Chapter 1: Basic Anatomy and Actions of the Extraocular (Eye) Muscles
A Patient & Parent Guide to Strabismus Surgery
George R. Beauchamp, M.D.
The muscles that move the eye are called the extraocular muscles. There are six of them on each eye. They work together in pairs—complementary (or yoke) muscles pulling the eyes in the same direction(s), and opposites (or antagonists) pulling the eyes in opposite directions. Below are some basic movements for each of the pairs.
Horizontal Rectus (Eye) Muscles
The medial rectus, or nose-side, muscles move the eyes inwardly; when working simultaneously, they converge, or cross, the eyes. The lateral rectus, or temple-side, muscles move the eyes outwardly; when they work simultaneously, they diverge, or splay apart, the eyes. When moving the eyes from side to side—their principal function, they work in teams, so that the normal movements are smooth and coordinated. For example, to gaze to the right, the nose side (medial rectus) muscle of the left eye, and the outside (lateral rectus) muscle of the right contract; their opposites (or antagonists)—the left lateral and right medial recti— relax.
Vertical Rectus (Eye) Muscles
The vertical rectus muscles—superior moving the eyes upwardly, and inferior, moving them downwardly—are teamed in a similar fashion to the above-described horizontal recti. Because of the angle of the muscles on the eye, these muscles also have an effect on horizontal and torsion (or rotation) movements of the eyes. These secondary and tertiary actions are generally only of importance in complicated strabismus.
Oblique (Eye) Muscles
The most complicated muscles—both in anatomy and action—are the oblique muscles, superior and inferior.These muscles, like the rectus muscles, also work in antagonist pairs; the inferiors contract while the superiors relax, and vice versa. The inferior oblique muscles turn the eyes upwardly when the eye is looking inwardly, and rotates outwardly (extorts) the eye when looking outwardly. The superior oblique muscle turns the eye downwardly when the eye is turned inwardly, and rotates the eye inwardly when the eye is directed outwardly.
Cranial Nerves Innervate The Extraocular (Eye) Muscles
The muscles move in response to nerve impulses carried from the base of the brain (brain stem) by three cranial nerves, named the oculomotor (or III nerve), the trochlear (or IV nerve) nerve, and the abducens (or VI nerve). The trochlear nerve enervates the superior oblique muscle, the abducens nerve enervates the lateral rectus muscle, and the oculomotor nerve enervates the remaining four, plus the levator (or lifting) muscle of the upper eye lid.
Read Chapter 2: What is Strabismus?
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