In this note, we are going to discuss the anatomy and function of the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). These cranial nerves control our eye movements.

Oculomotor nerve

The oculomotor nerve (n. oculomotorius) is the cranial nerve III.

According to the generally accepted classification of functional components, it contains general somatic efferent fibers (GSE) and general visceral efferent fibers (GVE). That means that it is a motor nerve.

The somatic efferent fibers arise from the oculomotor nucleus (nucleus nervi oculomotorii), which is located in the midbrain at the level of the superior colliculi.

The nucleus lies within the periaqueductal gray matter surrounding the cerebral aqueduct.

It consists of separate groups of cells, each of which is a source of fibers for the innervation of a certain extraocular muscle.

The visceral efferent fibers arise from the accessory oculomotor nucleus, also known as the Edinger-Westphal nucleus (nucleus accessorius nervi oculomotorii), which is a parasympathetic nucleus. It is located nearby.

Both the somatic and visceral efferent fibers exit the anterior surface of the brainstem as the oculomotor nerve.

Then it pierces the dura mater and runs through the roof and lateral wall of the cavernous sinus (sinus cavernosus).

Next, the oculomotor nerve enters the orbit through the superior orbital fissure (fissura orbitalis superior).

At this point, it divides into superior and inferior branches (ramus superior et ramus inferior).

The superior branch innervates the superior rectus muscle (m. rectus superior),

pierces it and innervates the levator palpebrae superioris muscle (m. levator palpebrae superioris).

The inferior branch innervates the medial rectus muscle (m. rectus medialis)

and inferior rectus muscle (m. rectus inferior),

as well as the inferior oblique muscle (m. obliquus inferior).

This helps us to direct our gaze superiorly, inferiorly, and medially.

The inferior branch of the oculomotor nerve also carries visceral efferent fibers to the ciliary ganglion (ganglion ciliare).

Postsynaptic fibers from the ciliary ganglion pass to the eyeball in the short ciliary nerves (nn. ciliares breves) to innervate the intrinsic muscles of the eye.

In particular, the sphincter pupillae (m. sphincter pupillae), which causes constriction of the pupil,

and the ciliary muscle (m. ciliaris),

Trochlear nerve

The trochlear nerve (n. trochlearis) is the cranial nerve IV.

According to the generally accepted classification of functional components, it contains general somatic efferent fibers (GSE). That means that it is a motor nerve.

It arises from the trochlear nucleus (nucleus nervi trochlearis), located within the periaqueductal gray matter of the midbrain, at the level of the inferior colliculi.

Then it emerges from the posterior surface of the midbrain, crosses the midline, and passes anteriorly around the contralateral side of the brainstem.

The trochlear nerve pierces the dura mater and passes anteriorly in the lateral wall of the cavernous sinus.

It enters the orbit through the superior orbital fissure (fissura orbitalis superior).

The trochlear nerve innervates the superior oblique muscle (m. obliquus superior), which abducts, depresses, and medially rotates the eye.

Abducens nerve

And finally, the abducens nerve (n. abducens), or cranial nerve VI.

According to the generally accepted classification of functional components, it contains general somatic efferent fibers (GSE). That means that it is a motor nerve.

It arises from the abducens nucleus (nucleus nervi abducentis), which is located in the dorsal part of the pons.

The nerve emerges from the brainstem in the sulcus between the pons and the medulla oblongata.

The abducens nerve pierces the dura mater and runs through the cavernous sinus.

It enters the orbit through the superior orbital fissure (fissura orbitalis superior),

and innervates the lateral rectus muscle (m. rectus lateralis), which abducts the eye to direct the pupil away from the nose.

Dictionary

CN III, IV, VI

Oculomotor nerve

n. oculomotorius

Nucleus of the oculomotor nerve

nucleus nervi oculomotorii

Cerebral aqueduct

aqueductus cerebri

Accessory nucleus of the oculomotor nerve

nucleus accessorius nervi oculomotorii

Sulcus of the oculomotor nerve

sulcus nervi oculomotrii

Cavernous sinus

sinus cavernosus

Superior orbital fissure

fissura orbitalis superior

Superior branch

ramus superior

Inferior branch

ramus inferior

Superior rectus muscle

m. rectus superior

Levator palpebrae superioris muscle

m. levator palpebrae superioris

Medial rectus muscle

m. rectus medialis

Inferior rectus muscle

m. rectus inferior

Inferior oblique muscle

m. obliquus inferior

Ciliary ganglion

ganglion ciliare

Short ciliary nerves

nn. ciliares breves

Sphincter pupillae muscle

m. sphincter pupillae

Ciliary muscle

m. ciliaris

Trochlear nerve

n. trochlearis

Nucleus of the trochlear nerve

nucleus nervi trochlearis

Inferior colliculus

colliculus inferior

Superior oblique muscle

m. obliquus superior

Abducens nerve

n. abducens

Nucleus of the abducens nerve

nucleus nervi abducentis

Facial colliculus

colliculus facialis

Lateral rectus muscle

m. rectus lateralis