The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
The myelin sheath is a fatty white covering which is wrapped around nerve fibers (axons) throughout the body, brain, and spinal cord. This covering is composed of the membranes of one of two different types of specialized cells. In the central nervous system (the brain and spinal cord), the myelin sheath is composed of cells called oligodendrocytes. Throughout the rest of the body, Schwann cells create the myelin sheath. When oligodendrocytes or Schwann cells spiral around a nerve's axon, several layers of their membranes serve to insulate the axon. Because myelin has a white color, fibers which are myelinated are referred to as white matter. Unmyelinated fibers are referred to as gray matter. Small areas of interruption along the myelin sheath are called nodes of Ranvier. These nodes of Ranvier occur approximately every millimeter along the myelin sheath.
The function of this insulating layer of myelin around a nerve fiber is to increase the speed of a nerve impulse's conduction along the fiber. Without myelin, a nerve impulse travels at a rate of several meters per second along the nerve axon. Nerve impulses along a myelinated axon travel at approximately 100 meters per second.
Multiple sclerosis is a disease which results in the destruction of the myelin sheath. As the myelin sheath is stripped from axons, nerve impulses slow throughout the body, and movement becomes slow and difficult. Eventually, scar tissue may replace the myelin along various axons, resulting in further disability, and eventually in paralysis.