The optic nerve has been classified as the second of twelve pairedcranial nerves, but it is technically a myelinated tract of thecentral nervous system, rather than a classical nerve of theperipheral nervous system because it is derived from an out-pouching of thediencephalon (optic stalks) during embryonic development. As a consequence, the fibers of the optic nerve are covered withmyelin produced byoligodendrocytes, rather thanSchwann cells of the peripheral nervous system, and are encased within themeninges.[3]Peripheral neuropathies likeGuillain–Barré syndrome do not affect the optic nerve. However, most typically, the optic nerve is grouped with the other eleven cranial nerves and is considered to be part of the peripheral nervous system.
The optic nerve is ensheathed in all three meningeal layers (dura,arachnoid, andpia mater) rather than theepineurium,perineurium, andendoneurium found in peripheral nerves.Fiber tracts of the mammalian central nervous system have only limited regenerative capabilities compared to the peripheral nervous system.[4] Therefore, in most mammals, opticnerve damage results in irreversibleblindness. The fibers from theretina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into theprimary visual cortex.
Afundus photograph showing the back of the retina. The white circle is the beginning of the optic nerve.
The optic nerve is composed ofretinal ganglion cell axons andglia. Each human optic nerve contains between 770,000 and 1.7 million nerve fibers,[5] which are axons of the retinal ganglion cells of one retina. In thefovea, which has high acuity, these ganglion cells connect to as few as 5photoreceptor cells; in other areas of the retina, they connect to thousands of photoreceptors.
The optic nerve leaves theorbit (eye socket) via theoptic canal, running postero-medially towards theoptic chiasm, where there is a partial decussation (crossing) of fibers from the temporalvisual fields (the nasal hemi-retina) of both eyes. The proportion of decussating fibers varies between species, and is correlated with the degree ofbinocular vision enjoyed by a species.[6] Most of theaxons of the optic nerve terminate in thelateral geniculate nucleus from where information is relayed to thevisual cortex, while other axons terminate in thepretectal area[7] and are involved in reflexiveeye movements. Other axons terminate in thesuprachiasmatic nucleus and are involved in regulating thesleep-wake cycle. Its diameter increases from about 1.6 mm within the eye to 3.5 mm in the orbit to 4.5 mm within the cranial space. The optic nerve component lengths are 1 mm in the globe, 24 mm in the orbit, 9 mm in the optic canal, and 16 mm in the cranial space before joining the optic chiasm. There, partial decussation occurs, and about 53% of the fibers cross to form the optic tracts. Most of these fibers terminate in the lateral geniculate body.[1]
Based on this anatomy, the optic nerve may be divided into four parts as indicated in the image at the top of this section (this view is from above as if you were looking into the orbit after the top of the skull had been removed): 1. the optic head (which is where it begins in the eyeball (globe) with fibers from the retina); 2. 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).[2]
From the lateral geniculate body, fibers of theoptic radiation pass to thevisual cortex in theoccipital lobe of the brain. In more specific terms, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in thelingual gyrus below thecalcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly, to thecuneus.[8]
The optic nerve transmits all visual information includingbrightness perception,color perception andcontrast (visual acuity). It also conducts the visual impulses that are responsible for two important neurological reflexes: thelight reflex and theaccommodation reflex. The light reflex refers to the constriction of bothpupils that occurs when light is shone into either eye. The accommodation reflex refers to the swelling of thelens of the eye that occurs when one looks at a near object (for example: when reading, the lens adjusts to near vision).[1]
The eye'sblind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.[1]
Damage to the optic nerve typically causes permanent and potentially severe loss ofvision, as well as an abnormal pupillary reflex, which is important for the diagnosis of nerve damage.
The type ofvisual 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 (see diagram above) will affect the areas of vision loss. Damage to the optic nerve that isanterior, or 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 orbitemporal hemianopsia (see image to the right). Such damage may occur with largepituitary tumors, such aspituitary adenoma. Finally, damage to theoptic tract, which is posterior to, or behind the chiasm, causes loss of the entire visual field from the side opposite the damage, e.g. if the left optic tract were cut, there would be a loss of vision from the entire right visual field.
Glaucoma is a group of diseases involving loss of retinal ganglion cells causingoptic neuropathy in a pattern ofperipheral vision loss, initially sparing central vision. Glaucoma is frequently associated with increased intraocular pressure that damages the optic nerve as it exits the eyeball. Thetrabecular meshwork assists the drainage ofaqueous humor fluid. The presence of excess aqueous humor, increases IOP, yielding the diagnosis and symptoms of glaucoma.[9]
Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, the most notable one beingmultiple sclerosis. The patient will likely experience varying vision loss and eye pain. The condition tends to be episodic.
Anterior ischemic optic neuropathy is commonly known as a "stroke of the optic nerve" and affects the optic nerve head (where the nerve exits the eyeball). There is usually a sudden loss of blood supply and nutrients to the optic nerve head. Vision loss is typically sudden and most commonly occurs upon waking up in the morning. This condition is most common in diabetic patients 40–70 years old.
Other optic nerve problems are less common.Optic nerve hypoplasia is the underdevelopment of the optic nerve resulting in little to no vision in the affected eye. Tumors, especially those of the pituitary gland, can put pressure on the optic nerve causing various forms of visual loss. Similarly,cerebral aneurysms, a swelling ofblood vessel(s), can also affect the nerve. Trauma can cause serious injury to the nerve. Direct optic nerve injury can occur from a penetrating injury to the orbit, but the nerve can also be injured by indirect trauma in which severe head impact or movement stretches or even tears the nerve.[1]
Ophthalmologists andoptometrists can detect and diagnose some optic nerve diseases butneuro-ophthalmologists are often best suited to diagnose and treat diseases of the optic nerve. The International Foundation for Optic Nerve Diseases (IFOND) sponsors research and provides information on a variety of optic nerve disorders.
^abcdeVilensky, Joel; Robertson, Wendy; Suarez-Quian, Carlos (2015).The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top". Ames, Iowa: Wiley-Blackwell.ISBN978-1118492017.
^Smith, Austen M.; Czyz, Craig N. (2021). "Neuroanatomy, Cranial Nerve 2 (Optic)".StatPearls. Treasure Island (FL): StatPearls Publishing.PMID29939684. Retrieved14 June 2021.
^Textbook of Veterinary Anatomy, 4th Edition. Dyce, Sack and Wensing
^Belknap, Dianne B.; McCrea, Robert A. (1988-02-01). "Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey".The Journal of Comparative Neurology.268 (1):13–28.doi:10.1002/cne.902680103.ISSN0021-9967.PMID3346381.S2CID21565504.
^"Vision".casemed.case.edu. Archived fromthe original on 2020-01-26. Retrieved2020-01-23.
online case history – Optic nerve analysis with both scanning laser polarimetry with variable corneal compensation (GDx VCC) and confocal scanning laser ophthalmoscopy (HRT II - Heidelberg Retina Tomograph). Also includes actual fundus photos.
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