All of the muscles of the eye are active in any eye movement. Muscles control right-to-left and up-and-down movement. Extrensic (extraorbital) muscles include the levator palpebrae superioris, medial and lateral rectus, superior and inferior rectus, and the superior and inferior oblique. Intrinsic (inside the pupil) muscles include the ciliary muscle, sphincter pupillae and dilator pupillae.
Levator Palpebrae Superioris
A collection of smooth muscle fibers, the levator palpebrae superioris controls eyelid movement. The levator palpebrae superioris connects to the eyelid in three locations. Cranial nerve III controls activation of the levator palpebrae superioris. Loss of function results in ptosis, a droopy eyelid.
Medial and Lateral Rectus
The medial and lateral rectus muscles are extrensic muscles that suspend the eyeball. The medial rectus pulls the eyeball toward the nose. The lateral rectus pulls the eyeball away from the nose. Resection of the medial and lateral rectus consists of surgery to move the location of the muscle to correct conditions such as strabismus. MedRounds reports the medial rectus as the largest ocular muscle. Cranial nerve III innervates the medial rectus and cranial nerve VI controls the lateral rectus.
Superior and Inferior Rectus
The superior and inferior rectus muscles, innervated by cranial nerve III, are extrensic muscles that control downward and upward eyeball movement. The superior rectus assists the eye to look upward, rotating inward toward the nose, and works in conjunction with the levator palpebrae superioris to lift the upper eyelid. Mission for Vision reports the inferior rectus, the shortest of the recti muscles, assists eye movement to look downward.
Superior and Inferior Oblique
The superior oblique, an extrensic muscle innervated by cranial nerve IV, rotates the eyeball downward and inward toward the nose. The inferior oblique, an extrensic muscle innervated by cranial nerve III, rotates the eyeball upward and away from the nose.
Ciliary Muscle
The ciliary muscle, an intrinsic muscle located inside the eyeball, controls the shape of the lens to focus. The amount of tension on the lens changes the shape. St. Luke's Cataract & Laser Institute reports the loss of contraction of the ciliary body muscles causes presbyopia, the inability to focus up close.
Sphincter and Dilator Pupillae
Two of the three intrinsic muscles,the sphincter and dilator pupillae, control the size of the iris (pupil). The sphincter pupillae constricts or decreases the size of the pupil by muscle contraction. The dilator pupillae increases or dilates the size of the pupil.
