The optic nerve begins anatomically at the ONH but physiologically and functionally within the RGC layer that covers the entire retina. The first portion of the optic nerve, representing the confluence of approximately 1.0–1.2 million RGC axons, traverses the sclera through the lamina cribrosa, which contains approximately 200–300 channels. The combination of small channels and a unique blood supply (largely from branches of the posterior ciliary arteries) probably plays a role in several optic neuropathies. Axonal transport—both anterograde and retrograde—of molecules, subcellular organelles, and metabolic products occurs along the length of the optic nerve and is an energy-dependent system requiring high concentrations of oxygen. The anterograde axonal transport system can be subdivided into slow, intermediate, and fast speeds. The axonal transport system is sensitive to ischemic, inflammatory, and compressive processes.
Just posterior to the sclera, the optic nerve acquires a dural sheath that is contiguous with the periorbita of the optic canal and an arachnoid membrane that supports and protects the axons and is contiguous with the arachnoid of the subdural intracranial space through the optic canal. This arrangement permits free circulation of CSF around the optic nerve up to the ONH. Just posterior to the lamina cribrosa, the optic nerve also acquires a myelin coating, which increases its diameter to approximately 3 mm (6 mm including the optic nerve sheath) from the 1.5 mm of the ONH. The myelin investment is part of the membrane of oligodendrocytes that join the nerve posterior to the sclera.
The intraorbital optic nerve extends approximately 30 mm to the optic canal. The extra length of the intraorbital optic nerve allows unimpeded globe rotation as well as axial shifts within the orbit. The CRA and CRV travel within the anterior 10–12 mm of the optic nerve. The CRA supplies only a minor portion of the optic nerve circulation; most of the blood supply comes from pial branches of the surrounding meninges, which are in turn supplied by small branches of the OphA (see Fig 1-10). An approximate topographic representation is maintained in the optic nerve. Peripheral retinal representation is found more peripherally, and the papillomacular bundle travels temporally and increasingly centrally within the nerve.
As the optic nerve enters the optic canal, its dural sheath fuses with the periorbita. It is also surrounded by the annulus of Zinn, which serves as the origin of the 4 rectus muscles and the superior oblique muscle. Within the canal, the optic nerve is accompanied by the OphA inferiorly and separated from the superior orbital fissure by the optic strut (the lateral aspect of the lesser wing of the sphenoid), which terminates superiorly as the anterior clinoid. Medially, the optic nerve is separated from the sphenoid sinus by bone that may be thin or dehiscent. The optic canal normally measures approximately 8–10 mm long and 5–7 mm wide but may be elongated and narrowed by processes that cause bone thickening (eg, fibrous dysplasia, intraosseous meningioma). The canal runs superiorly and medially. Within the canal, the optic nerve is relatively anchored and can easily be injured by shearing forces transmitted from blunt facial trauma (see Chapter 4).
At its intracranial passage, the optic nerve passes under a fold of dura (the falciform ligament) that may impinge on the nerve, especially if it is elevated by lesions arising from the bone of the sphenoid (tuberculum) or the sella. Once it becomes intracranial, the optic nerve no longer has a sheath. The anterior loop of the carotid artery usually lies just below and temporal to the nerve, and the proximal ACA passes over the nerve. The gyrus rectus, the most inferior portion of the frontal lobe, lies above and parallel to the optic nerves. The 8–12 mm intracranial portion of the optic nerve continues to the optic chiasm.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.