The Anatomy of the Optic Nerve

Made of nerve cells, the optic nerve is located in the back of the eye. Also known as the second cranial nerve or cranial nerve II, it is the second of several pairs of cranial nerves. It is a bundle of nerve cells that transmits sensory information for vision in the form of electrical impulses from the eye to the brain. The optic nerve has been studied heavily because it is a direct extension of the brain.

Anatomy

The optic nerve is mainly made up of the axons (nerve fibers) of the retinal ganglion cells from the retina. The optic disc or nerve head is the point where the axons from the retinal ganglion cells leave the eye.

The nerve head appears as a white circular structure in the back of the eye. There are no photoreceptors on this structure. As a result, humans have a natural blind spot.

Nerve cells travel from the nerve head through a structure called the lamina cribrosa that allows the nerve fibers to pass through many holes and into the extraocular (outside of the eyeball) space. As the fibers pass through, they become covered with a type of insulation called myelin. The nerve fibers become insulated with glial cells known as oligodendrocytes.

Location

As the optic nerves exit the eye they join together at the optic chiasm. At the optic chiasm, nerve fibers from half of the retina cross over to the opposite side of the brain. The fibers from the other half of the retina travel to the same side of the brain.

Because of this junction, each half of the brain receives visual signals from the visual fields of both eyes. The chiasm is located at the bottom of the brain.

After the chiasm, the nerve fibers extend to the lateral geniculate nucleus (LGN) in the thalamus. From there, the nerve fiber tracts extend from the LGN into optic radiation fibers that fan through different parts of the brain including the parietal lobe, temporal lobe, and occipital lobe.

The blood supply of the optic nerve is complex but is mainly provided by the posterior ciliary artery which is a branch of the internal carotid artery.

Knowledge of the pathway of the optic nerve from the eye to the brain is important because the origin of different diseases that affect vision can be localized based upon the location of the defect in vision or where in the visual field a defect may show up.

Function

The optic nerve produces all sorts of visual information.

The optic nerve is also responsible for the light reflex and the accommodation reflex. These are two important neurological reflexes. The light reflex allows both pupils to constrict when light is shone into one of the eyes. The accommodation reflex allows the eye to adjust for near vision by allowing the lens to swell.

Associated Conditions

There are several diseases that can affect the optic nerve, chiasma, and radiations, including:

Glaucoma

Glaucoma refers to a group of diseases that may cause damage to the optic nerve. The optic nerve fibers make up a part of the retina that gives us sight. This nerve fiber layer can be damaged when the pressure of the eye (intraocular pressure) becomes too high.

Over time, high pressure causes the nerve fibers to die, resulting in decreased vision. Vision loss and blindness will likely result if glaucoma is left untreated.

Optic Neuritis

Optic neuritis is an inflammation of the optic nerve. This often affects only one eye at a time and affects the part of the nerve before the optic chiasm. Because of the location of the inflammation, one would predict that problems will show up in the vision of just one eye.

Optic neuritis can be caused by a variety of sources such as multiple sclerosis, a viral illness, chemical exposure, or severe sinus disease.

Pituitary Adenoma

The pituitary gland is located underneath the optic chiasm. If the pituitary gland grows large or develops a mass or growth, it can press on the optic chiasm causing defects in both visual fields because the nerve fibers cross at the chiasm.

Vascular Infarcts and Aneurysms

Vascular diseases (diseases that affect blood vessels) can cause problems along the pathway of the optic radiations. Because the optic radiation nerve fibers pass through the parietal lobe, temporal lobe, and occipital lobe of the brain, defects or blind spots can develop in the visual field. The location of the defect in the visual field can tell doctors where in the brain to look for the problem.

Treatment

Treatment of optic nerve damage, chiasma or optic radiation damage depends on the cause. However, treatments for optic nerve damage may not restore lost sight. In most cases, measures are taken to stop further damage and worsening of symptoms. For example:

• Glaucoma is secondary to increased pressure inside the eye, so medications for glaucoma are aimed at reducing the pressure to a point where the disease process is stopped. Although glaucoma can be treated with surgery, laser, and oral medications, most glaucoma is treated with topical medication in the form of eye drops.
• Diseases such as optic neuritis are treated with oral and intravenous steroids to reduce the inflammation. Also, if the cause of the optic neuritis is known, the underlying condition will be treated.
• Diseases of the optic chiasm are often treated with neurosurgery and managed with medications or hormones. Depending on the severity of optic chiasm disease, such as a pituitary adenoma, sometimes simple observation is all that is needed.
• Vascular accidents, or stroke, are more difficult to treat unless the condition is diagnosed very quickly. Sometimes blood thinners are prescribed. Surgery could be involved if the disease process is caused by aneurysms.