Level 3 Unit 2 Part 29: Extraocular muscles

Eye movements are complex and have some unique terminology. Each globe is moved by six attached muscles called the extraocular muscles. The four rectus muscles are attached to the posterior orbit and are named for the side of the globe where they attach: superior, inferior, medial, and lateral. Contraction of the medial rectus muscle causes eye adduction (the pupil moves medially), and contraction of the lateral rectus muscle causes eye abduction (the pupil moves laterally).

The superior and inferior rectus muscles are oriented at about 45 degrees relative to the axis of the globe (a line through the pupil). These muscles are attached more medially in the posterior orbit and more laterally on the globe, therefore the eye movement that results from contraction of these muscles depends on the horizontal position of the globe. With the eye abducted, these muscles are about parallel to the axis of the globe, so that contraction of the superior rectus muscle causes eye elevation (the pupil moves superiorly) and contraction of the inferior rectus muscle causes eye depression (the pupil moves inferiorly). With the eye adducted, these muscles are about perpendicular to the axis of the globe, so that contraction of the superior rectus muscle causes eye intortion (the upper pole of the globe rotates medially) and contraction of the inferior rectus muscle causes eye extortion (the upper pole of the globe rotates laterally).

The other two extraocular muscles, the superior and inferior oblique muscles, are about perpendicular to the superior and inferior rectus muscles. The superior and inferior oblique muscles are oriented at about 45 degrees relative to the axis of the globe, being attached more medially in the anterior orbit and more laterally on the globe, therefore the eye movements that result from contraction of these muscles depends on the horizontal position of the globe. With the eye adducted, these muscles are about parallel to the axis of the globe, so that contraction of the superior oblique muscle causes eye depression and contraction of the inferior oblique muscle causes eye elevation. With the eye abducted, these muscles are about perpendicular to the axis of the globe, so that contraction of the superior oblique muscle causes eye intortion, and contraction of the inferior oblique muscle causes eye extortion.

All the extraocular muscles are innervated by the oculomotor nerve except for the superior oblique muscle, which is innervated by the trochlear nerve, and the lateral rectus muscle, which is innervated by the abducens nerve.

Next:

Level 3 Unit 2 Part 30: Motor control of eye movements

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