AnatomyLove

Cranial Nerve 3 – Oculomotor nerve

Cranial nerve 3 is called the oculomotor nerve because it has two motor targets – skeletal muscle (GSE fibers) and smooth muscle (GVE parasympathetic fibers) – and they are both in the orbit. This nerve is responsible for the direction of gaze (where we’re looking), the size of the pupil, and the shape of the lens.

Oculomotor nerve

The nuclei for these fibers lies in the midbrain, at the oculomotor motor nucleus (GSE) and the Edinger-Westphal nucleus (GVE). The oculomotor nerve exits the midbrain at the interpeduncular fossa, moves forward through the cavernous sinus (with all other extraocular nerves plus V1 and V2), and splits into superior and inferior divisions within the superior orbital fissure.

oculomotor nerve in cranial cavity
Oculomotor nerve (cut) as it enters the cavernous sinus.

Superior division – extra-ocular muscles

The superior division of the oculomotor nerve innervates the superior rectus and the levator palperprae superioris mm. (LVP elevates the eyelid, it does not move the eye).

Therefore, the only extraocular muscles that are NOT innervated by they oculomotor nerve are the superior oblique m. (CN 4) and lateral rectus m. (CN 6). This means that lesions of CN 3 leave the eye looking “down and out” through the combined actions of superior oblique and lateral rectus mm.

Inferior division – extra-ocular muscles, parasympathetics

The inferior division of the oculomotor nerve innervates the medial rectus, inferior rectus, and inferior oblique mm. These muscles adduct the eye (medial rectus), depress/adduct/extort the eye (inferior rectus), and elevate/abduct/extort the eye (inferior oblique). Again, lesions of CN 3 leave the eye looking “down and out” through the combined actions of superior oblique and lateral rectus mm.

Additionally, the inferior division of the oculomotor nerve also contains the GVE pre-ganglionic parasympathetic fibers that began in the Edinger-Westphal nucleus of the midbrain. These fibers use the inferior division of CN 3 to reach the ciliary ganglion, where they synapse. Post-ganglionic fibers then move through short ciliary nn. into the choroid (middle layer) of the eye.

The targets of CN 3’s GVE (parasympathetic) fibers are the smooth muscle fibers of the ciliary muscle and the pupillary constrictor m., which affect the shape of the lens and the size of the pupil.

Lens accommodation

The ciliary muscle is attached to the lens of the eye by suspensory ligaments. The ciliary muscle is circular around the lens, meaning when it contracts it moves closer to the lens, making the suspensory ligaments shorter and looser, allowing the lens to take on more of it’s inherently spherical shape.

Thus, the beam of light moving through it focuses more anteriorly, which is better for seeing objects that are near. To refocus on objects far away the ciliary body relaxes, which pulls the suspensory ligaments tight, pulling on the lateral edges of the lens, flattening it anterior to posteriorly. This moves the focus of the light beam back so that it hits the retina and produces a focused image. This process is called lens accommodation.

Pupillary constriction & convergence

The other target of the GVE (parasympathetic) fibers in CN 3 are the smooth muscles of the pupillary constrictor m. This band of muscle circles the inner portions of the iris, immediately forming the boundaries of the pupil. The radial-shaped dilator pupillary m. sits just outside of it, forming most of the outer iris (it’s innervation comes from sympathetic fibers).

Once they’ve synapsed in the ciliary ganglion, the post-ganglionic fibers use the short ciliary nn. to reach the choroid layer of the eye and course to the front as described for the ciliary muscle. Here, the circular fibers constrict, or tighten the size of the pupil.

pupillary reflex pathway
When light hits the retina it triggers a parasympathetic response via CN 3 to constrict the pupil.

 

This is good for bright light situations, as it protects the retina. It is also part of the convergence response, meaning that when an object moves towards us our eyes should come together towards the midline, the pupils should constrict, and the ciliary muscle adjusts the shape of the lens to accommodate the closer object.

 

The oculomotor nerve is important for surveying the world around you – allowing you to look around and take in your surroundings. While it isn’t the ONLY cranial nerve to innervate extra-ocular muscles, it does the majority of them. And, it is important for making changes to the shape of the lens so light focuses on the retina as it should, and the size of the pupil.

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