Chapter 33

The optic nerve is the structure at the back of the eye that carries visual information from the eye to the central nervous system (CNS). Approximately 120 million rods and cones sense light in the outer retina. This information is transmitted to approximately 1 million ganglion cells in the inner retina. These ganglion cells converge to form the optic nerve. The electrical impulses generated in the optic nerve are transmitted to the occipital lobe via the lateral geniculate nucleus. Posterior to the eye, the optic nerve is covered by a dural sheath and arachnoid membrane. This structure surrounds the nerve until it enters the brain, where it is contiguous with the subdural space (Figure 33–1).

###### FIGURE 33–1

Normal microscopic appearance of optic nerve as it exits the eyeball. The neurons of the inner ganglion cell layer of the retina combine to form the nerve. The sclera surrounds the nerve. A portion of the central retinal artery is visible in cross-section. The retina is artificially detached due to histological processing. (Photograph contributed by Morton Smith, MD.)

The only portion of the optic nerve that is visible on examination is the site where the nerve attaches to the eye at the posterior retina (Figure 33–2). The normal nerve has a central area through which the central retinal artery travels, branching into vessels lining the inner layer of the retina. The central portion is called the cup, and the entire area is called the disc. The cup:disc ratio describes the relationship between these 2 structures. The normal ratio ranges from 0.1 to 0.4, although larger cups may be normal in certain ethnic populations, particularly African Americans. Increased intraocular pressure may cause enlargement of the cup, and increased intracranial pressure (ICP) may cause edema and obscuration of the cup.

###### FIGURE 33–2

Normal view of optic nerve as seen with opththalmoscope. The cup:disc ratio of this nerve is 0.1.

Optic nerve problems in children occur infrequently. However, bilateral optic nerve hypoplasia (ONH) is one of the common causes of profound visual loss in infancy.

### Embryology

The optic nerves form from the optic stalk, the structure that connects the embryonic forebrain and optic vesicle. It contains an outer layer of cells from the neural crest, which differentiates into the structures of the sheath that surround the optic nerve (pia, arachnoid, and dura). An inner neuroectodermal layer regresses and is replaced by ganglion cells that originate in the inner layer of the retina and travel through the optic nerve to the lateral geniculate body. At 4 months of gestation, almost 4 million ganglion cells are present within the nerve. This number decreases to 1 million by the time of birth. Myelinization of the optic nerve ...

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