Book : Human Anatomy
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Posted by: CHELSEA
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Writer CHELSEA

Optic nerve

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The optic nerve is located in the back of the eye. It is also called the second cranial nerve or cranial nerve II. It is the second of several pairs of cranial nerves. The job of the optic nerve is to transfer visual information from the retina to the vision centers of the brain via electrical impulses. The optic nerve is made of ganglionic cells or nerve cells. It consists of over one million nerve fibers. Our blind spot is caused by the absence of specialized photosensitive (light-sensitive) cells, or photoreceptors, in the part of the retina where the optic nerve exits the eye.

It is made up of about 1 million small individual thread-like nerve fibers that come from the retina. The fibers bend about 90 degrees as they leave the retina and enter the front of the optic nerve (known as the optic nerve head). Normally, there is a small crater-like depression seen at the front of the optic nerve head. This depression is known as the cup. Its diameter is smaller than the diameter of the optic nerve. In the old days, when a doctor looked at the nerve with a monocular magnification device, the nerve head looked like a cup on a saucer.

The optic nerve is the second of twelve paired cranial nerves and is technically part of the central nervous system, rather than the peripheral nervous system because it is derived from an out-pouching of the diencephalon (optic stalks) during embryonic development. As a consequence, the fibers of the optic nerve are covered with myelin produced by oligodendrocytes, rather than Schwann cells of the peripheral nervous system, and are encased within the meninges. Peripheral neuropathies like Guillain–Barré syndrome do not affect the optic nerve. However, most typically the optic nerve is grouped with the other eleven cranial nerves and considered to be part of the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves. Fiber tracts of the mammalian central nervous system (as opposed to the peripheral nervous system) are incapable of regeneration, and, hence, optic nerve damage produces irreversible blindness. The fibres from the retina run along the optic nerve to nine primary visual nuclei in the brain, from which a major relay inputs into the primary visual cortex.

Function:

The optic nerve transmits all visual information including brightness perception, color perception and contrast (visual acuity). It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex. The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye; the accommodation reflex refers to the swelling of the lens of eye that occurs when one looks at a near object as in reading (lens adjusts to near vision).

Causes:

Most commonly, optic nerve atrophy occurs without a known or proven cause.

Causes leading to optic atrophy range from eye injury or trauma to systemic eye conditions and diseases. These include:

  • ¬ Optic neuritis occurs when the optic nerve is inflammed due to autoimmune conditions, viral, fungal and bacterial infections, parasitic diseases, toxins, allergies, digestive problems, diabetes, and/or simply poor circulation. The person might notice eye pain which is more severe when they move their eye. It mostly found in young to middle-aged women. Such inflammation attacks the myelin covering and the optic nerve becomes swollen and, over time, damaged. Some people with this condition may develop multiple sclerosis later on in life.
  • ¬ Leber's hereditary optic neuropathy is a hereditary eye disease mostly occurring in late teens to early 20s young men. It is characterized by development over a few weeks of painless, serious, central visual loss in one eye, followed months or weeks later by the same process in the other eye. In the beginning there might be slight swelling, but eventually the optic nerve atrophied resulting in, usually, permanent vision loss.
  • ¬ Toxic optic neuropathy. Optic atrophy can result from nutritional deficiencies, some medications, and toxins which damage the optic nerve causing gradual or sudden vision loss. The most common optic neuropathy from poisons and toxins is called as tobacco-alcohol amblyopia, thought to be caused by exposure to cyanide from tobacco smoking, and by low vitamin B12. Other toxins include methyl alcohol (moonshine), ethylene glycol (antifreeze), cyanide, lead, and carbon monoxide.
  • ¬ Nutritional optic neuropathy may be caused by protein, B vitamins, vitamin B12, in particular, and folic acid deficiencies which result from poor nutrition, starvation, poor absorption or alcoholism. Vitamin B12 deficiency damages the nerves, and drinking alcohol contributes to poor absorption of vitamin B12. Treatment with intramuscular B12 as well as oral B12 supplementation has been found to result in dramatic improvement in vision.1
  • ¬ Glaucoma Untreated, the increased pressure characteristic of glaucoma may eventually affect the optic nerve.
  • ¬ Compressive optic neuropathy results from a tumor or other lesion pressing on the optic nerve or from eye movement muscle enlargement which is seen in hyperthyroidism (Graves' disease) patients.
  • ¬ Syphilis, untreated, can also result in damage to the optic nerve.

Tumors or other growths as well as elevated intraocular pressure can mechanically compress the optic nerve resulting in compressive optic neuropathy.


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