The optic nerve is embryologically derived from the optic stalk and is continuous with the optic tract in the brain. Therefore, many diseases of the optic nerve reflect those of the central nervous system (CNS). The optic nerve measures 35–55 mm, extending from the eye to the optic chiasm, where the fibers decussate; from there, the fibers continue until they synapse in the lateral geniculate nucleus. The optic nerve is divided into 4 topographic areas (the length of each is given in parentheses):
intraocular (0.7–1.0 mm)
intraorbital (25–30 mm, with curvature and slack to accommodate eye movement)
intracanalicular (4–10 mm)
intracranial (average = 10 mm)
The clinically visible portion of the optic nerve inside the eye is known as the optic disc. The portion of the optic nerve inside the eye and anterior to the lamina cribrosa is known as the optic nerve head (ONH). These terms are often used interchangeably.
The optic nerve consists of retinal ganglion cell axons and glial cells (Fig 15-1). Glial cells (glia = glue), including oligodendrocytes, astrocytes, and microglial cells, make up the supportive tissue of the CNS. Their functions are listed in Table 15-1.
The optic nerve becomes myelinated by oligodendrocytes just posterior to the lamina cribrosa of the sclera, making it larger in diameter than the optic nerve head. The optic nerve is also encased in a meningeal sheath consisting of the following: the external dense connective tissue sheath, or dura (which merges with the sclera anteriorly and the bony optic canal and the dura of the brain posteriorly); the cellular arachnoid sheath (which forms part of the internal sheath of the optic nerve and is the continuation of the arachnoidea mater around the brain); and the fibrovascular pial sheath (which also forms part of the internal sheath and is the continuation of the pia mater around the brain). The pial vessels and connective tissue extend into the optic nerve and divide the nerve fibers into fascicles. The subarachnoid space of the optic nerve sheath contains cerebrospinal fluid (Fig 15-2).
Figure 15-1 Longitudinal section of normal optic nerve. Axons of the retinal ganglion cells (R) travel in the nerve fiber layer toward the optic disc and make a 90° turn posteriorly to become the axonal fibers of the optic nerve. Optic nerve axons pass through the fenestrations in the lamina cribrosa (arrowheads), a perforated area in the posterior sclera (S), and become myelinated, increasing the diameter of the retrolaminar nerve. Sclera surrounding the lamina cribrosa is continuous with the dura of the optic nerve (D). A = arachnoid; C = choroid; CRA = central retinal artery; CRV = central retinal vein; P = pial septa.
(Courtesy of Tatyana Milman, MD.)
Table 15-1 Supporting Cells in the Central Nervous System
The vascular supply of the optic nerve stems predominantly from pial vessels, which are branches of the ophthalmic and superior hypophyseal arteries; the short posterior ciliary arteries mainly supply the optic nerve head. See BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 5, Neuro-Ophthalmology, for additional discussion of the optic nerve and its vascular supply.
Figure 15-2 Cross section through a normal optic nerve. The axons of the optic nerve are segregated into fascicles by the delicate, fibrovascular pial septa. The nuclei of oligodendrocytes, astrocytes, and microglia are visible between the eosinophilic axons. The subdural space (asterisk) is relatively narrow in the normal optic nerve.
(Courtesy of Tatyana Milman, MD.)
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.