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Biology 202, Spring 2005
Second Web Papers
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Interrupted Nerve Fibers and Multiple Sclerosis

Sophia Louis

I am interested in what happens in extreme cases of nerve damage. More specifically, what happens when electrical impulses between nerve fibers are interrupted? The nervous system is a network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. The nervous system has two main divisions, the central and the peripheral nervous system. I am most concerned with the central nervous system. The central nervous system (CNS) consists of the brain and the spinal cord. Many analogies have been made and my favorite reads "the brain might be compared to a computer and its memory banks, the spinal cord to the conducting cable for the computer's input and output, and the nerves to a circuit supplying input information to the cable and transmitting the output to muscles and organs"(1). But, what happens if there is a malfunction in the transmission of messages? What happens if the cable is damaged?

When the cell body of a neuron is chemically stimulated, it generates an impulse that passes from the axon of one neuron to the dendrite of another through the synapse. Electrical impulses may pass directly from axon to axon, axon to dendrite, or from dendrite to dendrite. So-called white matter in the central nervous system consists primarily of axons coated with light-colored myelin produced by certain neuroglial cells. Myelin is a fatty substance, which protects the nerve fibers in the CNS and helps to conduct electrical impulses (2). Myelin is considered fatty because it is rich in protein and lipids, it forms layers around the nerve fibers and acts as insulation. Referring back to analogies, the nerve can be compared to an electrical cable, the axon (nerve fiber that transmits the nerve impulse) is like the wire, and the myelin sheath is like the insulation around the wire, protecting the flow of electrical impulses along the line. Nerve cell bodies that are not coated with white matter are known as gray matter. There are regular intervals along peripheral axons where the myelin sheath is interrupted. These areas, called nodes of Ranvier, are the points between which nerve impulses, in myelinated fibers, jump, rather than pass, continuously along the fiber (as is the case in unmyelinated fibers). Transmission of impulses is faster in myelinated nerves, varying from about 3 to 300 ft (191 m) per sec (3).

When extensive damage is suffered by myelin, a condition called Multiple Sclerosis occurs (MS). About 200 people are diagnosed every week. Worldwide, MS may affect 2.5 million individuals. Most people are diagnosed between the ages of 20 and 50 and two to three times as many women as men have MS. Multiple sclerosis (MS) is an autoimmune disease of the myelin in the central nervous system (CNS) that is clinically characterized by episodes of neurologic dysfunction separated by time and space (4). In MS, myelin is lost in multiple areas, leaving scar tissue called sclerosis. These damaged areas are also known as plaques or lesions (5). Sometimes the nerve fiber itself is damaged or broken.

In MS, an immune system reaction causes a breakdown in the myelin layer, or sheath. When any part of the myelin sheath is destroyed, nerve impulses to and from the brain are distorted or interrupted. These "shorts" in the system may impair bodily functions such as movement, speech, or sight, depending on where in the central nervous system they occur. The name itself explains this: "Multiple" because many areas of the brain and spinal cord are affected and "Sclerosis" because scleroses, or hardened patches of scar tissue, may form over the damaged myelin. Some people remember this more easily by thinking that MS is short for "many scars" (6, 7). It was long believed that no nerve damage accompanied damage to the myelin sheath in MS. Recent studies have, however, drawn this belief into question.

To answer my initial, one must have a deeper understanding of the function of myelin. Myelin not only protects nerve fibers, but also makes their job possible. When myelin or the nerve fiber is destroyed or damaged, the ability of the nerves to conduct electrical impulses to and from the brain is disrupted, and this produces the various symptoms of MS. Symptoms of MS are unpredictable and do vary from person to person and from time to time in the same person. For example, one person may experience abnormal fatigue, while another might have severe vision problems. A person with MS could have loss of balance and muscle coordination making walking difficult; another person with MS could have slurred speech, tremors, stiffness, and bladder problems. While some symptoms will come and go over the course of the disease, others may be more lasting (8). Recent studies suggest that the MS disease process starts long before symptoms begin, and by the time symptoms appear, there are already signs of brain and spinal cord atrophy. The cause of MS is unknown, and it cannot be prevented or cured. A test called the Expanded Disability Status Scale (EDSS) is used to rate the severity of symptoms. It is also used after a diagnosis to gauge the status of the disease, and score the effectiveness of treatments. The scale ranges from zero to ten with higher scores indicating more severe symptoms. There is no single test that can accurately diagnose MS and for this reason several laboratory procedures are necessary before a diagnosis can be made. These procedures usually include: an Analysis of Cerebrospinal Fluid (CFS); an Evoked Potential (EP) Test; and Magnetic Resonance Imaging (MRI) (10).

While there is no cure for MS, research is being done towards the future of myelin repair. There are efforts being made to reverse the damage caused by MS and restore "normal" function in people suffering from the disease. This is not the only treatment to consider though, because MS is an autoimmune disease, the body destroys its own myelin. So researchers need to find a say to stop the immune system from fighting and damaging its own CNS tissue. In 2003 several hundred researchers from around the world participated in an intensive three-day workshop organized by the National MS Society. They realized that the first step to repair and treatment is stopping the underlying immune attack from perpetuating damage to repaired tissue, which is why ongoing research to find better immune-modulating treatments for MS is vital (International Workshop). Advancements from that weekend show that the body can repair myelin by stimulating neighboring oligodendrocytes (the cells that make myelin) or by recruiting immature "progenitor" cells that move to the lesion and replace damaged myelin. Some participants noted that this natural repair process might actually be stimulated by the inflammation that occurs during MS attacks.

I mentioned before that women are more prone to getting MS than men.

Apparently, there are genetic difference between men and women, which could be the reason why MS strikes more women than men, says Dr. Brian Weinshenker from the Mayo Clinic. The online edition of "Genes and Immunity" explains that genes and environment are probably both involved in the development of MS. Doctors from the Mayo Clinic studied the genes of MS patients in the U.S, and at gene patterns in people from Northern Ireland and Belgium (1). Women with MS were more likely to have a variation of a gene that produces high levels of a protein called interferon gamma. Interferon gamma can aggravate MS by promoting inflammation and tissue damage (1).In the U.S. and Northern Ireland, men with the gene variation were more susceptible to MS. That was also true for Belgian men, but the effect wasn't significant there. The gene variation was less common among men. "That might explain why men are generally protected more from MS," says Weinshenker in a news release.

I think there is a bright future for patients suffering from MS. New advancements are on a steady rise. Scientists have discovered the ability to use potential cells found in skin and bone marrow to transform into brain cells. These cells can be used as a source of replacement cells useful in viable new tissue. Possible sources also include: skin derived cells, bone marrow, and umbilical cord blood cells, fetal cells, and Schwann cells from the PNS. This will be promising if the immune attack can be stopped. Then new ways to repair the nerve damage and restore nerve function will arise.

Why more women get MS
International Workshop
MS information
What is Multiple Sclerosis,
Multiple Sclerosis: Overview,
Personal Stories
General Information,
Multiple Sclerosis,
Detection of Neuroimmunologic Disorders

Myelin function-overview
Nervous System: Anatomy and Function

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