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. To understand the role that the optic nerve plays in our vision, let's 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.
But how do the images make it onto the computer in the first place? 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.
You've 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 is has a similar structure, except it is composed of 1.2 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 film in a 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 the fibers of the optic nerve.
One travels from the rear surface of each eye. They then 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 visual loss. OPTIC NEUROPATHY The 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.
If 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. Let's return to the electrical appliance and its cable. When an electrical 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 structure 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 the 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.
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. 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). Elevated intraocular pressure causes deformations in the web network of the lamina cribrosa, creating a sheering effect that is thought to damage the nerve fibers through pinching and eventual breakage.
When 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).
I see a similar image when I look through the pupil of your eye to see the retina. The optic disc, which is packed with nerve fibers extending from retinal ganglion cells is analogous to the outer rim of the teacup. 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. Nerve fibers surround it, 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.
Return from The Optic Nerve to Glaucoma Diagnosis
|
||||||||||||||
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?
Nerve fibers extending from retinal ganglion cells pass through this scaffolding prior to condensing to form the optic nerve. 
