Muscles of the Eyeball (Extra Ocular Muscles) Prof. Dr. Imran Qureshi

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1 Muscles of the Eyeball (Extra Ocular Muscles) Prof. Dr. Imran Qureshi There are six extrinsic muscles of the eyeball, namely the (S), Medial (M), (I), & Lateral (L) recti, and (SO) and (IO) Obliques. In the diagram given below, each muscle is represented as a heavy line placed in a position which best indicates its action. For easier analysis of the action or actions) of each muscle, the axes of elevation and depression, abduction and adduction and rotation have been added. The view is superior one. On the right side of the diagram are shown: The origins of the superior (S), lateral (L) and medial (M) recti muscles from the annular tendon and their insertions into the equator of the eyeball; The origin of superior (SO) muscle from the roof of the orbit, and its tendon curving around the trochlea and passing posterolaterally to attach to the sclera posterior to the equator of the eyeball. On the left side of the diagram are shown: The origin of the (I) from the annular tendon and its insertion into the sclera anterior to the equator of the eyeball (ly); The origin of the (IO) from the inferomedial angle of the orbit and its insertion into the sclera posterior to the equator. From the positions of attachments of each muscle and their relations to the axes, one should be able to deduce the action (or actions) of the six extrinsic muscles of the eyeball. Normal binocular vision depends on properly coordinated activity of the 12 muscles that move the two eyes. All ocular muscles have a high proportion of nerve fibers in relation to muscle fibers and also an unusually high proportion of muscle spindle. In terms of overall function, it is important to remember that three of the muscles (Medial, & ) are concerned with turning the eye in (not just medial as might be expected), and that the lateral and two Obliques ( & ) turn it out. The eyeball can be elevated and depressed, abducted and adducted, and medially and laterally rotated. These movements result from the action of six muscles, each of which has one 1 P age

2 attachment to the eyeball and a second attachment to the orbit. They are the extrinsic muscles of the eyeball. Before discussing their attachments and actions it is advisable to understand the position of the imaginary axis involved in each movement. Thus, the axis around which the eyeball moves in elevation and depression passes horizontally through the equator of the eyeball. In the figure, A is an anterior view and Aˊ, a lateral view. The arrow indicates the direction of elevation. Depression is obvious. The vertical line is the midline; the heavy inclined line is the roof of the orbit. The axis of abduction and adduction passes vertically through the equator of the eyeball. B shows the axis in anterior view; it parallels the midline. The arrow indicates the direction in which the eyeball moves in adduction, i.e., the cornea moves towards the midline. In abduction, the cornea moves away from the midline. Bˊ shows the axis in lateral view. The Axis of medial and lateral rotation passes anteroposteriorly through the center of the retina and the eyeball. Conventionally, rotation is designated as medial when the upper half of the iris rotates toward the midline as indicated by the arrow in C (which is an anterior view of the eyeball showing the axis of rotation on end). Lateral rotation is obvious. Cˊ shows the axis in lateral view. Clinically Intorsion is used instead of medial rotation and the word extorsion instead of lateral rotation. The six extrinsic ocular muscles are inserted into the scleral coat of the eye and move the eye and its fascial sheath within the orbit. The fibrous ring is attached to the posterior wall of the orbit and encloses the openings of the optic canal and the middle part of the superior orbital fissure. Because the diameter of the fibrous ring is small in comparison to the eye, the muscles which arise from it form an expanding cone as they pass forwards to the eye. 2 P age

3 The vessels and nerves which enter the orbit through either the optic canal or the middle compartment of the superior orbital fissure, lie initially inside the cone of muscles whereas structures which enter the orbit by other routes will lie outside the cone. The movements of the eye can be resolved into two components which may be visualized as occurring around vertical and transverse axes through the center of the eye (at the equator). The basic rotations of the eye may be named according to the displacements of the pupil that they produce. Thus elevation, depression, abduction and adduction of the eye move the pupil upwards, downwards, laterally and medially respectively. Since both the medial and lateral muscles traverse the transverse axis of the eye, they consequently produce no movement around it. However, the medial pulls the medial aspect of the sclera backwards around the vertical axis, thus adducting the eye, whereas the lateral pulls the lateral aspect of the sclera backwards round the vertical axis, thus abducting the eye. The superior and muscles both lie in the vertical plane which contains the long axis of the orbit. Consequently, the superior pulls the anterosuperior aspect of the sclera backwards above the transverse axis and medial to the vertical axis, and as a result, both elevates and adducts the eye. Similarly, the pulls the antero aspect of the sclera backwards beneath the transverse axis and medial to the vertical axis and as a result, both depresses and adducts the eye. The superior muscle runs directly forwards along the medial wall of the orbit above the medial. Its tendon passes through a cartilaginous pulley attached to the anterior part of the junction of the medial wall and roof of the orbit and this runs backwards and laterally 3 P age

4 between the eye and superior muscle (passing behind the vertical axis). It is inserted into the superolateral aspect of the sclera behind the equator. The muscle thus pulls this part of the sclera forwards above the transverse axis and produces depression and abduction of the eye. The muscle differs from the other extrinsic ocular muscles in arising in the anterior part of the orbit from the maxilla just lateral to the orifice of nasolacrimal canal. The muscle passes backwards and laterally below the eye and the muscle and, turning upwards, inserts into the inferolateral aspect of the sclera behind the equator. The muscle thus pulls this area of sclera forwards beneath the transverse axis and medial behind the vertical axis, so that it elevates and abducts the eye. Whereas abduction and adduction of the eye are each produced by a single muscle, pure elevation and depression of the eye requires the activity of two muscles acting in a concert. Elevation produced by the combined action of and superior, whereas depression is produced by the combined activities of superior and. 4 P age

5 MUSCLE ORIGIN INSERTION NERVE SUPPLY ACTIONS Rectus part of Rectus Medial Rectus Lateral Rectus Oblique Oblique part of Medial part of Lateral part of Body of Sphenoid, superomedial to the optic canal Near the anterior margin of the floor of orbit posterior to rim; orbital surface of maxilla lateral to nasolacrimal groove superiorly ly medially laterally The sclera behind the equator in its superolateral posterior quadrant The sclera behind the equator in its inferolateral posterior quadrant (III) (III) (III) Abducent nerve (VI) Trochlear (IV) nerve (III) upwards and inwards downwards and inwards Turn the eye inwards outwards downwards and outwards upwards and outwards 5 P age

6 Extraocular Muscle Function: Primary and Secondary Actions Muscle Primary Action Secondary Action Motor Innervation Antagonists Synergists Lateral Abduction None Abducens n (CNVI) Medial & Medial Adduction None division) Lateral & Elevation Adduction, Intorsion superior division) Medial &, Depression Adduction, Extorsion division) Medial & superior, & superior, Intorsion Depression, Abduction Trochlear n (CNIV) Lateral & 6 P age

7 Extorsion Elevation, Abduction division), Medial & superior 7 P age