The patient’s symptoms indicate damage to the oculomotor nerve, also known as cranial nerve III (CNIII). This nerve is composed of two different types of efferent fibers: general somatic efferent (GSE) fibers and general visceral efferent (GVE) fibers. The GSE fibers are responsible for innervating most of the muscles in the orbit that move the eye. These muscles are the levator palpebrae superioris, the superior rectus, the medial rectus, the inferior rectus, and the inferior oblique muscles. The GVE fibers are a part of the parasympathetic nervous system. These fibers innervate muscles in the ciliary body to focus the eye on nearby objects and the sphincter pupillae, which constricts the pupil (Drake, Mitchell, & Vogl, 2012).
The oculomotor nerve branches off the ventral surface of the brainstem (Dauzvardis, n.d.) between the pons and the midbrain. It then enters the tentorium cerebelli, after which it travels anteriorly via the anterior wall of the cavernous sinus (Drake et al., 2012). After exiting the cranial cavity through the superior orbital fissure, CNIII divides into superior and inferior divisions. GSE fibers can be found in both divisions, with the superior division innervating the levator palpebrae superioris and …show more content…
CNIII innervates a large majority of the extraocular muscles that move the eye, including the superior rectus, inferior rectus, medial rectus, and inferior oblique. Damage to this nerve would result in a lack of innervation to these muscles. The lateral rectus muscle, however, would still be fully functional because of its innervation by the abducens nerve (CNVI). Without function of the medial rectus, superior rectus, inferior rectus, and inferior oblique muscle, there is nothing to counteract the contraction of the lateral rectus, causing the eye to move laterally and inferiorly, creating the “down and out” eye position described (Goodwin,