The optic nerve is a bundle of more than 1 million nerve fibers that carry visual messages. You have one connecting the back of each eye (your retina) to your brain. Damage to an optic nerve can cause vision loss. The type of vision loss and how severe it is depends on where the damage occurs. It may affect one or both eyes.

There are many different types of optic nerve disorders, including:

• Glaucoma is a group of diseases that are the leading cause of blindness in the United States. Glaucoma usually happens when the fluid pressure inside the eyes slowly rises and damages the optic nerve.
• Optic neuritis is an inflammation of the optic nerve. Causes include infections and immune-related illnesses such as multiple sclerosis. Sometimes the cause is unknown.
• Optic nerve atrophy is damage to the optic nerve. Causes include poor blood flow to the eye, disease, trauma, or exposure to toxic substances.
• Optic nerve head drusen are pockets of protein and calcium salts that build up in the optic nerve over time

Contact your health care provider if you are having vision problems. Tests for optic nerve disorders may include eye exams, ophthalmoscopy (an examination of the back of your eye), and imaging tests. Treatment depends on which disorder that you have. With some optic nerve disorders, you may get your vision back. With others, there is no treatment, or treatment may only prevent further vision loss.

Rods and cones are connected (via several interneurons) to retinal ganglion cells. Axons from the retinal ganglion cells converge and exit through the back of the eye to form the optic nerve. The optic nerve carries visual information from the retina to the brain. There is a point in the visual field called the blind spot: Even when light from a small object is focused on the blind spot, we do not see it. We are not consciously aware of our blind spots for two reasons: First, each eye gets a slightly different view of the visual field; therefore, the blind spots do not overlap. Second, our visual system fills in the blind spot so that although we cannot respond to visual information that occurs in that portion of the visual field, we are also not aware that information is missing.

The optic nerve from each eye merges just below the brain at a point called the optic chiasm. As Figure 5.13 shows, the optic chiasm is an X-shaped structure that sits just below the cerebral cortex at the front of the brain. At the point of the optic chiasm, information from the right visual field (which comes from both eyes) is sent to the left side of the brain, and information from the left visual field is sent to the right side of the brain.

This illustration shows the optic chiasm at the front of the brain and the pathways to the occipital lobe at the back of the brain, where visual sensations are processed into meaningful perceptions.

Once inside the brain, visual information is sent via a number of structures to the occipital lobe at the back of the brain for processing. Visual information might be processed in parallel pathways which can generally be described as the “what pathway” and the “where/how” pathway. The “what pathway” is involved in object recognition and identification, while the “where/how pathway” is involved with location in space and how one might interact with a particular visual stimulus (Milner & Goodale, 2008; Ungerleider & Haxby, 1994). For example, when you see a ball rolling down the street, the “what pathway” identifies what the object is, and the “where/how pathway” identifies its location or movement in space.

Optic Tract

Nerve fiber originating from the OPTIC CHIASM that connects predominantly to the LATERAL GENICULATE BODIES. It is the continuation of the VISUAL PATHWAY that conveys the visual information originally from the RETINA to the optic chiasm via the OPTIC NERVES.

National Center for Biotechnology Information, U.S. National Library of Medicine

tract

bundle of axons in the central nervous system having the same function and point of origin

Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the CNS is called a tract whereas the same thing in the PNS would be called a nerve. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure). A similar situation outside of science can be described for some roads. Imagine a road called "Broad Street" in a town called "Anyville." The road leaves Anyville and goes to the next town over, called "Hometown." When the road crosses the line between the two towns and is in Hometown, its name changes to "Main Street." That is the idea behind the naming of the retinal axons.

Optic Nerve Versus Optic Tract

This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central.

Morning glory syndrome (MGS) is a birth (congenital) defect of the nerve of the eye (optic nerve) that resembles a flower known as "morning glory". It is characterized by an enlarged, funnel-shaped cavity of the optic disc, the point in the eye where the optic nerve fibers leave the retina. The disc is enlarged and has a white center (giving the appearance of a white pupil). The blood vessels are increased in number and curve as they come from the enlarged disc that have the white center, resulting in this malformation to be similar to petals on a flower. Most reported cases are unilateral (affecting only one eye), sporadic (without other cases in the family), and occurin females.

Symptoms include very poor vision in the affected eye with poor visual acuity. MGS may occur by itself or along with other eye abnormalities, such as crossed eyes (strabismus) or lazy eye (amblyopia), or other non-ocular problems like brain disorders. Typically, individuals with non-ocular findings also have a wide head, depressed nasal bridge, and mid upper lip defect or cleft. MGS seems to becaused from failure of the optic nerve to completely form when the baby is developing. The most severe complication is retinal detachment, and can occur in about 26–38% of people with MGS. The MGS is sometimes misdiagnosed as an optic nerve coloboma.

Treatment includes surgery and may result insome recovery of vision. Depending on the other associated abnormalities some patients require referrals to several specialties such as neurosurgery, interventional neuroradiology, otolaryngology, and dentistry. All people diagnosed with morning glory disc anomaly should have brain image exams.

Pigment-dispersion syndrome is an eye disorder that occurs when pigment granules that normally adhere to the back of the iris (the colored part of the eye) flake off into the clear fluid produced by the eye (aqueous humor). These pigment granules may flow towards the drainage canals of the eye, slowly clogging them and raising the pressure within the eye (intraocular pressure or IOP). This rise in eye pressure can cause damage to the optic nerve (the nerve in the back of the eye that carries visual images to the brain). If the optic nerve becomes damaged, pigment-dispersion syndrome becomes pigmentary glaucoma. This happens in about 30% of cases. Pigment-dispersion syndrome commonly presents between the second and fourth decades, which is earlier than other types of glaucoma. While men and women are affected in equal numbers, men develop pigmentary glaucoma up to 3 times more often than women. Myopia (nearsightedness) appears to be an important risk factor in the development of pigment-dispersion syndrome and is present in up to 80% of affected individuals.

The condition may be sporadic or follow an autosomal dominant pattern of inheritance with reduced penetrance. At least one gene locus on chromosome 7 has been identified. Pigment-dispersion syndrome can be treated with eye drops or other medications. In some cases, laser surgery may be performed.