Eye Movements Geometry of the Orbit The eye (oculus) is located in the anterior aspect of the orbit: the equator of the eye (defined by a coronal plane passing through its middle) lies at the margin of the lateral orbital margin. Relative to one another, the medial walls of the orbits are parallel, while the lateral walls are at a right angle. As a result, the long axes of the orbits, or orbital axes, diverge from midline by ~22 and from each other by about 45. The visual (optic) axes are aligned very close to the sagittal plane and are parallel in the primary position (looking straight ahead); this position is maintained by (tonic) muscular effort. The visual axis is defined as extending from the anterior pole of the eye (center of cornea) to the posterior pole (fovea centralis of the retina, the point on the retina of clearest or most acute vision). CNS pathways, including hard wiring between and among the nuclei of cranial nerves III, IV and VI ensure that the visual axis can always be aimed at whatever one wishes to see most clearly. Extraocular Muscles The extraocular muscles work together to move the upper eyelids and the eyes. Levator palpebrae superioris: this muscle arises from the apex of the orbit and inserts along the skin of the eyelid where it interdigitates with the (palpebral) fibers of the orbicularis oculi; the levator palpebrae superioris is innervated by CN III. Superior tarsal muscle: this bundle of smooth muscle extends from the distal tendon of the levator palpebrae superioris to the superior edge of the superior tarsal plate; this muscle is innervated by sympathetic motor neurons. Rectus muscles: all four rectus muscles (medial, lateral, superior, inferior) originate from a common tendinous ring at the apex of the orbit. Rectus muscles insert anterior to equator of eye, and are aligned with the orbital axis (not the visual axis). Eye Movements Page 1 of 5
Superior oblique: one of two oblique muscles, this muscle is oriented medial/superior in the orbit. Its tendon hooks through the trochlea (L. pulley) - a loop of connective tissue just posterior to the orbital margin between the roof and medial wall. The tendon courses obliquely back over the superior surface of sclera, and inserts laterally and posterior to the equator. Inferior oblique: this muscle originates from the anterior/medial aspect of floor of orbit and inserts laterally and posterior to the equator, in the same plane as the tendon of superior oblique. Innervation of Extraocular Muscles Motor innervation of the extrinsic muscles is supplied by cranial nerves III, IV and VI: The trochlear nerve (CN IV) innervates the superior oblique muscle (the muscle that passes through the trochlea). The abducens nerve (CN VI) innervates the lateral rectus muscle (the only action of this muscle is abduction of the eye). The oculomotor nerve (CN III) innervates the five remaining extrinsic muscles. While CNs IV and VI are purely somatic motor nerves, CN III has both somatic motor and visceral motor (parasympathetic) components. The parasympathetic component, associated with the ciliary ganglion, controls intrinsic muscles of the eye, the constrictor of the pupil, sphincter pupillae, and the ciliary muscle that controls accommodation of the lens (focusing on close objects). Constriction of the ciliary muscle relaxes tension on the elastic lens, allowing it to become more rounded, thus decreasing the focal length of the eye (more on intrinsic muscles and focal length in the Neuroscience resources). Eye Movements Movements of the eye are defined on three axes of rotation which intersect in the middle of the eye: Movements on the vertical axis = abduction and adduction (of the anterior pole) Movements on the transverse axis = elevation and depression (of the anterior pole) Movements on the sagittal axis = intorsion (medial rotation) and extorsion (lateral rotation) (defined by movement of the superior surface of the eye). Important Concept: If a muscle is oriented at 90 o to an axis, all of its force is applied to movement on that axis. If a muscle is oriented at 0 o to an axis (in the same plane), none of its force can be applied to movement on that axis. As the angle between a muscle and an axis changes, the amount of force the muscle applies to movement on that axis changes correspondingly. Eye Movements Page 2 of 5
Lateral and medial recti are in the same (horizontal) plane as the transverse and sagittal axes and have no actions on these axes. Instead, they are capable only of rotating the eye about the vertical axis (abduction and adduction). The other extrinsic muscles have both primary and secondary actions due to their orientations with respect to the axes of movement: o o The superior and inferior recti act primarily on the transverse axis (elevation and depression), but also have minor actions on the other two axes. When the oblique muscles contract, their insertions on the posterior/lateral portion of the sclera move towards their more medial and anterior origin (inferior oblique) or anchoring point (trochlea- superior oblique). Due to the orientation of these muscles in a plane that is relatively close to transverse, the two oblique muscles are the main rotators of the eye (sagittal axis), but also have significant action on the transverse axis (elevation and depression) and to a lesser extent are abductors. It is not necessary (or desirable) to memorize all of the actions of the extrinsic muscles. If you understand the geometry of the orbit and the muscles, their actions will be evident. Eye Movements Page 3 of 5
**Keep in mind that in the real world, the extrinsic muscles do not act in isolation; they all have some tone most of the time. Also, the muscles obviously exert all of their actions whenever they contract. However, most students find it useful to start by understanding the individual actions of each muscle as if they could occur in isolation.** Using this information clinically. The eye exam. The eye cannot be abducted or adducted fully without the actions of the lateral and medial recti. When the eye is abducted, the transverse axis is aligned perpendicular to the plane of the superior and inferior recti, so their actions predominate in elevation and depression. (Abducting the eye aligns the sagittal axis nearly perpendicular to the plane of the obliques, which emphasizes the rotation action of the obliques.) When the eye is adducted, the transverse axis is aligned nearly perpendicular to the plane of the obliques, so they predominate in elevation and depression. (Adducting aligns the sagittal axis nearly perpendicular to the plane of the recti, which emphasizes their rotation action.) Eye Movements Page 4 of 5
Therefore, to test the function of the superior rectus and inferior rectus muscles, start with the eye abducted, then test elevation and depression. To test the superior and inferior oblique muscles, start with the eye adducted, then test elevation and depression. This sequence defines the H-test. There is a very good animated simulation of the H-test showing the muscles in anatomical orientation at: http://brodel.med.utoronto.ca/~jodie/anatomia/media/extraocular_eval.swf (see the link on this page of the MBB website). Eye Movements Page 5 of 5