Nerves of Orbit

The large optic nerves convey purely sensory nerves that transmit impulses generated by optical stimuli. They are cranial nerves (CN II) by convention, but develop as paired anterior extensions of the forebrain and are actually central nervous system (CNS) fiber tracts formed of second-order neurons. The optic nerves begin at the lamina cribrosa of the sclera, where the unmyelinated nerve fibers pierce the sclera and become myelinated, posterior to the optic disc. They exit the orbits via the optic canals. Throughout their course in the orbit, the optic nerves are surrounded by extensions of the cranial meninges and subarachnoid space, the latter occupied by a thin layer of CSF. The intra-orbital extensions of the cranial dura and arachnoid constitute the optic nerve sheath, which becomes continuous anteriorly with the fascial sheath of the eyeball and the sclera. A layer of pia mater covers the surface of the optic nerve within the sheath.

In addition to the optic nerve (CN II), the nerves of the orbit include those that enter through the superior orbital fissure and supply the ocular muscles: oculomotor (CN III), trochlear (CN IV), and abducent (CN VI) nerves. A memory device for the innervation of the extra-ocular muscles moving the eyeball is similar to a chemical formula: LR₆SO₄AO₃ (lateral rectus, CN VI; superior oblique, CN IV; all others, CN III). The trochlear and abducent nerves pass directly to the single muscle supplied by each nerve. The oculomotor nerve divides into a superior and an inferior division. The superior division supplies the superior rectus and levator palpebrae superioris. The inferior division supplies the medial and inferior rectus and inferior oblique and carries presynaptic parasympathetic fibers to the ciliary ganglion. The movements are stimulated by the oculomotor, trochlear, and abducent nerves, starting from the primary position in the right and left orbits, and produce binocular vision.

The three terminal branches of the ophthalmic nerve, CN V₁ (the frontal, nasociliary, and lacrimal nerves), pass through the superior orbital fissure and supply structures related to the anterior orbit (e.g., lacrimal gland and eyelids), face, and scalp.

The ciliary ganglion is a small group of postsynaptic parasympathetic nerve cell bodies associated with CN V₁. It is located between the optic nerve and the lateral rectus toward the posterior limit of the orbit. The ganglion receives nerve fibers from three sources:

• Sensory fibers from CN V₁ via the sensory or nasociliary root of the ciliary ganglion.

• Presynaptic parasympathetic fibers from CN III via the parasympathetic or oculomotor root of the ciliary ganglion.

• Postsynaptic sympathetic fibers from the internal carotid plexus via the sympathetic root of the ciliary ganglion.

The short ciliary nerves arise from the ciliary ganglion and are considered to be branches of CN V₁. They carry parasympathetic and sympathetic fibers to the ciliary body and iris. The short ciliary nerves consist of postsynaptic parasympathetic fibers originating in the ciliary ganglion, afferent fibers from the nasociliary nerve that pass through the ganglion, and postsynaptic sympathetic fibers that also pass through it. Long ciliary nerves, branches of the nasociliary nerve (CN V₁) that pass to the eyeball, bypassing the ciliary ganglion, convey postsynaptic sympathetic fibers to the dilator pupillae and afferent fibers from the iris and cornea.

The posterior and anterior ethmoidal nerves, branches of the nasociliary nerve arising in the orbit, exit via openings in the medial wall of the orbit to supply the mucous membrane of the sphenoidal and ethmoidal sinuses and the nasal cavities, as well as the dura of the anterior cranial fossa.