Optic Nerve (Cranial Nerve II)

Synonyms

Cranial nerve II (CN II); Optic disk; ; Optic nerve head

Definition

The optic nerve ON (cranial nerve II) is an afferent sensory nerve that transmits visual input from the eye to the occipital cortex. The second of 12 paired cranial nerves, the optic nerve is unique because it is part of the central nervous system and develops from the diencephalon. The ON is surrounded by the meninges of the optic nerve sheath and is myelinated by oligodendrocytes. The ON consists of an average of 1.2 million axons originating from the retinal ganglion cells in the retinal nerve fiber layer which receives visual input from photoreceptors in the retina. The ON is approximately 1.5 mm in diameter and enlarges to 3.5 mm posterior to the lamina cribrosa because of the myelin nerve sheath, which protects and insulates the nerve. The optic nerve about is 45–50 mm long and consists of four portions: intraocular, intraorbital, intracanalicular, and intracranial. The intraocular portion (1 mm), also...