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the lipid substance forming a sheath (the myelin sheath) around the axons of certain nerve fibers; it is an electrical insulator that serves to speed the conduction of nerve impulses in these nerve fibers, which are called myelinated or medullated fibers. adj., adj myelin´ic. 

Myelinated nerve fibers occur predominantly in the cranial and spinal nerves and compose the white matter of the brain and spinal cord. Unmyelinated fibers are abundant in the autonomic nervous system. The term gray matter refers to areas in the nervous system in which the nerve fibers are unmyelinated. In unmyelinated nerves impulses are conducted by the propagation of the action potential along the membrane of the axon. In myelinated nerves impulses are transmitted by an entirely different process, called saltatory conduction, in which the impulse jumps from one node of ranvier to the next. Impulses in myelinated nerves are transmitted hundreds of times faster and require much less energy than in unmyelinated nerves.
Structure of a typical myelin sheath. From Applegate, 2000.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


1. The lipoproteinaceous material, composed of regularly alternating membranes of lipid lamellae (for example, cholesterol, phospholipids, sphingolipids, and phosphatidates) and protein, of the myelin sheath.
2. Droplets of lipid formed during autolysis and postmortem decomposition.
Farlex Partner Medical Dictionary © Farlex 2012


(mī′ə-lĭn) also


(-lĭn, -lēn′)
A white fatty material, composed chiefly of lipids and lipoproteins, that encloses certain axons and nerve fibers. Also called medulla.

my′e·lin′ic adj.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


1. The lipoproteinaceous material of the myelin sheath, composed of alternating membranes of lipid and protein.
2. Droplets of lipid formed during autolysis and postmortem decomposition.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


The fatty, white material forming a sheath around most nerve fibres and acting as an insulator. See also DEMYELINATION.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005


a white phospholipid. See MYELIN SHEATH.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


A whitish fatty substance that acts like an electrical insulator around certain nerves in the peripheral nervous system. It is thought that the loss of the myelin surrounding the vestibular nerves may influence the development of Ménière's disease.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


Lipoproteinaceous material, composed of regularly alternating membranes of lipid lamellae (e.g., cholesterol, phospholipids, sphingolipids, and phosphatidates) and protein, of the myelin sheath.
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about myelin

Q. What is Myelin?

A. As part of the nervous system, myelin lines nerve fibers to protect and insulate neurons. Myelin aids in the quick and accurate transmission of electrical current carrying data from one nerve cell to the next. When myelin becomes damaged, the process involves numerous health conditions, including multiple sclerosis.

Dysfunction in the myelin of nerve fibers causes the interruption of smooth delivery of information. Either nerve impulses can be slowed, such that we can't pull our hand away in time to avoid being burned, or mixed up, so we aren't able to determine if a pan is hot in the first place. This is akin to a pet chewing on a wire, causing the device to dysfunction. When problems arise in nerves of the PNS, neuropathy might result, and when injury affects the nerves of the CNS, multiple sclerosis is often diagnosed.

More discussions about myelin
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References in periodicals archive ?
Intriguingly, the composition/distribution of some low-abundance myelin proteins that regulate functions such as cytoskeletal dynamics, energy metabolism, vesicular trafficking, or adhesion is also changed in these mice, supporting a role of these lipids in intracellular trafficking [83].
Another variable in assays of T-cell reactivity to myelin antigens, particularly when dealing with myelin proteins extracted from brain tissue, is the species of the donor tissue.
Huang, "Autoimmune T cell repertoire in optic neuritis and multiple sclerosis: T cells recognising multiple myelin proteins are accumulated in cerebrospinal fluid," Journal of Neurology Neurosurgery and Psychiatry, vol.
They also indicate that ancestors of other key myelin proteins, including MBP, [P.sub.0], CNP, and olig 1, most likely developed rather late, possibly with the first vertebrates.
Appearance of myelin proteins during vertebrate evolution.
Included are compositionally prominent (20%-50% of protein mass) myelin basic protein (MBP), protein zero ([P.sub.0]), and proteolipid protein (PLP) and "myelin-related" 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), peripheral myelin protein 22 (PMP22), myelin and lymphocyte protein (MAL), and plasmolipin.
They showed that MBP mRNA behaves differently from mRNAs for other myelin proteins when brain samples are subjected to subcellular fractionation.
It's possible that each person with MS has T-cells that recognize different myelin proteins. If T-cells create tolerance by secreting TGF BEta, then the road to a therapy for people might be smoother, Weiner thinks.
"We expected it to be safe, and the animal studies confirmed that." His group has undertaken a small double-blind pilot study in which people with early relapsingremitting MS were given specially prepared myelin proteins by mouth, daily, for one year.
Two families present mutations in the myelin protein zero gene (MPZ).
Several genetic loci and genes have been related with primary demyelinating neuropathy, including altered dosage of the peripheral myelin protein 22 gene (PMP22).