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Biology 202
2000 First Web Report
On Serendip

A Look at Myelin and Myelin-Related Disorders

Henrike Blumenfeld

Myelin is the protective sheath around axons in the nervous system, and is often referred to as 'white matter.' It acts as an insulator to the electrical signal that is conducted down the axon as a neuron fires, and can well be compared to the insulation around an electrical wire. The myelin sheath contains a variety of fatty substances (lipids), and contains at least ten distinct chemicals (2). . From what is known, axons are not fully myelinated at birth, but myelin is formed from birth on into the second decade of life (3). The way myelination takes place is through glial cells wrapping around the axons in a spiral fashion. The axons are originally embedded in these glial cells, and the cell's membranes extend to wrap the axon tightly and repeatedly. 'Glia' comes from Greek, meaning 'glue,' hinting at the way the cell's membrane sticks to the axon, and how the different layers of the extended membrane stick to each other. There are different glial cells that form myelin: in the central nervous system, these cells are referred to as oligodendrocytes, and in the peripheral nervous system they are Schwann cells. While in the peripheral nervous system the glial cell wraps around a single axon to insulate it, in the central nervous system a glial cell may wrap around several axons at once (4)..

The characteristics and specific functions of myelin have not fully been explored by researchers, and as far as I can tell most current research efforts are initiated by groups that research myelin-related disorders, such as the Myelin Profect. In demyelinating diseases, the myelin sheath around some axons is targeted. These diseases fall into two main groups: acquired diseases and hereditary metabolic disorders. The most common disease in the first group is multiple sclerosis (MS), which is suspected to be linked to a viral infection where myelin in the central nervous system is attacked in an autoimmune response. After patches of myelin are taken off the axon, oligodendrocytes 'repair' the damage, but in the process cause scar tissue (gliotic plaques). These hard plaques then begin to interfere with the flow of electrical impulses that move through the axon (6).. In most cases, the disease is characterized by alternating phases of relapsing and remission, where the remission phases get increasingly shorter. The symptoms include spasticity, double-sightedness (diplopia), and an unsteady gait (5).. This may suggest that it is not the case that myelin in the CNS in general is targeted, but that rather myelin in specific regions is affected. In case of the symptoms above, we might suspect demyelination of some motor neurons, or neurons related to the function of the visual cortex.

The second group of demyelinating diseases,hereditary metabolic disorders, are degenerative diseases and include, among others, the eight identified leukodystrophies (5). Here, 'leuko' means 'white', and refers to the white matter (myelin sheaths) of the central nervous system, and 'dystrophy' means 'imperfect growth or development' (2).. The majority of the leukodystrophies are storage disorders, where the absence or malfunctioning of an enzyme results in the toxic accumulation of chemical substances (5) In each of the so far identified leukodystrophies only one of the chemical substances that constitute the myelin sheath is affected. This means that leukodystrophy of one type does not increase the risk of leukodystrophy of a different type (2). The leukodystrophies, as well as other hereditary demyelinating disorders, generally have their onset in infancy or early childhood, although they can also occur later. Children that have previously developed normally are deprived of sight, hearing, speech and movement in generally short periods of time. Their life expectancy after onset is only a few years.

Neither multiple sclerosis nor the leukodystrophies currently have a cure. Rather, symptoms of the diseases are treated and alleviated as much as possible. Both groups of diseases cause either a blockage or reduction of the electrical impulse traveling down the axon. One can imagine the effect this has on the transport of messages in the nervous system in general when imagining that while impulses normally travel around 225 miles an hour, they are slowed down to about half that rate or less wherever a section of myelin is destroyed (5). Thus impulses are either significantly slower than normally or completely blocked. This causes considerable disruption to the overall 'symphony' of firing neurons that create meaningful patterns together, either caused by the absence of a signal or by a late signal which results in imperfect timing with other neurons. The Myelin Project is currently conducting research on possible myelin regrowth strategies, with the goal to either considerably slow down demyelinating diseases for which there is no cure, or restoring full function for those diseases that do have cures (5). McNorris and McKinnon strive to apply findings on the factors that contribute to the development of the myelin-forming oligodendrocytes to the treatment of neurological diseases (7).

As we have seen, the production, make-up and functions of myelin are very complex, and they are interrupted in very specific ways in myelin-related disorders, some of which were described here. These different ways of affecting the myelin lead to different symptoms in the patient, which explains the multitude of demyelinating disorders that have so far been identified These different disorders often have very fine distinctions. For example, although we saw that multiple sclerosis and the leukodystrophies are fundamentally very different disorders, even an experienced clinician may have difficulty telling them apart (2). Since the chemical make-up of myelin is as complex as it is, and since changes in this make-up seem to lead to different kinds of results, further questions may pose themselves, such as whether the function of myelin in affecting the electrical impulse in the axon is more than merely that of an insulator.

http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?CMD=Text&DB=PubMed

WWW Sources

1)NINDS Leukodystrophy Information Page , information about the leukodystrophies

2)United Leukodystrophy Foundation Website , information about the leukodystrophies

3) Myelin Disorders Menu <a/, a forum, largely for people affected by demyelinating diseases sharing their experiences, questions and concerns.

(4) Principles of Neural Science, Eric R. Kandel and James H. Schwartz, Elsevier, 1985, New York.

5) The Myelin Project webpage <a/, contains information about demyelinating diseases, as well as information on current research.

6) About Multiple Sclerosis <a/, A brief summary of MS at the neurological level.

7) Research summary <a/, a brief summary of research related to remyelination.




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