part of


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FUSIONAL/POSITIVEFUSIONALCONVERGENCE

To ensure similar retinal images areprojected onto corresponding retinalareas

Stimulus-bitemporal retinal imagedisparity

Optomotor reflex-forms a kind of motorfusion


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ACCOMODATIVE

Stimulus blured retinal image

Part of triad synkinetic near reflexcomplex,other twoare,accomodation&miosis

PROXIMAL Induced by proximity of the object

Stimulus when sub. Believes he is

looking at near object


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DIVERGENCE

FUSIONAL/NEGATIVEFD

Stimulus binasal retinal disparity

Optomotor reflex

Amp. N near-8-12 D

Dist -4-6 D

End point for near-when acc. Is relaxedto prevent retinal disparity,causingblurring of vision

-

V ti l


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Vertical vergence

Disjugate vertical moovements of 2eyes in opp. Dir.

Stimulus retinal image disparity Amp. Is smaller than hori. Vergence

To correct vertical heterophoria

avg.vertical FV is 3-8D.

+ve vertical divergence-when rt. Eyerotates up in relation to lt.& vice versa


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cyclovergence

Disjugate torsional movements,wherevertical meridians of 2 eyes move inopp. Dir.

Stimulus retinal image disparity,

To correct cyclophoria

6-10deg. Amp. For incyclovergence&4-8deg. For excyclovergence

Slow ,low amp.

Cortical centers for


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Cortical centers forsaccades

Frotal oculomotor area for bothhori.&vertical saccades

frontal eye field(FEF) Supplementary eye field(SEF)

Dorsolateral prefrontal cortex(DLPFC)

Post. Eye field(PEF)

SUBCORTICAL : HORIZANTAL GAZE

Paramedian pontine reticular


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