There are six extraocular muscles that together move the pupil up, down, side to side and in combinations of these gazes. As its name denotes, the inferior rectus muscle attaches to the sclera — the white “ball” of the eyeball — at the bottom of the structure and primarily works to move the gaze downward in a process known as depression. The other extraocular muscles are the superior rectus, medial rectus, lateral rectus, superior oblique and the inferior oblique. All but one of these muscles form a conical shape in each eye orbit, with the mouth of the cone where the muscles connect to the eyeball and the sharp point of the cone where the muscles converge together in a ring-shaped tendon. The optic nerve, cranial nerve II, runs through this ring-shaped tendon through the cone to the pupil.
The four inferior and superior extraocular muscles each have three functions of varying responsibility. The primary function of the inferior rectus muscle is the downward movement or depression of the pupil. A secondary function of the inferior rectus muscle is the rotation of the top of the eyeball away from the nose in a process known as extorsion. Finally, the tertiary function of this muscle is to move the pupil inward, or medially, in adduction.
Some eye motions are the result of just one extraocular muscle contraction while others require two muscles working in conjunction. The muscle primarily responsible for the pupil’s movement is referred to as the agonist. Other extraocular muscles that assist in the motion are known as synergists. Those extraocular muscles that move the pupil in an opposite direction are antagonists. Thus, the inferior rectus muscle is the agonist for a pupil’s downward motion; the synergist for extorsion or adduction; and the antagonist for the superior rectus muscle’s movement of the eye upward during elevation.
These descriptions refer to the movement of one eye only, which is called a duction. For a single pupil’s gaze downward, this downward depression motion is also known as infraduction. When two eyes work together, however, the eye movement is known as a version. To look down with both eyes, performing either an infraversion or deorsumversion, both the right and left inferior rectus muscles — or recti — must contract to force the motion. In addition, the right and left superior oblique recti work as synergists to the inferior rectus muscle to move each eye in the same direction.