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The left optic nerve and the optic tracts.
|Anatomical terms of neuroanatomy|
The optic nerve, also known as cranial nerve II, is a paired nerve that transmits visual information from the retina to the brain. 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. In humans, the optic nerve extends from the optic disc to the optic chiasm and continues as the optic tract to the lateral geniculate nucleus, pretectal nuclei, and superior colliculus.
The optic nerve is the second of twelve paired cranial nerves and the only one to be considered part of the central nervous system, as it is derived from an out-pouching of the diencephalon (optic stalks during embryonic development. As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells of the peripheral nervous system, and are encased within the meninges. Peripheral neuropathies like Guillain-Barré syndrome do not affect the optic nerve.
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 (as opposed to the peripheral nervous system) are incapable of regeneration, and, hence, optic nerve damage produces irreversible blindness. The fibres 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.
The optic nerve is composed of retinal ganglion cell axons and glial cells. Each human optic nerve contains between 770,000 and 1.7 million nerve fibers, 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 retina, they connect to many thousand photoreceptors.
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 fibres from the temporal visual fields (the nasal hemi-retina) of both eyes. The proportion of decussating fibers varies between species, and is correlated with the degree of binocular vision enjoyed by a species. Most of the axons of the optic nerve terminate in the lateral geniculate 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. 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 fibres terminate in the lateral geniculate body.
From the lateral geniculate body, fibers of the optic radiation pass to the visual cortex in the occipital lobe of the brain. In more specific terms, fibers carrying information from the contralateral superior visual field traverse Meyer's loop to terminate in the lingual gyrus below the calcarine fissure in the occipital lobe, and fibers carrying information from the contralateral inferior visual field terminate more superiorly, to the cuneus.
The optic nerve transmits sensory information regarding brightness perception, red-green color perception, contrast (visual acuity), and visual fields. The optic nerve contains special somatic afferent sensory fibers.
There are five components to the clinical examination of the optic nerve:
- Visual acuity
- Color vision
- Visual fields
- Pupillary response to light (tests both cranial nerves II and III, plus sympathic nerve fibers)
Damage to the optic nerve typically causes permanent and potentially severe loss of vision, as well as an abnormal pupillary reflex, which is diagnostically important. The type of visual field loss will depend on which portions of the optic nerve were damaged. In general:
- Damage to the optic nerve anterior to the optic chiasm causes loss of vision in the eye on the same side as the damage.
- Damage in the optic chiasm causes loss of vision laterally in both visual fields (bitemporal hemianopsia). It may occur in large pituitary adenoma.
- Damage to the optic tract posterior to the chiasm causes loss of vision in the visual field on the side opposite to the damage.
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).
Anterior Ischemic Optic Neuropathy is a particular type of infarct that affects patients with an anatomical predisposition and cardiovascular risk factors.
Optic nerve hypoplasia is the underdevelopment of the optic nerve causing little to no vision in the affected eye.
Ophthalmologists and optometrists can detect and diagnose some optic nerve diseases but, those sub-specialists that are neuro-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 information on a variety of optic nerve disorders and may provide general direction.
The ophthalmic artery and its branches. (optic nerve is yellow)
|Wikimedia Commons has media related to Optic nerve.|
- This article uses anatomical terminology; for an overview, see anatomical terminology.
- Jonas, Jost B. et al. (May 1992). "Human optic nerve fiber count and optic disc size". Investigative Ophthalmology & Visual Science 33 (6).
- Textbook of Veterinary Anatomy, 4th Edition. Dyce, Sack and Wensing
- "GLAUCOMA AND RETINAL NEURODEGENERATIVE RESEARCH GROUP". http://www.ucl.ac.uk/.
- The optic nerve on MRI
- Stained brain slice images which include the "optic%20nerve" at the BrainMaps project
- 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.
- Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)
- lesson3 at The Anatomy Lesson by Wesley Norman (Georgetown University) (orbit4)
- cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (II)
- Notes on Optic Nerve