The optic nerve is the pathway that directly connects the eye to the brain. It is a sensory nerve that is responsible for the transmission of all visual information. It is the second of the 12 paired cranial nerves and, therefore, is also known as cranial nerve II.
There are approximately 1.2 million nerve cell axons in each optic nerve. The optic nerve acts like a cable connecting the eye to the brain. The diameter of the optic nerve varies from about two to four millimeters. The optic nerve is mainly composed of axons from the ganglion cells of the retina, making it a pure white matter tract. This means it is actually more like brain tissue than nerve tissue, and therefore, its development and functions are similar to other typical central nervous system brain cells. Thus, oligodendrocytes cover the axons with a myelin sheath, astrocytes give biochemical and homeostatic support to the optic nerve, and finally, microglia, a type of resident macrophage used for immune protection of the brain, scavenge the nerve for extracellular components and infectious agents. The optic nerve is also covered with all three layers of the specialized brain coverings called the meninges: dura, arachnoid, and pia maters.
The optic nerve can be broken down into three parts: the optic nerve, optic chiasma (or chiasm), and optic tract. The nerve fibers of the retina exit the eye through the optic nerve, and there is no retinal tissue over the head of the optic nerve. The optic nerve head or optic disc is oval shaped and is around 1.5 to 1.75 millimeters in circumference. It leaves the back of the eye socket through the optic canal. It is approximately 30 millimeters in length from the retina to the optic canal. The nerve then runs 5 to 12 millimeters through the canal. Once it enters the cranium, the length varies from 8 to 19 millimeters as it runs posterior and medial to the optic chiasma.
The optic chiasma is an “X-shaped” intersection of both left and right optic nerves and is located directly below the hypothalamus and directly above the pituitary gland. As the two optic nerves meet, around half of the nerve fibers cross over and continue on the opposite side. For example, the nerves that originate on the nose or nasal half of the left retina cross over at the chiasma and will proceed onto the right side of the brain. On the other hand, the nerve fibers that originate on the lateral or temporal side of the left retina will proceed onto the left side of the brain. This causes a partial crossing of the fibers from the visual fields of both eyes. In other words, visual information from the nasal part of each eye ultimately reaches the visual centers of the opposite side of the brain.
Anatomically, the optic nerve ends at the chiasma, but the retinal ganglion cell axons continue on to make up the optic tract. As the nerve ends leave the chiasma, the optic tract moves posterior and laterally toward its terminations. The optic nerve has four main terminations within the brain: the lateral geniculate nucleus (LGN), the pretectal nucleus and pulvinar, the superior colliculus, and the hypothalamus.
The main function of the optic nerve is to carry visual information from the retina to various parts of the brain. Approximately 90 percent of all optic nerve fibers terminate at the LGN of the thalamus, as its main role is to regulate the flow of visual information to the primary visual cortex. Other functions are (1) to aid in the pupillary response, (2) to help coordinate head and eye movements based on visual information sent to the superior colliculus, and (3) to aid circadian control of sleep-wake cycles, temperature, and other systematic functions.
In a healthy optic nerve, all of the nerve fibers that receive light signals from the outside world pass the signals to the brain, meaning that the entire field of vision should be seen. In any disease or disorder, functioning nerve fibers decrease at a rate much faster than would occur through the normal aging process. This decrease may be due to increased intraocular pressure, lack of blood flow, some other mechanism, or a combination of mechanisms. When these nerve fibers die, they leave an empty space where they used to be. When enough nerve fibers die, an empty space on the optic disc becomes visible through examination. These traumas to the optic nerve may cause severe or even permanent loss of vision. Since each eye has a left and right visual hemifield, the particular loss to these visual fields mainly depends on which parts of the optic nerve are damaged.
The most common injuries to the optic nerve are from glaucoma, optic neuritis, and anterior ischemic optic neuropathy. Glaucoma is not just a single disease but a group of eye conditions. It results in optic nerve damage, which may ultimately cause loss of vision. Abnormally high pressure inside the eye is the usual cause of this damage. Glaucoma cannot be cured and neither can damage caused to the optic nerve, but treatment may prevent vision loss in early glaucoma. The main way to treat glaucoma is to lower pressure in the eye. This may be done by directly lowering the eye pressure, increasing drainage of fluid in the eye, or lowering the amount of fluid produced by the eye.
Optic neuritis is inflammation of the optic nerve. It is mainly found in those younger than 50 years of age. It is most commonly associated with diseases that involve demyelination, such as multiple sclerosis. Other causes of optic neuritis include infection, autoimmune disorders, inflammatory bowel disease, and diabetes. An ophthalmologist will treat optic neuritis by prescribing eye drops containing antibiotics and/or steroids to stop the infection and inflammation. Anterior ischemic optic neuropathy is a condition involving damage to the optic nerve from insufficient blood supply and usually affects patients that are over the age of 50. The most common congenital optic nerve damage is optic nerve hypoplasia, which is the underdevelopment of the optic nerve causing little to no vision. This hypoplasia occurs during fetal development and has no cure.
Mario J. Perez
See also: Circadian Rhythm; Cranial Nerves; Optic Nerve Hypoplasia; Retina; Thalamus; Visual System
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