Optic nerve

Nerve: Optic Nerve
The left optic nerve and the optic tracts.
Inferior view of the human brain, with the cranial nerves labelled.
Latin	nervus opticus
Gray's	subject #197 882
MeSH	Optic+Nerve

The optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.

Anatomy

The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development. Consequently, the fibres are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system and are encased within the meninges. Therefore the distinction of nerve is technically a misnomer, as the optic system lies within the central nervous system and nerves exist, by definition, within the peripheral nervous system. Therefore 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. Fibre tracks 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, whence a major relay inputs into the primary visual cortex.

The optic nerve is composed of retinal ganglion cell axons and support cells. It leaves the orbit (eye) 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 of both eyes. 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 and 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. More specifically, 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.

Physiology

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.

Each optic nerve contains around 1.2 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.

Role in disease

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. Generally speaking:

• Damage before the optic chiasm causes loss of vision in the visual field of the same side only.
• Damage in the chiasm causes loss of vision laterally in both visual fields (bitemporal hemianopia). It may occur in large pituitary adenomata.
• Damage after the chiasm causes loss of vision on one side but affecting both visual fields: the visual field affected is located on the opposite side of the lesion.

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.

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

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 under-development of the optic nerve causing little to no vision in the affected eye.

Ophthalmologists, particularly those sub specialists who 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.

Additional images

MRI scan of human eye showing optic nerve.	The ophthalmic artery and its branches. (optic nerve is yellow)	Dura mater and its processes exposed by removing part of the right half of the skull, and the brain.	Tentorium cerebelli from above.
Superficial dissection of brain-stem. Lateral view.	Dissection of brain-stem. Lateral view.	Mesal aspect of a brain sectioned in the median sagittal plane.	Scheme showing central connections of the optic nerves and optic tracts.
The fornix and corpus callosum from below.	Nerves of the orbit. Seen from above.	Nerves of the orbit, and the ciliary ganglion. Side view.	The arteries of the choroid and iris. The greater part of the sclera has been removed.
The veins of the choroid.	The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.	Human brain dura mater (reflections)

See also

• List of mnemonics for the cranial nerves

References

External links

• The optic nerve on MRI
• BrainMaps at UCDavis optic%20nerve

• IFOND
• 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.
• lesson3 at The Anatomy Lesson by Wesley Norman (Georgetown University) (orbit4)
• cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (II)