The nervus opticus, or optic nerve, is the second cranial nerve and carries visual stimuli from the eyes to the brain. It is technically not a nerve but a part of the central nervous system. The central nervous system is made up of the spinal cord and brain, and is responsible for making sure the body runs properly. Nerves are a part of the peripheral nervous system, which is comprised of the automatic nervous system, spinal nerves, and cranial nerves. There are 12 pairs of cranial nerves that emerge directly from the brain, rather than the spinal cord, like the 31 pairs of spinal nerves.
The optic nerve carries sensory nerve impulses from the retina to the visual centers of the brain. The retina is the light-sensitive layer of tissue that lines the interior of the eyeball. The nerve is attached to the optic disk, which is an oval-shaped area at the back of the eye, near the nasal cavity. This area is also referred to as the blind spot because it does not contain photoreceptors.
All of the ganglion fibers, which are collections of nerve cells, converge on the optic disk. The nervus opticus runs from the optic disk, through the optic canal, and goes further into the cranium to form an X-shaped structure called the optic chiasm. The nerve fibers then join and proceed back under the brain, to attach to the occipital lobe on each side. The occipital lobe of the brain contains most of the visual cortex, and is the visual processing center. It is located at the rear of the head, at the back of the skull.
When vision problems occur, doctors can study the patterns of lost vision to understand where damage may have occurred. This is so because damage at different points of the nerve produces different patterns of lost sight. For instance, a tumor pressing on the optic chiasm can produce vision problems such as a loss of color in vision, blind spots, and loss of vision within a matter of days.
Infections, like optic neuritis, can affect the nervus opticus. Optic neuritis is the inflammation of the nervus opticus, and can be caused by a viral or bacterial infection; complications from other inflammatory diseases, like meningitis; a reaction to something toxic; or by physical trauma. Another common problem that can affect the optical nerve is glaucoma. Glaucoma is the loss of vision due to high pressure within the eye — this pressure often squeezes the nervus opticus, and results in vision difficulties.