Walsh & Hoyt: General Anatomy

The optic nerve can be separated into four sections: the intraocular, intraorbital, intraosseous, and intracranial portions. The intraocular segment of the optic nerve is primarily formed by coalescence of RGC axons, with the addition of three nonneural elements: astrocytes, capillary-associated cells, and fibroblasts. The ONH is a major zone of transition because the nerve fibers pass from an area of relatively high intraocular pressure to the lower-pressure zone of the retro-orbital segment of the optic nerve, which is (usually) equivalent to the intracranial pressure; leave the blood supply of the central retinal artery to receive blood from branches of the posterior ciliary and ophthalmic arteries; make a 90-degree turn and enter the tight confines of the lamina cribrosa; and become myelinated just behind the lamina cribrosa.