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.