Anatomy of the optic nerve. The optic nerve consists of approximately 1 million nerve fibers arising from retinal ganglion cells. These axons extend from the retinal ganglion cells to the optic nerve as the nerve fiber layer of the retina. The axons forming the optic nerve maintain the retinal relationships. Supertemporal and nasal fibers form the superior optic nerve, and inferior axons form the inferior optic nerve. The fibers arising from the foveal and parafoveal ganglion cell enter the temporal portion of the optic nerve at the papillomacular bundle.

The intraocular optic nerve measures 1.5 mm by 1 mm in diameter and 1 mm in length. These axons turn posteriorly to exit the globe perpendicular to the surface layer. They are divided into bundles by Müller cells in the retina and continue as bundles separated by astrocytes as they pass through the prelaminar region of the optic nerve. Retina and choroid surround this region. The nerve fibers pass through the multiple fenestrations of the lamina cribrosa, which is formed by connective tissue continuous with the surrounding sclera and distal dura mater (Du) of the optic nerve.

The eye and optic nerve receive their blood supply from branches of the ophthalmic artery (Ar). The ophthalmic artery arises from the internal carotid artery lateral to the optic nerve. Passing inferior to the optic nerve, it enters the orbit through the optic canal. The central retinal artery arises from the ophthalmic artery and penetrates the substance of the optic nerve 6 to 12 mm from the globe. The major branch extends anteriorly through the substance of the optic nerve to nourish the retina, along the way providing minor branches to the core of the retrolaminar optic nerve and the surface of the optic disc. The blood supply of the optic nerve head is derived from the short posterior ciliary arteries. These vessels nourish the choroid, prelaminar optic nerve, and lamina cribrosa and form a pial plexus along the exterior of the retrolaminar optic nerve.

Blood is drained from all regions of the optic nerve head, predominantly into the central retinal vein. The choroidal circulation is involved to a lesser extent. 1a—inner limiting membrane of retina; 1b—inner limiting membrane of Elschnig; 2—central meniscus of Kuhnt; 3—border tissue of Jacoby; 5—intermediary tissue of Kuhnt; 6—anterior portion of lamina cribrosa; 7—posterior portion of lamina cribrosa; GIC—cell bodies; GIM—glial mantle of Fuchs; Pia—pia matter; Sep—septum. (Adapted from Anderson and Hoyt [1]; with permission.)