During the active awake state, the primary muscles used to open the eyes are the levator palpebrae muscles (Figure 188.1). The levator palpebrae muscles are innervated by the common ocular motor nerve. The oculomotor nucleus complex is located in the dorsal midbrain at the level of the superior colliculi. The oculomotor nucleus complex receives bilateral cortical innervation and innervation from the cerebellum and other brainstem nuclei. The oculomotor nucleus complex has two single midline structures and two lateral structures (Figure 188.1).

Figure 188.1.— Schematic representation of the muscle of the left eye, their brainstem nuclei, and innervation. III: cranial nerve III nucleus; IV: cranial nerve IV nucleus; VI: cranial nerve III nucleus. OD: right eye; OS: left eye; SR: superior rectus; IR: inferior rectus; MR: medial rectus; IO: inferior oblique; around the pupil: sphincter muscle; LPM: levator palpebrae muscle. The color of the brainstem nuclei matches the color of the muscles in the left eye: LPMN: nucleus of the levator palpebrae muscle (vertical brown and yellow); EWN: Edinger-Westphal nucleus (blue). Innervation of the LPM and sphincter muscle arises from a midline single structures (that innervates both eyes). The left SR is innervated by the right SR nucleus. The left SO is innervated by the right fourth cranial nerve nucleus (IV). All other muscles are innervated by the brainstem nuclei on the same side.

The midline structures are the nucleus for the levator palpebrae and the Edinger-Westphal nucleus (Figure 188.1). The axons from the neurons in these nuclei split into two bundles. One bundle goes to the right to innervate the right levator palpebrae muscle and the right pupillary sphincter; and the other bundle goes to the left to innervate the left levator palpebrae muscle and the left pupillary sphincter. These bundles join the axons from the two lateral neuronal structures destined to innervate the superior, inferior, and medial recti, and the inferior oblique muscles. The fibers for the superior rectus arise from the contralateral nucleus (the neurons for the left superior rectus arise from the right oculomotor nucleus complex). The fibers for the inferior and medial recti, and inferior oblique arise from the ipsilateral oculomotor nucleus.