Anatomy of the oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves


Imagine a world where we had no control over the movements of our eyes.

We would have to move our entire head in that direction to look at something new, something that would surely cause neck issues when playing video games.

Luckily, we do have control over our eye movements, and the cranial nerves which innervate the extrinsic ocular muscles that move the eyeball are the oculomotor, trochlear, and abducens nerve - or cranial nerves III, IV and VI.

First up, the oculomotor nerve has two main motor functions: a somatic motor function and a visceral motor or parasympathetic function, and there are different motor nuclei that control these two functions.

So, the somatic motor function is controlled by the oculomotor nucleus which is located in the midbrain, and the visceral motor function is controlled by the accessory oculomotor nucleus, or Edinger-Westphal nucleus, also located in the midbrain.

The oculomotor nerve innervates four of the six extraocular muscles, namely the superior rectus, medial rectus, inferior rectus and inferior oblique muscle.

Thanks to this cranial nerve, it mainly helps us to direct our gaze superiorly, inferiorly, and medially.

The oculomotor nerve also innervates the levator palpebrae superioris muscle, which lifts the superior eyelid.

Parasympathetic innervation is provided through the ciliary ganglion to the smooth muscle of the sphincter pupillae, which causes constriction of the pupil, and to the ciliary body which produces accommodation for near vision by relaxing the lens and allowing it become more rounded.

Now, the oculomotor emerges from the midbrain, pierces the dura mater lateral to the sellar diaphragm or diaphragm sellae, and then runs through the roof and lateral wall of the cavernous sinus, and the oculomotor nerve enters the orbit through the superior orbital fissure.

At this point, it divides into a superior division to innervate the superior rectus and levator palpebrae superioris muscles, and an inferior division to innervate the inferior rectus, medial rectus, and inferior oblique muscles.

The inferior division also carries visceral efferent fibers to the ciliary ganglion, where they synapse.

Postsynaptic fibers from the ciliary ganglion pass to the eyeball in the short ciliary nerves to innervate the ciliary body and the sphincter pupillae.

Now, the optic nerve and the oculomotor nerve work together for two reflexes of the eye, which are the pupillary light reflex and the accommodation reflex.

The optic nerve serves as the sensory or afferent pathway, and the oculomotor serves as the motor or efferent pathway.

The pupillary light reflex is a reflex that controls the diameter of the pupil in response to light.

When light enters either retina, it is sensed by the optic nerve, travels to the optic chiasm then to the optic tract.

The optic tract then sends a signal to a nucleus located in the midbrain called the pretectal nucleus.

This nucleus then sends signals to both the right and left Edinger-Westphal nuclei located in the midbrain, and those signals travel along preganglionic parasympathetic fibers that exit with the oculomotor nerve and synapse with postganglionic parasympathetic neurons in the ciliary ganglion.

Postganglionic nerve fibers leave the ciliary ganglion via short ciliary nerves to innervate the ciliary sphincter pupillae muscle of the iris to cause constriction.

Therefore, shining a light in one eye should cause a reflexive constriction of that same pupil called the direct pupillary light reflex, in addition to causing constriction of the contralateral pupil called the consensual pupillary light reflex.

This pupillary reflex is meant to constrict the pupil when there’s plenty of light, so as to prevent more light from reaching the retina, and the strength of this reflex is dependent on the intensity of the light shining into the eye.

The accommodation reflex, on the other hand, refers to the change in the shape of the lens of the eye in response to focusing on an object near or far away, similar to how a photographer changes their lens to take pictures on their camera.