The optic nerve carries the greater than one million axons that derive from the retinal ganglion cells and project to eight visual nuclei. Actually an extension or tract of the central nervous system, not a true peripheral nerve, the conventional designation of the optic nerve applies to that portion of this tract that extends from the eye to the optic chiasm.
This topic will review functional anatomy of the optic nerve and the differential diagnosis of optic nerve pathologies. Specific disorders are discussed in detail separately. (See "Optic neuritis: Pathophysiology, clinical features, and diagnosis" and "Nonarteritic anterior ischemic optic neuropathy: Clinical features and diagnosis".)
The optic nerve has four anatomic segments: the intraocular, intraorbital, intracanalicular, and intracranial sections [1,2].
●The intraocular portion, or the optic disc, consists of the unmyelinated retinal ganglion cell axons and astrocytes. As the nerve exits the globe, crossing the collagenous lamina cribrosa, it increases in diameter as it becomes myelinated by oligodendrocytes.
●The intraorbital segment of the optic nerve is approximately 25 to 30 mm. Within the orbital apex, the optic nerve is surrounded by dura, arachnoid, and pia mater. The subarachnoid space contains cerebrospinal fluid continuous with that of the brain and spinal cord. The dural covering fuses with the sclera around the optic nerve and more posteriorly with the periosteum of the optic canal. The nerve courses through the annulus of Zinn, which contains the connective tissue origins of the superior, medial, lateral, and inferior rectus muscles.