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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<. 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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THANK YOU