Eye Motor Physiology Dr Besharati MD Axes of Fick, Center of Rotation A movement of the eye around a...
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Transcript of Eye Motor Physiology Dr Besharati MD Axes of Fick, Center of Rotation A movement of the eye around a...
Axes of Fick, Center of RotationA movement of the eye around a theoretical center ofrotation is described with specific terminology. Two helpful concepts are the axes of Fick and Listing'splane. The axes of Fick are designated as x, y, and z. The x-axis is a transverse axis passing through the centerof the eye at the equator; vertical rotations of the eyeoccur about this axis. The y-axis is a sagittal axis passing through the pupil;involuntary torsional rotations occur about this axis. The z-axis is a vertical axis; horizontal rotations occurabout this axis.
listing's Plane
Listing's equatorialplane contains thecenter of rotation andincludes the x and zaxes. The y-axis isperpendicular toListing's plane.
Positions of GazePrimary position is the position ofthe eyes when fixating straight ahead.Secondary positions are straight up,straight down, right gaze, left gazeTertiary positions are the 4 obliquepositions of gaze: up and right, upand left, down and right, down and left.Cardinal positions are up and right, upand left, right, left, down and right,down and left .
Extraocular Muscle ActionThe 4 rectus muscles havetraditionally been thought of as fixedstraight strings running directly fromthe orbital apex to the muscleinsertions. The oblique muscles, historically, werethought to simply attach obliquely to theglobe. In light of ongoing discoveries thatlend support to the active pulleyhypothesis, some of the older conceptsand descriptions of extraocular musclesand their actions are currentlyundergoing revision.
Arc of contactThe point of effective, or physiologic, insertionis the tangential point where the muscle firstcontacts the globe. The action of the eyemuscle may be considered a vector of forcethat acts at this tangential point to rotate theeye. The length of muscle actually in contactwith the globe constitutes the arc of contact.The traditional concepts of arc of contact andmuscle plane, based on straight-line 2dimensional models of orbital anatomy, do nottake into account the recently discoveredeffective muscle pulleys and their effect onlinearity of muscle paths.
Primary, secondary, and tertiary action
With the eye in primary position,the horizontal rectus muscles arepurely horizontal movers aroundthe z-axis (the vertical axis), andthey have a primary action only.The vertical rectus muscles have adirection of pull that is mostlyvertical as their primary action,but the angle of pull from originto insertion is inclined 23° to thevisual axis, giving rise which is definedas any rotation of the vertical cornealmeridians.
torsionIntorsion is the secondary action forthe superior rectus; extorsion is the secondary action forthe inferior rectus; and adduction isthe tertiary action for both muscles.Because the oblique muscles areinclined 51 0 to the visual axis,torsion is their primary action.Vertical rotation is their secondaryand horizontal rotation their tertiaryaction
Field of actionThe term field of action hasBeen used in 2 ways to describeEntirely separate and distinctconcepts:• to indicate the direction ofrotation of the eye from primaryposition if the muscle was theonly one to contract.• to refer to the gaze position
(oneof the cardinal positions) in whichthe effect of the muscle is mostreadily observed.
direction of rotationFor the LR, the direction of rotation and thegaze position are both abduction; for theMR, they are both adduction. However, thedirection of rotation and the gaze positionare not the same for all muscles. Forexample, the IO, acting alone, is an abductorand elevator, pulling the eye up and out; butits elevation action is best observed inadduction. Similarly, the SO, actingalone, is an abductor and depressor, pullingthe eye down and out; but its depressionaction is best observed in adduction.
fields of actionThe clinical significance of fields of action is that a deviation(strabismus) that increases with gaze in some directions may resultfrom the weakness of the muscle normally pulling the eye in thatdirection. For example, an acute left sixth nerve palsy in an adult canbe diagnosed by asking the patient with diplopia by 3 questions:1. Is the diplopia horizontal or vertical? Patient's answer: Horizontal 2-Is the diplopia worse at distance or at near? Patient's answer: distance (implicating the lateral recti )3. Is the diplopia worse on looking to the left or to the right? Patient'sanswer: Looking to the left (the field of action of the left lateral rectus )
Changing muscle action with different gaze positions
The gaze position determines the effect of EOM contractions on the rotationof the eye. The different positions are primary gaze and the 6 cardinalpositions. In each of these 6 cardinal positions, each of the 6 EOM hasdifferent effects on the eye rotation based on the relationship between thevisual axis of the eye and the orientation of the muscle plane to the visualaxis. Each cardinal position minimizes the angle between the visual axis andthe muscle plane of the muscle being tested, thus maximizing the horizontaleffect of the medial or lateral rectus or the vertical effect of the SR, IR, SO, orIO. By having the patient move the eyes to the 6 cardinal positions, theclinician can isolate and evaluate the ability of each of the 6 EOM to move
theeye
muscle actionsWith the eye in primary position, thehorizontal rectus muscles share a commonhorizontal plane that contains the visualaxis .The relative strength of the horizontalrectus muscles can be assessed byobserving the horizontal excursion of theeye as it moves medially from primaryposition to test the MR and laterally to testthe LR.The muscle actions of the vertical rectusmuscles and the oblique muscles are morecomplex because, in primary position, themuscle axes are not parallel with the visualaxis
muscle actionsIn primary position, the superior and inferior rectus muscle planes form an angle of23° with the visual axis (y-axis) and insert slightly anterior to the z-axis .Therefore, from primary position, the contraction of the SR has 3 effects:Primary elevation around the x-axis, secondary intorsion around the y-axis, andAdduction around the z-axis. The relative strength of the SR muscle can be most readily observed by aligning thevisual axis parallel to the muscle plane axis-that is, when the eye is rotated 23° inabduction. In this position, the SR becomes a pure elevator and its elevating action ismaximal. To minimize the elevation action of the SR, the visual axis should beperpendicular to the muscle axis at a position of 67° of adduction. In this position, theSR would become a pure intorter. Because the globe cannot be adducted this far,there is still a SR elevating action in maximal adduction.
muscle actionsThe action of the IR is similar tothat of the SR. Because the IR isattached to the globe inferiorly,its action from primary positionis primarily depression,secondarily extorsion, andtertiarily adduction . Its actionas a depressor is maximallydemonstrated in 23° ofabduction and minimizedin adduction.
muscle actions
The 2 oblique muscle planes coursein a direction from the anteromedialaspect of the globe to theposterolateral, forming an angle ofapproximately 51 ° with the visualaxis .Because of the large angleformed in primary position, thePrimary action of the SO is intorsion,with a secondary depression andtertiary abduction.
muscle actions
As the muscle plane is alignedwith the visual axis in extremeadduction, the SO action canbe seen as a depressor. WithAbduction of the eye, thevisual axis becomesperpendicular to the muscleplane, and the muscle action isone of intorsion.
muscle actions
The action of the IO is similarto that of the SO. In primaryposition, the primary action isextorsion, with secondaryelevation and tertiaryabduction. The IO action as anelevator is best seen inadduction and, as an extorter,In abduction.
Motor UnitsAn individual motor nerve fiber and itsseveral muscle fibers constitute amotor unit.EMG records motor unit electricalactivity. An EMG is useful ininvestigating normal and abnormalinnervation and can be helpful indocumenting paralysis, recovery fromparalysis, and abnormalities ofinnervation in myasthenia gravisand muscle atrophy. However, this
testis not helpful in ordinary comitantstrabismus.
Recruitment during fixation or following movement
As the eye moves farther intoabduction, more and more lateralrectus motor units are activatedand brought into play by the brainto help pull the eye. This process iscalled recruitment. In addition, asthe eye fixates farther intoabduction, the frequency of activityof each motor unit increases untilit reaches a peak.
Monocular Eye MovementsDuctions are monocular rotations of theeye. Adduction is movement of the eyenasally; Abduction is movement of theeye temporally. Elevation is an upwardrotation of the eye; Depression is adownward rotation of the eye. Intorsionis defined as a nasal rotation of thesuperior portion of the vertical cornealmeridian. Extorsion is a temporalrotation of the superior portionof the vertical corneal meridian
DuctionsThe following terms relating to the musclesused in monocular eye movements are alsoimportant:Agonist: the primary muscle moving the eyein a given directionSynergist: the muscle in the same eye as theagonist that acts with the agonist to produce agiven movement (eg, the IO is a synergist withthe agonist SR for elevation of the eye)Antagonist: the muscle in the same eye asthe agonist that acts in the direction oppositeto that of the agonist; the MR and LR areantagonists
Sherrington's lawSherrington's law of reciprocalinnervation states that increasedinnervation and contractionof a given EOM are accompaniedby a reciprocal decrease in innervationand contraction of its antagonist. Forexample, as the right eye abducts, theright LR receives increased innervationwhile the right MR receives decreasedinnervation.
Binocular Eye Movements
When binocular eye movements areconjugate and the eyes move in thesame direction, such movementsare called versions. When the eye movements aredisconjugate and the eyes move inopposite directions, suchmovements are known as vergences(eg, convergence and divergence).
Versions
Right gaze (Dextroversion) is movement of both eyes to thepatient's right. Left gaze (Levoversion) is movement of both eyes to the patient's left.Elevation, (Sursumversion), is an upward rotation of both eyes;Depression, (Deorsumversion), is a downward rotation of both eyes.Dextrocycloversion, both eyes rotate so that the superior portion ofthe vertical corneal meridian moves to the patient's right.Levocycloversion is movement of both eyes so that the superiorportion of the vertical corneal meridian rotates to the patient's left.
yoke musclesThe term yoke muscles is used todescribe 2 muscles (1 in each eye)that are the prime movers of theirrespective eyes in a given positionof gaze. For example, when theeyes move or attempt to move intoright gaze, the right LR and the leftMR are simultaneously innervatedand contracted. These muscles aresaid to be "yoked" together.
yoke musclesEach EOM in 1 eye has a yoke muscle in the other eye.Because the effect of a muscle is usually best seen in a givendirection of gaze, the concept of yoke muscles is used toevaluate the contribution of each EOM to eye movement. Hering's law of motor correspondence states that equal andsimultaneous innervation flows to yoke muscles concernedwith the desired direction of gaze. The most useful applicationof this law is in evaluating binocular eye movements and, inparticular, the yoke muscles involved.
Hering's lawHering's law has important clinical implications, especially when thepractitioner is dealing with a paralytic or restrictive strabismus.Because the amount of innervation to both eyes is always determinedby the fixating eye, the angle of deviation varies according to whicheye is fixating. When the normal eye is fixating, the amount ofmisalignment is called the primary deviation. When the paretic orrestrictive eye is fixating, the amount of misalignment is called thesecondary deviation. The secondary deviation is larger than theprimary deviation because of the increased innervation necessary tomove the paretic or restrictive eye to the position of fixation.
Hering's lawHering's law is also necessary to explain the following example. If aPatient has a right SO paresis and uses the right eye to fixate an objectthat is located up and to the patient's left, the innervation of the rightIO required to move the eye into this gaze position is reduced becausethe right IO does not have to overcome the normal antagonistic effectof the right SO. Therefore, according to Hering's law, less innervation isalso received by the right IO muscle's yoke muscle, the left SR. Thisdecreased innervation could lead to the incorrect impression that theleft SR is paretic
VergencesConvergence is movement of both eyes nasally relative toa given starting position; Divergence is movement of both eyes temporally relativeto a given starting position. Incyclovergence is a rotation of both eyes so that the superiorportion of each vertical corneal meridian rotates nasally;Excyclovergence is a rotation of both eyes so that the superiorpole of each vertical corneal meridian rotates temporally.Vertical vergence movement, though less frequentlyencountered, can also occur: 1 eye moves upward and theother downward
Tonic convergence The constant innervational tone to the EOMwhen a person is awake and alert. Because
ofthe anatomical shape of the bony orbits andthe Position of the rectus muscle origins, thealignment of the eyes under completemuscle paralysis is divergent. Therefore,convergence tone is necessary in the awakestate to maintain straight eyes even in theabsence of strabismus.
Accommodative convergence Accommodative convergence of the visual axes Part of the synkineticnear reflex. A fairly consistent increment of accommodativeconvergence (AC) occurs for each diopter of accommodation(A), giving the accommodative convergence/accommodation (AC/A)ratio.Abnormalities of this ratio are common, and they are an importantcause of strabismus.With an abnormally high AC/ A ratio, the excess convergence tends toproduce esotropia during accommodation on near targets. Anabnormally low AC/ A ratio tends to make the eyes exotropic when theperson looks at near targets.
Other ConvergenceVoluntary convergence aconscious application of the nearsynkinesis.Proximal (instrument) convergencean induced convergence movementcaused by a psychologicalawareness of near; this movementis particularly apparent when aperson looks through an instrumentsuch as a binocular microscope.
Fusional convergence
A movement to converge and position the eyesso that similar retinal images project oncorresponding retinal areas. Fusionalconvergence is accomplished without changingthe refractive state of the eyes and is promptedby bitemporal retinal image disparity
Fusional divergence
The only clinically significant form of divergence. Itis an optomotor reflex to diverge and align the eyesso that similar retinal images project onCorresponding retinal areas. Fusional divergence isaccomplished without changing the refractive stateOf the eyes and is prompted by binasal retinalimage disparity.
Supra nuclear Control Systems for Eye Movement
There are several supra nuclear eye movement systems. The saccadicsystem generates all fast (up to 400o-500o/sec) eye movements, suchas eye movements of refixation. This system functions to place theimage of an object of interest on the fovea or to move the eyes fromone object to another. Saccadic movements require a sudden strongpulse of force from the EOM to move the eye rapidly against theviscosity produced by the fatty tissue and the fascia in which the globelies. The study of saccadic velocity is of practical value in determiningparesis of muscles and abnormal innervation.
Supra nuclear Control Systems for Eye Movement
The smooth pursuit system generates allfollowing, or pursuit, eye movements.Pursuit latency is shorter than forsaccades, but the maximum peakvelocity of these slow Pursuitmovements is limited to 30o - 60o/sec.The vergence system controlsdisconjugate eye movement, as inconvergence or divergence.Supranuclear control of vergence eyemovements is not yet fully understood.
nonoptic reflex systems
The nonoptic reflex systems integrateeye movements and body movements.The most clinically important of thesesystems is the labyrinthine reflex system,which involves the semicircular canals ofthe inner ears. Other, less important,systems involve the utricle and sacculeof the inner ears. The cervical, or neck,receptors also provide input for thisnonoptic reflex control.
