FIGURE 12-8

Sagittal section through brainstem showing trigeminal, ­oculomotor, trochlear, abducens, and facial nuclei.

arrangement of the subnuclei are postulated primarily on the basis of animal models.23,25-27 The nucleus for the medial rectus is located toward the lower border of the oculomotor nucleus; the inferior rectus nucleus lies toward the upper border, with the nucleus for the inferior oblique between. The nucleus of the superior rectus lies in the medial and caudal two thirds of the oculomotor nucleus. Each of these subnuclei are found in the right and left oculomotor nucleus. The nucleus for the levator muscle is single and is located centrally in the caudal area (Figure 12-9).

Fibers to the inferior rectus, inferior oblique, and medial rectus muscles supply the ipsilateral eye; fibers innervating the superior rectus muscle decussate and supply the contralateral eye. The decussating fibers pass through the opposite superior rectus nucleus; thus damage to the right oculomotor nucleus might have bilateral superior rectus muscle involvement.27-30 The centrally placed caudal nucleus provides innervation for both levator muscles.

An autonomic nucleus, the accessory third nerve nucleus (Edinger-Westphal nucleus), supplies parasympathetic innervation to the ciliary and iris sphincter muscles. It is located in the rostral, ventral portion of the oculomotor nucleus30,31 (see Figure 12-9).

Oculomotor Nerve Pathway

Fibers from each of the individual nuclei join, forming the fascicular part of the nerve that passes through the red nucleus and the decussating fibers of the superior cerebellar peduncle.32 These fibers emerge just medial to the cerebral peduncles and within the interpeduncular

Edinger-Westphal Superior rectus Levator

Inferior oblique

Medial rectus

Inferior rectus

A B

FIGURE 12-9

Oculomotor nerve nuclei. A, Lateral view. B, Dorsal view.

fossa on the anterior aspect of the midbrain as the oculomotor nerve. The nerve passes between the superior cerebellar and posterior cerebral arteries as it runs forward, lateral to, and slightly inferior to the posterior communicating artery of the circle of Willis (Figure 12-10). The nerve pierces the roof of the cavernous sinus and runs within the two dural layers of its lateral wall above the trochlear nerve2,14,16 (see Figure 12-6). While in the cavernous sinus, the oculomotor nerve sends small sensory branches (likely proprioceptive) to the ophthalmic nerve and receives sympathetic fibers from the plexus around the internal carotid artery.2,19

The oculomotor nerve exits the sinus and enters the orbit through the superior orbital fissure, having divided into superior and inferior divisions; both divisions are located within the oculomotor foramen. The superior branch runs medially above the optic nerve and enters the superior rectus on its inferior surface; additional fibers either pierce the muscle or pass around its border to innervate the levator14,33 (Figure 12-11).

The inferior branch runs below the optic nerve and divides into three branches. One branch enters the medial rectus on its lateral surface, and one enters the inferior rectus on its upper surface (see Figure 12-11). The third branch gives off parasympathetic fibers that form the parasympathetic root extending to the ciliary ganglion; then it runs along the lateral border of the inferior rectus, crossing it to enter the inferior oblique muscle near its ­midpoint.5,14,34,35

TROCHLEAR NERVE: CRANIAL NERVE IV

The trochlear nerve innervates the superior oblique muscle.

Trochlear Nucleus

The trochlear nucleus is located in the midbrain, at the level of the inferior colliculus, anterior to the cerebral aqueduct, dorsal to the medial longitudinal

Posterior cerebral artery

Superior cerebellar artery

Posterior communicating artery

Abducens nerve

Ophthalmic division

Maxillary division (turned forwards)

Internal carotid artery

FIGURE 12-10

Trochlear Superior orbital nerve

fissure

FIGURE 12-11

Orbital apex with the globe removed, showing the origin of the rectus muscles at the annulus of Zinn and the relationship between superior orbital fissure and common tendinous ring.

fasciculus, and below the oculomotor nucleus32 (see Figure 12-8). The fibers travel dorsally and decussate. CN IV is the only cranial nerve to cross; thus the trochlear nucleus innervates the contralateral superior oblique muscle.

Trochlear Nerve Pathway

Of the cranial nerves, the trochlear nerve is the only one that leaves the dorsal aspect of the CNS. It is the most slender of the cranial nerves, and its attachment is very delicate. (The small diameter of the nerve probably reflects the fact that it supplies only one muscle, the most slender of the extraocular muscles.) As the trochlear nerve emerges from the dorsal midbrain immediately below the inferior colliculus, it decussates and curves around the cerebral peduncle at the upper border of the pons, approximately paralleling the superior cerebellar and posterior cerebral arteries. It passes between these two vessels and runs forward lateral to the oculomotor nerve (see Figure 12-10).