Nerves of Orbit and Cavernous Sinus

TROCHLEAR NERVE

The trochlear nerve is slender, and its nucleus of origin is located in the midbrain just caudal to the oculomotor nuclei. The trochlear fibers curve posterolaterally and slightly caudally around the cerebral aqueduct to reach the upper part of the superior medullary velum; here the nerve fibers from opposite sides decussate before emerging on either side of the frenulum veli, below the inferior colliculi. No other cranial nerves emerge from the dorsal aspect of the brainstem.

Each trochlear nerve winds forward around the midbrain below the free edge of the tentorium cerebelli, passes between the superior cerebellar and posterior cerebral arteries and above the trigeminal nerve, and pierces the inferior surface of the tentorium near its attachment to the posterior clinoid process to run forward in the lateral wall of the cavernous sinus between the oculomotor and ophthalmic nerves. The nerve enters the orbit through its superior fissure, immediately lateral to the common annular tendon, and passes medially between the orbital roof and the levator palpebrae superioris to supply the superior oblique muscle. Proprioceptive fibers are transferred through a communication with the ophthalmic nerve to the trigeminal nerve. The trochlear nerve usually receives sympathetic filaments from the internal carotid nerve plexus.

ABDUCENS NERVE

The abducens nerve arises from the abducens nucleus, which is located in the pons, subjacent to the facial colliculus in the upper half of the floor of the fourth ventricle. The nucleus is encircled by fibers of the homolateral facial nerve. The abducens nerve fibers pass forward to emerge near the midline through the groove between the pons and the pyramid of the medulla oblongata. Each abducens nerve then inclines upward in front of the pons, usually behind the inferior cerebellar artery. Near the apex of the petrous part of the temporal bone, the nerve bends sharply forward above the superior petrosal sinus to enter the cavernous sinus, where it lies adjacent to the internal carotid artery. There the abducens may transfer proprioceptive fibers to the ophthalmic branch of the trigeminal nerve and receive sympathetic filaments from the internal carotid nerve plexus. The abducens nerve enters the orbit through the superior orbital fissure, within the common annular tendon, and ends by supplying the lateral rectus muscle.

The abducens has a relatively long intracranial route in the posterior cranial fossa and cavernous sinus. Consequently, it is vulnerable to increases in intracranial pressure and to pathologic or traumatic lesions affecting nearby parts of the brain, skull, or sinus.

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