Optic nerve

Optic nerve is the second of the twelve paired cranial nerves (i.e. Cranial nerve II). It transmits visual information from retina to the brain, and is thus also known as “nerve of sight”. It is about 1.6 inches long.

Course:

Starting from the retina, the optic nerve leaves the orbital cavity by passing through the optic canal, in company with the ophthalmic artery, and enters the cranial cavity.

The nerves from both sides then join to form the optic chiasma. From the optic chiasma, two tracts, called optic tracts, originate.

The nerve fibers that originate from medial (nasal) half of retina cross the midline and join the optic tract of the opposite side. So the nerve fibers from the medial (nasal) half of the retina of left eye join the right optic tract and vice versa.

The nerve fibers that originate from the lateral (temporal) half of retina join the optic tract of same side. So the nerve fibers from the lateral (temporal) half of the retina of left eye join the left optic tract and vice versa.

Each optic tract then passes backward, around the lateral side of the midbrain, to reach the lateral geniculate body. From the lateral geniculate body, a radiation of fibers called optic radiation curves backward to reach the visual cortex of cerebral hemisphere.

In the optic tracts, there are few nerve fibers that are associated with pupillary and ocular reflexes. These fibers bypass the lateral geniculate body and go directly to the pretectal nucleus and superior colliculus.

Branches:

Optic nerve has no branches.

Optic nerve as part of the CNS:

Optic nerve is considered a part of central nervous system instead of the peripheral nervous system.  This is because embryologically, it is derived from an outpouching of the diencephalon. As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells.

Furthermore, the optic nerve is ensheathed in the three meningeal layers (dura, arachnoid and pia mater), instead of the epineurium, perineurium and endoneurium found in peripheral nerves. In this way, an extension of subarachnoid space is carried into the orbital cavity.

Functions:

From the prior discussion of optic nerve, it can be concluded that optic nerve has two primary functions

  • Carrying visual impulses from retina to brain
  • Nervous pathway for pupillary and ocular reflexes