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.
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.
EXTRAOCULAR MOVEMENTS I: MUSCLES AND THEIR INNERVATION
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:
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)
Schematic of the extraocular muscles and eye movements. A: The attachments of the extraocular muscles on the left eye, viewed laterally from the left side. B: The actions of the superior oblique as shown from above on the right eye (see text). C: Principal functions of the extraocular muscles demonstrated for the right eye. Note that the superior oblique and inferior oblique act maximally to depress and elevate the eye in the adducted position. Reproduced with permission from Aminoff M, Greenberg D, Simon R: Clinical Neurology, 9th ed. New York: McGraw-Hill Education; 2015.
TABLE 11–1Innervation and Actions of the Extraocular Muscles. ||Download (.pdf) TABLE 11–1 Innervation and Actions of the Extraocular Muscles.
| ||Cranial Nerve Innervation ||Principal Action ||Secondary Action ||Effect of Weakness |
|Superior rectus ||CN 3 ||Elevation ||Intorsion in adducted position ||Impaired upgaze |
|Inferior rectus ||CN 3 ||Depression ||Extorsion in adducted position ||Impaired downgaze |
|Medial rectus ||CN 3 ||Adduction ||– ||Impaired medial gaze |
|Lateral rectus ||CN 6 ||Abduction ||– ||Impaired lateral gaze |
|Superior oblique ||CN 4 ||Intorsion ||Depression in adducted position || |
Impaired depression ...