Understanding the Oculomotor Nerve: Anatomy and Function

José Sánchez introduces the topic of the third cranial nerve, also known as the oculomotor nerve. It is the main motor nerve of the eye, responsible for moving the eyeball and controlling pupil dilation and lens accommodation.

The oculomotor nerve is distinguished from the optic and trochlear nerves by being purely motor in function. It is the third cranial nerve and is essential for eye movement, along with the trochlear and abducens nerves.

The oculomotor nerve has a real origin where motor fibers originate and an apparent origin where it emerges from the brain. Understanding the real and apparent origins is crucial for grasping the nerve's pathway.

The oculomotor nerve has two nuclei located in the midbrain. These nuclei include the somatic motor nucleus responsible for moving eye muscles and the accessory nucleus, also known as the visceral nucleus, which controls pupillary constriction and lens accommodation.

The motor nuclei consist of two main components: the common motor nucleus and the accessory or Edinger-Westphal nucleus. The common motor nucleus comprises two subnuclei, one purely motor and one parasympathetic. These nuclei are located around the cerebral aqueduct, anterior to the superior colliculi of the quadrigeminal plate in the midbrain.

The common motor nucleus is responsible for controlling the extrinsic eye muscles, specifically the lateral rectus muscle innervated by the abducens nerve and the superior oblique muscle innervated by the trochlear nerve. Each muscle has its own subnucleus within the common motor nucleus.

Most fibers originating from the common motor nucleus are ipsilateral, meaning they project to muscles on the same side of the body. Damage to the common motor nucleus on one side affects only the ocular muscles on that side due to the predominantly ipsilateral fiber distribution.

The accessory nucleus of Edinger-Westphal is located posterior and medial to the somatomotor nucleus. It contains preganglionic parasympathetic fibers that innervate the pupillary dilator muscle, responsible for dilating the pupil. Additionally, it innervates the ciliary muscle for accommodation.

The Edinger-Westphal nucleus plays a crucial role in pupillary function. Stimulation of this nucleus causes pupillary constriction, reducing the amount of light entering the eye. Loss of sympathetic innervation leads to pupillary dilation, as the sympathetic system controls pupil dilation.

The oculomotor nerve originates anterior and lateral to the common nucleus, with connections to the somatomotor nucleus and the accessory nucleus of Edinger-Westphal. It receives corticonuclear fibers for voluntary control of striated muscles, such as those responsible for eye movement. These corticonuclear fibers transmit primary voluntary motor signals from the cortical motor centers to the oculomotor nucleus.

The oculomotor nerve communicates with various structures, including the optic pathway, the cochlear pathway, and the brain. It also receives input from the primary sensory pathway. The nerve's somatomotor nucleus is involved in body position control and originates from the anteromedial aspect of the cerebral peduncles in the midbrain.

The oculomotor nerve traverses three key regions: from its origin to the cavernous sinus, within the cavernous sinus, and through the superior orbital fissure. It starts in the interpeduncular cistern anterior to the midbrain peduncles, between the posterior cerebral artery and the superior cerebellar artery. The nerve then courses obliquely forward, outward, laterally, and upward towards the cavernous sinus.

In the frontal view, the common motor nucleus is superior to the trochlear nerve and lateral to the abducens nerve, which is the sixth cranial nerve. It is also lateral to the internal carotid artery. The cavernous sinus and the common motor nucleus can be observed in this frontal cut, with the abducens nerve and internal carotid artery located laterally.

Upon reaching the superior orbital fissure, the common motor nucleus descends obliquely, passing through the superior orbital fissure. It then divides into superior and inferior terminal branches within the orbit. These branches may separate before or immediately upon entering the superior orbital fissure. Both branches pass through the common tendinous ring and have a relationship with the accessory nerve, abducens nerve, and ophthalmic nerve.

The distribution of the common motor nucleus is relatively short, with two terminal branches. The superior terminal branch innervates the superior rectus and levator palpebrae superioris muscles. The inferior terminal branch is more complex, providing innervation to the medial rectus, inferior rectus, and inferior oblique muscles of the eye.

The discussion delves into the five muscles and two nerves related to eye anatomy. Motor fibers originating from the motor nucleus travel continuously to the muscles. Additionally, the accessory parasympathetic nucleus is highlighted, showing how sympathetic fibers travel with the third cranial nerve to reach the ciliary ganglion.

The speaker explains that sympathetic fibers marked with dots pass to the lower division, which then branches out to reach the ciliary and auxiliary ganglia. This pathway allows for sympathetic fibers to synapse at the auxiliary ganglion before continuing to the postganglionic sympathetic fibers within the eye globe.

The conversation shifts to the functionality within the eye globe, focusing on the sphincter muscle of the pupil and the ciliary muscle. The ciliary muscle, when contracted through parasympathetic effects, alters the shape of the lens to facilitate near vision by making it more convex. Conversely, relaxing the ciliary muscle flattens the lens for distant vision.

The importance of the parasympathetic system, traveling with the third cranial nerve, is emphasized for its role in adjusting the lens shape for focusing on near or distant objects. The communication of sympathetic fibers through the internal carotid plexus to the eye is also highlighted, showcasing the intricate neural pathways involved in visual function.

The discussion concludes by underlining the significance of sympathetic communication through the third cranial nerve, serving as a conduit for sympathetic fibers from the internal carotid plexus. This communication is crucial for coordinating sympathetic responses within the eye anatomy.