The Oculomotor System

The oculomotor system is comprised of the intrinsic and extrinsic muscles of the eye along with motor nuclei and higher cortical centers that serve to control the position of the eye as well as the shape of the lens and the size of the pupil. The main purpose of the system is to aid in vision by keeping the visual target focused on the fovea, the area ofcentral retina that has the highest visual acuity. Diplopia (double vision), blurred vision, or loss of depth perception can occur as a result of lesions at various points within the neuronal pathways that subserve the oculomotor system. The pathways are generally well defined and well known, so that it is possible to determine the location of a lesion in the oculomotor system by careful analysis of the types of visual deficits expressed and by direct observation of eye position and reflex eye movements. Tests of oculomotor function are an important part of most physical examinations of patients.

Cranial Nerve Innervations of Eye Muscles

Three pairs of extraocular muscles move the globe along three axes. The medial and lateral rectus muscles rotate the eye in the horizontal plane. The medial rectus muscle is innervated by cranial nerve III (the oculomotor nerve), and the lateral rectus muscle is innervated by cranial nerve VI, the abducens nerve. Both muscles attach to a common tendinous ring around the apex of the orbit and to the eyeball at points parallel to the floor of the orbit. Thus, the force they generate results in horizontal eye movement or horizontal gaze. The medial rectus adducts the eye (i.e., rotates the eye toward the nose along a vertical axis). The lateral rectus abducts the eye (rotates the eye away from the nose) (Fig. 17).

The superior and inferior rectus muscles are responsible for vertical and torsional movement of the eyeball. Both muscles are innervated by cranial nerve III. The muscles attach to the tendinous ring and to the globe at points 23° off the midsagittal plane. The superior rectus elevates but

A. The muscles of the right eye and their crainal nerves levator palpebrae -superior rectus III

lateral rectus VI

inferior oblique III

B. Horizontal gaze inferior oblique III

superior oblique IV

medial rectus III

inferior rectus III

superior oblique IV

medial rectus III

inferior rectus III

B. Horizontal gaze

C. Vertical gaze

superior oblique IV

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