Optic Nerve 

The optic nerve, also known as cranial nerve II, is a paired cranial nerve that transmits visual information from the retina to the brain.

Structure

The optic nerve has been classified as the second of twelve paired cranial nerves but it is technically part of the central nervous system. The fibers of the optic nerve are covered with myelin produced by oligodendrocytes and are encased within the meninges.

The optic nerve leaves the orbit (eye socket) via the optic canal, running postero-medially towards the optic chiasm, where there is a partial decussation (crossing) of fibers from the temporal visual fields of both eyes. Most of the axons of the optic nerve terminate in the lateral nucleus from where information is relayed to the visual cortex, while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements. Other axons terminate in the suprachiasmatic nucleus and are involved in regulating the sleep-wake cycle.

Based on this anatomy, the optic nerve may be divided in the four parts as indicated in the image at the top of this section 1. the optic head, which is where it begins in the eyeball (globe) with fibers from the retina; 2.the orbital part (which is the part within the orbit). 3. intracanicular part (which is the part within a bony canal known as the optic canal), and, 4. cranial part (the part within the cranial cavity, which ends at the optic chiasm).

Function

The optic nerve transmits all visual information including brightness perception, color perception and contrast (visual acuity). It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex. The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye. The accommodation reflex refers to the swelling of the lens of eye that occurs when one looks at a near object.

Clinical Significance

Damage to the optic nerve typically causes permanent and potentially severe loss of vision.

The type of visual field loss will depend on which portions of the optic nerve were damaged. In general, the location of the damage in relation to the optic chiasm will affect the areas of vision loss. Damage to the optic nerve that is in front of the optic chiasm (toward the face) causes loss of vision in the eye on the same side as the damage. Damage at the optic chiasm itself typically causes loss of vision laterally in both visual fields. Finally, damage to the optic tract, which is posterior to, or behind the chiasm, causes loss of the entire visual field from the side opposite the damage.By far, the three most common injuries to the optic nerve are from glaucoma; optic neuritis, especially in those younger than 50 years of age.

Glaucoma is a group of diseases involving loss of retinal ganglion cells causing optic neuropathy in a pattern of peripheral vision loss. Glaucoma is frequently associated with increased intraocular pressure that damages the optic nerve.

Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, the most notable one being multiple sclerosis.

Anterior ischemic optic neuropathy is commonly known as “stroke of the optic nerve” and affects the optic nerve head. There is usually a sudden loss of blood supply and nutrients to the optic nerve head.

Source: Wikipedia – January 2021.

Clarification: This text is an excerpt from a published Wikipedia article.