The optic nerve, also called cranial nerve II, is the nerve that transmits visual information from the retina to the brain.
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The optic nerve, also called cranial nerve II, is the nerve that 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 fibers are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system. Similarly, 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. This is an important issue, as fiber 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 fibers 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 Portort 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 fibers from the nasal 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. 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 (and an unexplained anomaly)
The eye's blind spot is a result of the absence of retina where the optic nerve leaves the eye. This is because there are no photoreceptors in this area.
The optic nerve contains 1.2 million nerve fibers. This number is low compared to the roughly 100 million photoreceptors in the retina, and implies that substantial pre-processing takes place in the retina before the signals are sent to the brain through the optic nerve.fact: date=August 2007 There is however an alternative explanation which might conceivably account for this apparent large discrepancy. In commercial communication-channels, there are ways of vastly increasing the signal-capacity (e.g. by using multiple-or-changed modes or frequencies), and there is a case for believing that may be occurring here. See the Appendix, below. In any case, this is an anomaly — and that arguably calls for investigation along any promising lines-of-enquiry.
