Cranial Nerves Affecting The Eye
There are four cranial nerves that affect the eye. The tissue and cell effects of these nerves can be projected with the use of a teaching microscope.
THE OCULOMOTOR NERVE (III) The oculomotor nerve leaves the brainstem between the cerebral peduncles and passes near the posterior comŽmunicating artery of the circle of Willis. Lateral to the pituitary gland, it is closely approximated to the optic tract, and here it pierces the dura to course in the lateral wall of the cavernous sinus, as seen in a teaching microscope. As the nerve leaves the cavŽernous sinus, it divides into superior and inferior diviŽsions. The superior division enters the orbit within the annulus of Zinn at its highest point and adjacent to the trochlear nerve.
The inferior division enŽters the annulus of Zinn low and passes below the optic nerve to supply the medial and inferior rectus muscles. A large branch from the inferior division extends forŽward to supply the inferior oblique. A small twig from the proximal end of the nerve to the inferior oblique carries parasympathetic fibers to the ciliary ganglion.
THE TROCHLEAR NERVE (IV) Although the thinnest of the cranial nerves when seen in a teaching microscope, the trochlear nerve has the longest intracranial course, and it is also the only nerve to originate on the dorsal surface of the brain stem. The fibers decussate before they emerge from the brainstem just below the inferior colliculi, where they are subject to injury from the tentorium. The nerve pierces the dura behind the sella turcica and travels within the lateral walls of the cavernous sinus to enter the superior orbital fissure meŽdial to the frontal nerve. From this point it travels within the periorbita of the roof over the levator muscle to the upper surface of the superior oblique muscle.
THE TRIGEMINAL NERVE (V) The trigeminal nerve originates from the pons, and its sensory roots form the trigeminal ganglion. The first (ophthalmic) of the three divisions passes through the lateral wall of the cavernous sinus and divides into the lacrimal, frontal, and nasociliary nerve. The lacrimal nerve passes through the upper lateral aspect of the suŽperior orbital fissure, outside the annulus of Zinn, and continues its lateral course in the orbit to terminate in the lacrimal gland, providing its sensory innervation. Slightly medial to the lacrimal nerve within the superior orbital fissure is the frontal nerve, which is the largest of the first division of branches of the trigeminal nerve. It also crosses over the annulus of Zinn and follows a course over the levator to the medial aspect of the orbit, where it divides into the supraorbital and supraŽtrochlear nerves. These provide sensation to the brow and forehead. The nasociliary nerve is the sensory nerve of the eye. After entering through the medial portion of the annulus of Zinn, it lies between the superior rectus and the optic nerve. Branches to the ciliary ganglion and those forming the ciliary nerves provide sensory supply to the cornea, iris, and ciliary body. The termiŽnal branches are the infratrochlear nerve, which supŽplies the medial portion of the conjunctiva and eyelids, and the anterior ethmoidal nerve, which provides sensation to the tip of the nose. Thus, the skin on the tip of the nose may be affected with vesicular lesions prior to the onset of herpes zoster ophthalmicus.
The second (maxillary) division of the trigeminal nerve passes through the foramen rotundum and enŽters the orbit through the inferior orbital fissure. It passes through the infraorbital canal, becoming the inŽfraorbital nerve, and exits via the infraorbital foraŽmen, supplying sensation to the lower lid and adjacent cheek. It is frequently damaged in fractures of the orŽbital floor.
THE ABDUCENS NERVE (VI) The abducens nerve originates between the pons and medulla and pursues an extended course up the clivus to the posterior clinoid, penetrates the dura, and passes within the cavernous sinus and can be magnified using a teaching microscope. (All other nerves course through the lateral wall of the cavernous sinus.) After passing through the superior orbital fissure within the annulus of Zinn, the nerve continues laterŽally to innervate the lateral rectus muscle. Read more on this subject.
