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Each eye is moved by six muscles: four rectus muscles and two oblique muscles. These muscles are controlled by three nerves: cranial nerves (CNs) 3, 4, and 6. These cranial nerves all originate from brainstem nuclei that communicate with one another through the medial longitudinal fasciculus (MLF) to coordinate movements between the left and right eyes. These nuclei are controlled by brainstem gaze centers that coordinate the eyes to move together horizontally or vertically, and these gaze centers are stimulated by cortical eye fields. From the top down, the cortical eye fields stimulate the gaze centers in the brainstem, the brainstem gaze centers communicate with the cranial nerve nuclei of CN 3, CN 4, and CN 6, and CN3, CN 4, and CN 6 activate the extraocular muscles.
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The vestibular system also interacts with the eyes to coordinate eye movements with head movements. This pathway involves CN 8 and the cerebellum, and is discussed further in Chapter 12.
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EXTRAOCULAR MOVEMENTS I: MUSCLES AND THEIR INNERVATION
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The six muscles that control each eye are the four rectus muscles (superior, inferior, medial, lateral) and the two oblique muscles (superior and inferior) (Fig. 11–1 and Table 11–1). CN 4 controls the superior oblique, CN 6 controls the lateral rectus, and CN 3 controls the rest (superior, inferior, and medial recti and inferior oblique). The principal eye movements performed by the rectus muscles are easy to understand:
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Lateral rectus (CN 6) moves the eye laterally (abducts)
Medial rectus (CN 3) moves the eye medially (adducts)
Superior rectus (CN 3) primarily moves the eye superiorly (elevates)
Inferior rectus (CN 3) primarily moves the eye inferiorly (depresses)
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