Optic nerve

The optic nerve sends visual signals from the eye to the brain. Vital for human sight, it is connected to the retina at the back of the eye and to the brain. The optic nerve is also known as the second cranial nerve, or cranial nerve II. It is one of twelve cranial nerves that help the body function.

Background

The optic nerve is one of twelve cranial nerves that send information to and from the brain. As its name suggests, the optic nerve sends signals about vision. It is needed for vision, but many steps take place inside the eye before the nerve transmits visual information to the brain. People need light to see. Light rays reflect off surfaces and enter the human eye through the cornea, which is the clear, curved coating covering the eye. The cornea bends the light as it enters the eye. The bent light goes into the pupil, which is at the center of the iris. The pupil becomes larger or smaller depending on the amount of light reflecting into the eye. The lens further bends and focuses the light onto the retina, which is a layer of tissue at the back of the eye packed with light-sensitive cells. The light stimulates cells inside the retina called rods and cones. Rods help with peripheral vision, which is what people see on the side while they are looking ahead. They also help people see in low light and sense motion. The cones help people see in bright light and see color. The rod and cone cells change light into electrical impulses. Then, the optic nerve picks up the electrical impulses and sends them to the brain. The brain uses the electrical impulses to create an image. Although the process includes many body parts, it happens extremely quickly.

Overview

The optic nerve begins at the back of the eye. It is attached to the eye through nerve cells in the retina. Cells called retinal ganglion cells are in the retina. These cells have long, finger-like structures called axons. Axons are parts of nerve cells that transmit information from the cell to other locations. Axons can be very long. The axons of the retinal ganglion cells extend outside the retina toward the brain. These axons form the optic nerve. The optic nerve includes roughly 1.2 million axons. The axons extend and meet in a central location called the optic disc. This is the beginning of the optic nerve.

The optic nerve then runs through the lamina cribrosa, which is part of the sclera (the white part of the eyeball). After the bundled axons pass through the sclera, they are covered in myelin. (Myelin is a substance that coats axons and other parts of nerve cells. It is made up of lipids, or fats, and proteins. The myelin protects the cells and helps transmit electrical impulses.) The myelin coating on the nerve makes it wider than the optic disc. The nerve measures roughly 3.5 millimeters after its fibers are coated in myelin.

The optic nerve continues back toward the brain. It exits the orbit, which is the part of the skull in which the eyes are located, through the optic canal. The nerve then enters the middle cranial fossa. At this point, some of the fibers of the two optic nerves—one extending from the left retina and one extending from the right retina—cross over each other and form an X. This location is called the optic chiasma/chiasm. The two nerves continue past the chiasma to enter the brain in two spots, the largest projection to the left and right lateral geniculate nuclei. The axons in the nerve form synapses with the brain to relay the visual information that has been carried from the retina. Each fiber of the optic nerve is responsible for sending different types of signals to the brain. Most of the optic nerve is bathed in cerebrospinal fluid, which is a clear, colorless liquid that helps protect the brain and some nerves from injury.

Medical professionals can use different tests and diagnostic procedures to determine the health of a person’s optic nerve. They usually view the optic nerve through an ophthalmoscope, which is an instrument that helps examine the eye. The end of the nerve at the back of the eye is called the optic disc, which is about 1.8 millimeters in diameter where it meets the eye. The edges of the optic nerve should appear crisp. The nerve should be a specific color and slightly indented. If any of these attributes is irregular, a medical professional might run further tests to see if a person is experiencing a medical problem with the eye, the optic nerve, or even the brain.

One common abnormality that can be identified through this diagnostic procedure is papilledema, which is the swelling of the optic disc. This swelling usually happens because of raised intracranial pressure. The brain is surrounded by fluid that helps protect it from injury. But an injury or illness can increase the amount of fluid. Too much fluid in the brain can raise the intracranial pressure to a level that is too high and can damage the brain. Increased fluid can also put pressure on the optic disc and make it swell. If the disc swells, it can cut off the blood supply to and from the eye. Intracranial pressure can also increase because of masses such as tumors or blood clots, inflammation, or impaired circulation of the cerebrospinal fluid. Therefore, papilledema can be a serious problem for many reasons.

Optic nerve hypoplasia (ONH) is another optic nerve disorder. People with ONH have an optic nerve that is underdeveloped or too small. People with ONH have impaired vision or a complete loss of vision. Although treatments are available, this disorder is incurable and cannot be reversed. Other disorders include glaucoma, optic neuritis, anterior ischemic optic neuropathy, optic nerve atrophy, and optic nerve head drusen. Scientists continue to investigate several types of stem cells for their ability to promote optic nerve regeneration, including mesenchymal stem cells, embryonic stem cells, and induced pluripotent stem cells. Researchers at the University of Connecticut experimented with a fibronectin peptide injection to treat mice with severed optic nerves. The test results found that the peptide stimulated nerve cells to regrow from the injury site to the optic chiasm in the brain. When combined with gene therapy, the observed benefits were further enhanced. This research offered positive advances in promoting vision restoration in cases of optic nerve damage.

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