The Optic Nerve

The complex process of sight begins when light reflecting off an object enters the clear outer portion of the eyeball (cornea) and passes through the lens, which brings the light into focus on the nerve cell layer or retina. When light hits the retina, electrical impulses are generated and carried along the optic nerve to the brain, where the impulses are converted into visual information.

The optic nerve is the second of twelve paired cranial nerves. It's considered to be part of the central nervous system (CNS) as it is derived from an outpouching of the diencephalon during embryonic development. The fibers are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system (PNS).

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Similarly, the optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endperiodism found in peripheral nerves. This is an important issue, as fiber tracks of the CNS 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 eye via the optic canal, running postero-medially towards the optic chiasm where there's a partial decussation (crossing) of fibers 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. 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.

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.

Inflammation of the optic nerve causes loss of vision usually because of the swelling and destruction of the myelin sheath covering the optic nerve. Direct axonal damage may also play a role in nerve destruction in many cases.