The optic nerve (CN II) consists of more than 1 million axons that originate in the ganglion cell layer of the retina and extend toward the lateral geniculate nucleus. The optic nerve begins anatomically at the optic nerve head (ONH) but physiologically and functionally within the ganglion cell layer that covers the entire retina and continues to the optic chiasm. It may be divided into the following 4 topographic areas (Fig 3-4, Table 3-1):
The optic nerve originates directly from the diencephalon and, developmentally, is part of the brain and central nervous system. Its fibers are surrounded not by Schwann cells but by myelin produced by oligodendrocytes. The intraorbital portion is approximately 25–30 mm long, which is greater than the distance between the back of the globe and the optic canal (18 mm). For this reason, when the eye is in the primary position, the optic nerve runs a sinuous course. Axial proptosis secondary to thyroid eye disease or a retrobulbar tumor will first lead to straightening of the intraorbital optic nerve. Further elongation can lead to stretching of the optic nerve, which may cause chronic nerve injury and optic neuropathy.
The ONH is the principal site of many congenital and acquired ocular diseases. Its anterior surface is visible ophthalmoscopically as the optic disc, an oval structure whose size reflects some ethnic and racial variance. The size of the ONH varies widely, averaging 1.76 mm horizontally and 1.92 mm vertically. The central depression, or cup, is located slightly temporal to the geometric center of the nerve head and represents an axon-free region. Results of studies have found that the cup maintains its size or enlarges throughout life. The main branches of the central retinal artery (CRA) and the central retinal vein (CRV) pass through the center of the cup.
The ONH can be divided into 4 topographic areas (Fig 3-5):
These are discussed in the following sections. Note: The term optic disc has been used interchangeably in the literature to refer to the superficial NFL and the prelaminar area, or to the entire ONH. This book uses the term optic nerve head to refer to all 4 parts.
Garway-Heath DF, Wollstein G, Hitchings RA. Aging changes of the optic nerve head in relation to open angle glaucoma. Br J Ophthalmol. 1997;81(10):840–845.
Jonas JB, Gusek GC, Naumann GO. Optic disc, cup and neuroretinal rim size, configuration and correlations in normal eyes. Invest Ophthalmol Vis Sci. 1988;29(7):1151–1158.
Superficial nerve fiber layer
As the unmyelinated ganglion cell axons enter the nerve head, they retain their retinotopic organization, with fibers from the upper retina superiorly and those from the lower retina inferiorly. Fibers from the temporal retina are lateral; those from the nasal side are medial. Macular fibers, which constitute approximately one-third of the nerve, occupy the immediate temporal aspect of the ONH. All other temporal fibers with origins distal to the macula are laterally displaced above or below the macular fibers (Fig 3-6).
The ganglion cell axons that enter the nerve head are supported by a “wicker basket” of astrocytic glial cells and are segregated into bundles, or fascicles, that pass through the lamina cribrosa (see Fig 3-5). These astrocytes invest the optic nerve and form continuous circular tubes that enclose groups of nerve fibers throughout their intraocular and intraorbital course, separating them from connective tissue elements at all sites. At the edge of the nerve head, the Müller cells that make up the internal limiting membrane (ILM) are replaced by astrocytes. Astrocytes constitute 10% of the nerve head volume and form a membrane that not only covers the surface of the nerve head but is continuous with the ILM of the retina.
The pigment epithelium may be exposed at the temporal margin of the ONH to form a narrow, pigmented crescent. When the pigment epithelium and choroid fail to reach the temporal margin, crescents of partial or absent pigmentation may be noted. The relationship between the choroid and the prelaminar portion of the optic nerve partly accounts for the staining of the ONH normally observed in late phases of fluorescein fundus angiography. The ONH vessels do not leak, but the choroidal capillaries are freely permeable to fluorescein, which can therefore diffuse into the adjacent optic nerve layers.
Figure 3-5 Schematic representation of the optic nerve head (ONH). The temporal retina has a thicker layer of ganglion cells, representing the increased ganglion cell concentration found in the macula. Müller glia traverse the neural retina to provide both structural and functional support. Where the retina terminates at the ONH edge, the Müller cells are continuous with the astrocytes, forming the internal limiting membrane. The border tissue of Elschnig is the dense connective tissue that joins the sclera with the Bruch membrane, enclosing the choroid and forming the scleral ring that defines the margin of the ONH. At the posterior termination of the choroid on the temporal side, the border tissue of Elschnig lies between the astrocytes surrounding the optic nerve canal and the stroma of the choroid. On the nasal side, the choroidal stroma is directly adjacent to the astrocytes surrounding the nerve. This collection of astrocytes surrounding the canal is known as the border tissue, which is continuous with a similar glial lining at the termination of the retina. The nerve fibers of the retina are segregated into approximately 1000 fascicles by astrocytes. On reaching the lamina cribrosa (upper dashed line), the nerve fascicles and their surrounding astrocytes are separated from each other by connective tissue. The lamina cribrosa is an extension of scleral collagen and elastic fibers through the nerve. The external choroid also sends some connective tissue to the anterior part of the lamina. At the external part of the lamina cribrosa (lower dashed line), the nerve fibers become myelinated, and columns of oligodendrocytes and a few astrocytes are present within the nerve fascicles. The bundles continue to be separated by connective tissue septa (derived from pia mater and known as septal tissue) all the way to the chiasm. A mantle of astrocytes, continuous anteriorly with the border tissue, surrounds the nerve along its orbital course. The dura, arachnoid, and pia mater are shown. The nerve fibers are myelinated. Within the bundles, the cell bodies of astrocytes and oligodendrocytes form a column of nuclei. The central retinal vessels are surrounded by a perivascular connective tissue throughout its course in the nerve. This connective tissue, known as the central supporting connective tissue strand, blends with the connective tissue of the lamina cribrosa. 1 = superficial nerve fiber layer; 2 = prelaminar area; 3 = laminar area; 4 = retrolaminar area.
(Illustration by Mark Miller.)
Figure 3-6 The pattern of the nerve fiber layer of axons from retinal ganglion cells to the ONH. Temporal axons originate above and below the horizontal raphe (HR) and take an arching course to the ONH. Axons arising from ganglion cells in the nasal macula project directly to the ONH as the papillomacular bundle (PM).
(Reproduced from Kline LB, Foroozan R, eds. Optic Nerve Disorders. 2nd ed. Ophthalmology Monographs 10. New York: Oxford University Press, in cooperation with the American Academy of Ophthalmology; 2007:5.)
The lamina cribrosa comprises approximately 10 connective tissue plates, which are integrated with the sclera and whose pores transmit the unmyelinated axon bundles of the retinal ganglion cells before they exit as the optic nerve. The openings are wider superiorly than inferiorly, which may imply less protection from the mechanical effects of pressure in glaucoma. The lamina contains type I and type III collagens, abundant elastin, laminin, and fibronectin. Astrocytes surround the axon bundles, and small blood vessels are present.
The lamina cribrosa serves the following 3 functions:
scaffold for the optic nerve axons
point of fixation for the CRA and CRV
reinforcement of the posterior segment of the globe
Optical coherence tomography and scanning laser ophthalmoscopy are being used to facilitate anatomical study of the lamina cribrosa in pathologic states such as glaucoma and retinal vascular disease.
As a result of myelination of the nerve fibers and the presence of oligodendroglia and the surrounding meningeal sheaths (internal, arachnoid, and external) (see Fig 3-5), the diameter of the optic nerve increases to 3 mm behind the lamina cribrosa. The retrolaminar nerve transitions to the intraorbital part of the optic nerve, to the apex of the orbit. The axoplasm of the neurons contains neurofilaments, microtubules, mitochondria, and smooth endoplasmic reticulum.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.