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Optic nerve

In neuroanatomy , the optic nerve , also known as the second cranial nerve , cranial nerve II , or simply CN II , is a paired cranial nerve that transmits visual information fro...

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In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain.[1] In humans, the optic nerve is derived from optic stalks during the seventh week of development and is composed of retinal ganglion cell axons and glial cells; it extends from the optic disc to the optic chiasma and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.[1][2]

Structure

The optic nerve has been classified as the second of twelve paired cranial nerves, but it is technically a myelinated tract of the central nervous system, rather than a classical nerve of the peripheral nervous system because it is derived from an out-pouching of the diencephalon (optic stalks) during embryonic development.[1] As a consequence, the fibers of the optic nerve are covered with myelin produced by oligodendrocytes, rather than Schwann cells of the peripheral nervous system, and are encased within the meninges.[1]Peripheral neuropathies like Guillain–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, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves. Fiber tracts of the mammalian central nervous system have only limited regenerative capabilities compared to the peripheral nervous system.[3] Therefore, in most mammals, optic nerve damage results in irreversible blindness. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the primary visual cortex.

A fundus photograph showing the back of the retina. The white circle is the beginning of the optic nerve.

The optic nerve is composed of retinal ganglion cell axons and glia.[1] Each human optic nerve contains between 770,000 and 1.7 million nerve fibers,[4] which are axons of the retinal ganglion cells of one retina. In the fovea, which has high acuity, these ganglion cells connect to as few as 5 photoreceptor cells; in other areas of the retina, they connect to thousands of photoreceptors.

El nervio óptico sale de la órbita (cavidad ocular) a través del canal óptico , dirigiéndose posteromedialmente hacia el quiasma óptico , donde se produce una decusación parcial (cruce) de fibras de los campos visuales temporales (la hemirretina nasal) de ambos ojos. La proporción de fibras decusadas varía entre especies y se correlaciona con el grado de visión binocular que posee una especie. [ 5 ] La mayoría de los axones del nervio óptico terminan en el núcleo geniculado lateral desde donde se transmite la información a la corteza visual , mientras que otros axones terminan en el área pretectal [ 6 ] y participan en los movimientos oculares reflejos . Otros axones terminan en el núcleo supraquiasmático y participan en la regulación del ciclo sueño-vigilia . Su diámetro aumenta de aproximadamente 1,6  mm dentro del ojo a 3,5  mm en la órbita y a 4,5  mm dentro del espacio craneal. Las longitudes de los componentes del nervio óptico son de 1  mm en el globo ocular, 24  mm en la órbita, 9  mm en el canal óptico y 16  mm en el espacio craneal antes de unirse al quiasma óptico. Allí, se produce una decusación parcial y aproximadamente el 53 % de las fibras se cruzan para formar los tractos ópticos. La mayoría de estas fibras terminan en el cuerpo geniculado lateral. [ 2 ]

Basándonos en esta anatomía, el nervio óptico se puede dividir en cuatro partes, como se indica en la imagen de la parte superior de esta sección (esta vista es desde arriba, como si se observara la órbita después de haber retirado la parte superior del cráneo): 1. la cabeza óptica (donde comienza en el globo ocular con fibras de la retina); 2. la parte orbitaria (que se encuentra dentro de la órbita); 3. la parte intracanalicular (que se encuentra dentro de un canal óseo conocido como canal óptico); y 4. la parte craneal (que se encuentra dentro de la cavidad craneal y termina en el quiasma óptico). [ 1 ]

Desde el cuerpo geniculado lateral, las fibras de la radiación óptica pasan a la corteza visual en el lóbulo occipital del cerebro. [ 1 ] En términos más específicos, las fibras que transportan información del campo visual superior contralateral atraviesan el asa de Meyer para terminar en la circunvolución lingual debajo de la fisura calcarina en el lóbulo occipital, y las fibras que transportan información del campo visual inferior contralateral terminan más superiormente, en el cuneus . [ 7 ]

Función

The optic nerve transmits all visual information including brightness perception, color perception and contrast (visual acuity).[1] It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex.[1] 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 the eye that occurs when one looks at a near object (for example: when reading, the lens adjusts to near vision).[2]

The eye's blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.[2]

Clinical significance

Disease

Damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is important for the diagnosis of nerve damage.[1]

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 (see diagram above) will affect the areas of vision loss. Damage to the optic nerve that is anterior, 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 or bitemporal hemianopsia (see image to the right). Such damage may occur with large pituitary tumors, such as pituitary adenoma. 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, e.g., if the left optic tract were cut, there would be a loss of vision from the entire right visual field.[1]

Injury to the optic nerve can be the result of congenital or inheritable problems like Leber's hereditary optic neuropathy, glaucoma, trauma, toxicity, inflammation, ischemia, infection (very rarely), or compression from tumors or aneurysms. 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; and anterior ischemic optic neuropathy, usually in those older than 50.

Glaucoma is a group of diseases involving loss of retinal ganglion cells causing optic neuropathy in a pattern of peripheral vision loss, initially sparing central vision. Glaucoma is frequently associated with increased intraocular pressure that damages the optic nerve as it exits the eyeball. The trabecular meshwork assists the drainage of aqueous humor fluid. The presence of excess aqueous humor, increases IOP, yielding the diagnosis and symptoms of glaucoma.[8]

Optic neuritis is inflammation of the optic nerve. It is associated with a number of diseases, the most notable one being multiple 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 of blood 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.[2]

Ophthalmologists and optometrists can detect and diagnose some optic nerve diseases but neuro-ophthalmologists are often best suited to diagnose and treat diseases of the optic nerve.

Additional images

See also

References

  1. 1234567891011Smith AM, Czyz CN (7 November 2022). "Neuroanatomy, Cranial Nerve 2 (Optic)". StatPearls, US National Library of Medicine. Retrieved 11 July 2026.
  2. 12345Vilensky, 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. ISBN 978-1118492017.
  3. ↑Benowitz, Larry; Yin, Yuqin (August 2010). "Optic Nerve Regeneration". Archives of Ophthalmology. 128 (8): 1059–1064. doi:10.1001/archophthalmol.2010.152. ISSN 0003-9950. PMC 3072887. PMID 20697009.
  4. ↑Jonas, Jost B.; et al. (May 1992). "Human optic nerve fiber count and optic disc size". Investigative Ophthalmology & Visual Science. 33 (6): 2012–8. PMID 1582806.
  5. ↑Textbook of Veterinary Anatomy, 4th Edition. Dyce, Sack and Wensing
  6. ↑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. ISSN 0021-9967. PMID 3346381. S2CID 21565504.
  7. ↑"Vision". casemed.case.edu. Archived from the original on 2020-01-26. Retrieved 2020-01-23.
  8. ↑"The Eye's Drainage System, the Trabecular Meshwork & Glaucoma | BrightFocus Foundation". www.brightfocus.org. Retrieved 2022-01-10.
  • The optic nerve on MRI