Optic (II) Nerve

The optic nerve is also known as cranial nerve II. It transmits visual information from the retina to the brain. 

Each human optic nerve contains between 770,000 and 1.7 million nerve fibers. 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.

Optic nerve

An illustration of the brain highlighting the optic nerve and optic tract. 

This is an illustration of the brain that highlights the optic nerve and optic tract. It shows how the eyes are connected to the optic nerve and optic tract, with these nerves moving back into the brain by crossing in the optic chiasm and connecting to the midbrain and occipital lobes.

The optic nerve is the second of twelve paired cranial nerves. It is considered by physiologists to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development.

As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells that are found in the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in the peripheral nerves. 

The fiber tracks of the mammalian central nervous system are incapable of regeneration. As a consequence, optic nerve damage produces irreversible blindness.

The optic nerve leaves the orbit, which is also known as an eye socket, via the optic canal, running posteromedially toward 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 (where information is relayed to the visual cortex), while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements.

The optic nerve transmits all visual information including brightness perception, color perception, and contrast. It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex. 

The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye; the accommodation reflex refers to the swelling of the lens of the eye that occurs when one looks at a near object, as in reading.