2. Eye Motor Physiology Dr Besharati MD

3. Axes of Fick, Center of Rotation A movement of the eye around a theoretical center of rotation is described with specific terminology. Two helpful concepts are the axes of Fick and Listing's plane. The axes of Fick are designated as x, y, and z. The x-axis is a transverse axis passing through the center of the eye at the equator; vertical rotations of the eye occur 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 occur about this axis.

4. listing's Plane Listing's equatorial plane contains the center of rotation and includes the x and z axes. The y-axis is perpendicular to Listing's plane.

5. Positions of Gaze Primary position is the position of the eyes when fixating straight ahead. Secondary positions are straight up, straight down, right gaze, left gaze Tertiary positions are the 4 oblique positions of gaze: up and right, up and left, down and right, down and left. Cardinal positions are up and right, up and left, right, left, down and right, down and left.

6. Extraocular Muscle Action The 4 rectus muscles have traditionally been thought of as fixed straight strings running directly from the orbital apex to the muscle insertions. The oblique muscles, historically, were thought to simply attach obliquely to the globe. In light of ongoing discoveries that lend support to the active pulley hypothesis, some of the older concepts and descriptions of extraocular muscles and their actions are currently undergoing revision.

7. Arc of contact The point of effective, or physiologic, insertion is the tangential point where the muscle first contacts the globe. The action of the eye muscle may be considered a vector of force that acts at this tangential point to rotate the eye. The length of muscle actually in contact with the globe constitutes the arc of contact. The traditional concepts of arc of contact and muscle plane, based on straight-line 2 dimensional models of orbital anatomy, do not take into account the recently discovered effective muscle pulleys and their effect on linearity of muscle paths.

8. Primary, secondary, and tertiary action With the eye in primary position, the horizontal rectus muscles are purely horizontal movers around the z-axis (the vertical axis), and they have a primary action only. The vertical rectus muscles have a direction of pull that is mostly vertical as their primary action, but the angle of pull from origin to insertion is inclined 23° to the visual axis, giving rise which is defined as any rotation of the vertical corneal meridians.

9. torsion Intorsion is the secondary action for the superior rectus; extorsion is the secondary action for the inferior rectus; and adduction is the tertiary action for both muscles. Because the oblique muscles are inclined 51 0 to the visual axis, torsion is their primary action. Vertical rotation is their secondary and horizontal rotation their tertiary action

10. Field of action The term field of action has Been used in 2 ways to describe Entirely separate and distinct concepts: to indicate the direction of rotation of the eye from primary position if the muscle was the only one to contract. to refer to the gaze position (one of the cardinal positions) in which the effect of the muscle is most readily observed.

11. direction of rotation For the LR, the direction of rotation and the gaze position are both abduction; for the MR, they are both adduction. However, the direction of rotation and the gaze position are not the same for all muscles. For example, the IO, acting alone, is an abductor and elevator, pulling the eye up and out; but its elevation action is best observed in adduction. Similarly, the SO, acting alone, is an abductor and depressor, pulling the eye down and out; but its depression action is best observed in adduction.

12. fields of action The clinical significance of fields of action is that a deviation (strabismus) that increases with gaze in some directions may result from the weakness of the muscle normally pulling the eye in that direction. For example, an acute left sixth nerve palsy in an adult can be 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's answer: Looking to the left (the field of action of the left lateral rectus )

13. Changing muscle action with different gaze positions The gaze position determines the effect of EOM contractions on the rotation of the eye. The different positions are primary gaze and the 6 cardinal positions. In each of these 6 cardinal positions, each of the 6 EOM has different effects on the eye rotation based on the relationship between the visual axis of the eye and the orientation of the muscle plane to the visual axis. Each cardinal position minimizes the angle between the visual axis and the muscle plane of the muscle being tested, thus maximizing the horizontal effect of the medial or lateral rectus or the vertical effect of the SR, IR, SO, or IO. By having the patient move the eyes to the 6 cardinal positions, the clinician can isolate and evaluate the ability of each of the 6 EOM to move the eye

14. muscle actions With the eye in primary position, the horizontal rectus muscles share a common horizontal plane that contains the visual axis.The relative strength of the horizontal rectus muscles can be assessed by observing the horizontal excursion of the eye as it moves medially from primary position to test the MR and laterally to test the LR. The muscle actions of the vertical rectus muscles and the oblique muscles are more complex because, in primary position, the muscle axes are not parallel with the visual axis

15. muscle actions In primary position, the superior and inferior rectus muscle planes form an angle of 23° 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, and Adduction around the z-axis. The relative strength of the SR muscle can be most readily observed by aligning the visual axis parallel to the muscle plane axis-that is, when the eye is rotated 23° in abduction. In this position, the SR becomes a pure elevator and its elevating action is maximal. To minimize the elevation action of the SR, the visual axis should be perpendicular to the muscle axis at a position of 67° of adduction. In this position, the SR would become a pure intorter. Because the globe cannot be adducted this far, there is still a SR elevating action in maximal adduction.

16. muscle actions The action of the IR is similar to that of the SR. Because the IR is attached to the globe inferiorly, its action from primary position is primarily depression, secondarily extorsion, and tertiarily adduction. Its action as a depressor is maximally demonstrated in 23° of abduction and minimized in adduction.

17. muscle actions The 2 oblique muscle planes course in a direction from the anteromedial aspect of the globe to the posterolateral, forming an angle of approximately 51 ° with the visual axis.Because of the large angle formed in primary position, the Primary action of the SO is intorsion, with a secondary depression and tertiary abduction.

18. muscle actions As the muscle plane is aligned with the visual axis in extreme adduction, the SO action can be seen as a depressor. With Abduction of the eye, the visual axis becomes perpendicular to the muscle plane, and the muscle action is one of intorsion.

19. muscle actions The action of the IO is similar to that of the SO. In primary position, the primary action is extorsion, with secondary elevation and tertiary abduction. The IO action as an elevator is best seen in adduction and, as an extorter, In abduction.

20. Eye Movements

21. Motor Units An individual motor nerve fiber and its several muscle fibers constitute a motor unit. EMG records motor unit electrical activity. An EMG is useful in investigating normal and abnormal innervation and can be helpful in documenting paralysis, recovery from paralysis, and abnormalities of innervation in myasthenia gravis and muscle atrophy. However, this test is not helpful in ordinary comitant strabismus.

22. Recruitment during fixation or following movement As the eye moves farther into abduction, more and more lateral rectus motor units are activated and brought into play by the brain to help pull the eye. This process is called recruitment. In addition, as the eye fixates farther into abduction, the frequency of activity of each motor unit increases until it reaches a peak.

23. Monocular Eye Movements Ductions are monocular rotations of the eye. Adduction is movement of the eye nasally; Abduction is movement of the eye temporally. Elevation is an upward rotation of the eye; Depression is a downward rotation of the eye. Intorsion is defined as a nasal rotation of the superior portion of the vertical corneal meridian. Extorsion is a temporal rotation of the superior portion of the vertical corneal meridian

24. Ductions The following terms relating to the muscles used in monocular eye movements are also important: Agonist: the primary muscle moving the eye in a given direction Synergist: the muscle in the same eye as the agonist that acts with the agonist to produce a given movement (eg, the IO is a synergist with the agonist SR for elevation of the eye) Antagonist: the muscle in the same eye as the agonist that acts in the direction opposite to that of the agonist; the MR and LR are antagonists

25. Sherrington's law Sherrington's law of reciprocal innervation states that increased innervation and contraction of a given EOM are accompanied by a reciprocal decrease in innervation and contraction of its antagonist. For example, as the right eye abducts, the right LR receives increased innervation while the right MR receives decreased innervation.

26. Binocular Eye Movements When binocular eye movements are conjugate and the eyes move in the same direction, such movements are called versions. When the eye movements are disconjugate and the eyes move in opposite directions, such movements are known as vergences (eg, convergence and divergence).

27. Versions Right gaze (Dextroversion) is movement of both eyes to the patient'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 of the vertical corneal meridian moves to the patient's right. Levocycloversion is movement of both eyes so that the superior portion of the vertical corneal meridian rotates to the patient's left.

28. yoke muscles The term yoke muscles is used to describe 2 muscles (1 in each eye) that are the prime movers of their respective eyes in a given position of gaze. For example, when the eyes move or attempt to move into right gaze, the right LR and the left MR are simultaneously innervated and contracted. These muscles are said to be "yoked" together.

29. yoke muscles Each EOM in 1 eye has a yoke muscle in the other eye. Because the effect of a muscle is usually best seen in a given direction of gaze, the concept of yoke muscles is used to evaluate the contribution of each EOM to eye movement. Hering's law of motor correspondence states that equal and simultaneous innervation flows to yoke muscles concerned with the desired direction of gaze. The most useful application of this law is in evaluating binocular eye movements and, in particular, the yoke muscles involved.

30. Hering's law Hering's law has important clinical implications, especially when the practitioner is dealing with a paralytic or restrictive strabismus. Because the amount of innervation to both eyes is always determined by the fixating eye, the angle of deviation varies according to which eye is fixating. When the normal eye is fixating, the amount of misalignment is called the primary deviation. When the paretic or restrictive eye is fixating, the amount of misalignment is called the secondary deviation. The secondary deviation is larger than the primary deviation because of the increased innervation necessary to move the paretic or restrictive eye to the position of fixation.

31. Hering's law Hering's law is also necessary to explain the following example. If a Patient has a right SO paresis and uses the right eye to fixate an object that is located up and to the patient's left, the innervation of the right IO required to move the eye into this gaze position is reduced because the right IO does not have to overcome the normal antagonistic effect of the right SO. Therefore, according to Hering's law, less innervation is also received by the right IO muscle's yoke muscle, the left SR. This decreased innervation could lead to the incorrect impression that the left SR is paretic

32. Vergences Convergence is movement of both eyes nasally relative to a given starting position; Divergence is movement of both eyes temporally relative to a given starting position. Incyclovergence is a rotation of both eyes so that the superior portion of each vertical corneal meridian rotates nasally; Excyclovergence is a rotation of both eyes so that the superior pole of each vertical corneal meridian rotates temporally. Vertical vergence movement, though less frequently encountered, can also occur: 1 eye moves upward and the other downward

33. Tonic convergence The constant innervational tone to the EOM when a person is awake and alert. Because of the anatomical shape of the bony orbits and the Position of the rectus muscle origins, the alignment of the eyes under complete muscle paralysis is divergent. Therefore, convergence tone is necessary in the awake state to maintain straight eyes even in the absence of strabismus.

34. Accommodative convergence Accommodative convergence of the visual axes Part of the synkinetic near reflex. A fairly consistent increment of accommodative convergence (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 important cause of strabismus. With an abnormally high AC/ A ratio, the excess convergence tends to produce esotropia during accommodation on near targets. An abnormally low AC/ A ratio tends to make the eyes exotropic when the person looks at near targets.

35. Other Convergence Voluntary convergence a conscious application of the near synkinesis. Proximal (instrument) convergence an induced convergence movement caused by a psychological awareness of near; this movement is particularly apparent when a person looks through an instrument such as a binocular microscope.

36. Fusional convergence A movement to converge and position the eyes so that similar retinal images project on corresponding retinal areas. Fusional convergence is accomplished without changing the refractive state of the eyes and is prompted by bitemporal retinal image disparity

37. Fusional divergence The only clinically significant form of divergence. It is an optomotor reflex to diverge and align the eyes so that similar retinal images project on Corresponding retinal areas. Fusional divergence is accomplished without changing the refractive state Of the eyes and is prompted by binasal retinal image disparity.

38. Supra nuclear Control Systems for Eye Movement There are several supra nuclear eye movement systems. The saccadic system generates all fast (up to 400 o -500 o /sec) eye movements, such as eye movements of refixation. This system functions to place the image of an object of interest on the fovea or to move the eyes from one object to another. Saccadic movements require a sudden strong pulse of force from the EOM to move the eye rapidly against the viscosity produced by the fatty tissue and the fascia in which the globe lies. The study of saccadic velocity is of practical value in determining paresis of muscles and abnormal innervation.

39. Supra nuclear Control Systems for Eye Movement The smooth pursuit system generates all following, or pursuit, eye movements. Pursuit latency is shorter than for saccades, but the maximum peak velocity of these slow Pursuit movements is limited to 30 o - 60 o /sec. The vergence system controls disconjugate eye movement, as in convergence or divergence. Supranuclear control of vergence eye movements is not yet fully understood.

40. nonoptic reflex systems The nonoptic reflex systems integrate eye movements and body movements. The most clinically important of these systems is the labyrinthine reflex system, which involves the semicircular canals of the inner ears. Other, less important, systems involve the utricle and saccule of the inner ears. The cervical, or neck, receptors also provide input for this nonoptic reflex control.