THE OPTIC NERVEThe optic nerve is part of the central nervous system. Damage to it results in glaucoma. Find out here how it functions and why it is so sensitive. The optic nerve, also known as cranial nerve II, is 5-6 cm in length. Approximately, two-thirds it resides inside the orbit of the eye, with the remaining travelling through the brain to the visual cortex. This nerve can be subdivided into four segments on the basis of location.
To understand the role that the optic nerve plays in our vision, I am going turn to photography to help us out. If you own a computer, chances are that at some point you have used it to view photos taken with a digital camera. These may be photos that you have taken yourself or they could be images that someone else has sent to you. It doesn't matter. Once an image is processed by the computer, we can view it, edit it, and send it to others.
A cable is required to connect the digital camera that has taken the photo to a computer. The computer then translates the electrical impulses into an image and displays it. Our system of vision functions in a similar way. The optic nerve serves as the cable that connects the eye to the brain. Damage to it means that nerve impulses or signals do not properly reach the brain for translation into images. The result either is reduced vision or blindness.
NERVE FIBERSYou have probably seen cables like the one shown here. They actually are hundreds of tiny wires, often of different colors, bound together to form what appears to be a single large cable. The optic nerve has a similar structure, except that it is composed of between 1.0 and 2.2 million fibers! Each fiber originates in a special cell located in the retina called a ganglion cell. Together these cells form the innermost layer of the retina. The
retina
has a function that is similar to the electronic image sensor in a digital camera. Light striking the ganglion cells of the retina activates nerve impulses. These are then transmitted to the various visual centers of the brain by fibers of the optic nerve.
Yes!One travels from the rear surface of each eye, where they join together to form the optic chiasm. (Things get a little complicated from this point, so stay with me.) After the combined fibers leave the optic chiasm they are called optical tracts. These transmit the nerve impules to a structure called the lateral geniculate nucleus located inside the thalamus of the brain. From here, visual input travels as optic radiations to an area of the brain called the occipital lobe.
Any problem that impairs or interrupts this pathway can lead to loss of vision. OPTIC NEUROPATHYThe technical term for conditions that damage the optic nerve is optic neuropathy. Injury, tumors, infection,
inflammation,
problems with blood flow (vascular), metabolic conditions, and hereditary disorders all can cause optic neuropathies. The consequence of these disease processes is either temporary or permanent loss of central and/or peripheral vision.
NERVE INSULATIONIf we think about a cable connected to an electrical appliance, the part of the cable that is visible to us is covered with insulation. This material protects the fibers inside from damage and, most important, it ensures that electrical impulses transmitted through the fibers reach their destination without escaping. Without intact insulation, the electrical current would be interrupted an the appliance would not function. A similar structure exists for the optic nerve. It is covered with an insulating material called a myelin sheath that is composed of about 80% lipid (fat) and 20% protein. In addition to serving a protection function, the main purpose of the myelin sheath is to increase the speed at which impulses move across the nerve fibers.Returning to the electrical appliance and its cable, when the cable enters its appliance the insulation is stripped and the individual wires are divided and assigned to serve various functions in the machinery to make it work. Again, a similar arrangement exists in the eye. The optic nerve enters the eye through a structure called the lamina cibrosa. Once inside the eye, it loses its insulating myelin sheath and the individual fibers extend out to the ganglion cells of the retina. Because they lack the protection of the myelin sheath, it is at this point that the nerve fibers are most vulnerable to damage.
THE LAMINA CRIBROSATo better understand this, we are going to change our orientation from thinking about the optic nerve entering the eye to viewing how its fibers condense to exit the eye. In glaucoma, the part of the visual pathway that is most affected is where the nerve fibers leave the eye through a structure called the lamina cribrosa. This is a multi-layered web of collagen fibers located in the external layers at the rear of the eye. Nerve fibers extending from retinal ganglion cells pass through this scaffolding prior to condensing to form the optic nerve.This point of convergence is referred to as the optic disc. The area defined by the optic disc is devoid of light sensitive cells, which results in a break in the visual field called the blind spot. The web-like structure of the lamina cribrosa that allows nerve fibers to pass out of the eye nerve also makes it structurally weaker than the surrounding tissue
(sclera).
OPTIC DISC CUPPINGWhen I examine your eyes using an ophthalmoscope, one of the things that I am looking at is the cup-to-disc ratio (C/D ratio).
At the center of the optic disc there is a smaller circle in which the color changes to a brighter yellow. This represents the bottom of the teacup in our analogy. The cup area, also called the disc excavation, is empty space through which the central retinal artery and central retinal vein traverse. Nerve fibers surround the disc excavation, but none run through it.
The C/D ratio compares the diameter of the optic cup to the diameter of the optic disc. For example, a ratio of 0.3 or less represents a healthy optic nerve. This means that 30% or less of the total optic disc area is occupied by the cup. The loss of nerve fibers from glaucoma causes the cup to grow progressively larger. The cup size increases because there is less space occupied by the remaining nerve fibers.
The visual pathway is far more complex than the overview that I have presented here. For those that yearn to know more, please consult the references listed below. REFERENCES:Chan JW. Optic Nerve Disorders: Diagnosis and Management. New York: Springer Science and Business Media, 2007. Remington LA. Clinical Anatomy and Physiology of the Visual System, 3rd Ed. St. Louis, MO: Butterworth Heinemann Elsevier, 2012. Tombran-Tink J, Barnstable KJ, and Shields MB. Mechanisms of Glaucoma: Disease Processes and Therapeutic Modalities. Totowa, New Jersey: Humana Press, 2008.
Return from The Optic Nerve to Glaucoma Diagnosis
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This arrangement allows a single cable to perform all of the tasks required for a particular function. 
Does each eye have its own optic nerve?
