Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 11 - Fundamentals of the Nervous System and Nervous Tissue - Review Questions - Page 426: 15a


Myelin is a compound that is made up primarily of lipids and proteins. It conducts electrical impulses poorly and forms an insulating layer around some axons ( fibers) in both the central nervous system(CNS) and the peripheral nervous system (PNS). In the the PNS the myelination of axons is effected by a kind of neuroglia called Schwann cells . The axons that are wrapped in Schwann membranes are called myelinated fibers. Those that are not wrapped by Schwann cells are called unmyelinated fibers, Myelin wrapping gives nerve tissue a whitish appearance. Each Schwann cells wraps a certain length of axon but some areas are unwrapped areas..The wrapped areas are called inter nodes and the unwrapped lengths of axons are designated Nodes of Ranvier. In the PNS as in the CNS, impulse conduction along myelinated axons is hundreds of times faster than along unmyelinated fibers. Myelination in the CNS is carried out by oligodendrocytes. The myelin is similar in composition and it serves the same purpose of insulating parts of axons. The effect is also the same: myelinated fibers by saltatory conduction transmit impulses much faster than unmyelinated fibers. Whereas one Schwann cell insulates only one internode of an axon, each oligodendrocyte has many processes and may simultaneously wrap and insulate many internodes.. In CNS myelinated axons, impulses also travel by skipping from node to node, that is, by saltatory conduction.

Work Step by Step

Myelin is compound composed of lipids and fat. Some of the proteins are myelin basic protein, and oligoprotein. . Sphingomyelis and glycolipids are also components of this substance. Both in the CNS and the PNS neuroglia wrap some axons with myelin membranes. This gives the wrapped fibers a whitish color, hence the term white matter. More importantly, myelin insulates certain areas of nerve fibers. This changes the method of transmission of nerve impulses to a much faster one of saltatory conduction -- the impulse skips from node to node. The PNS axons are wrapped by Schwann cells and the CNS axons are wrapped by oligodendrocytes. One Schwann can wrap only one length of a neuron, but a single oligondendrocyte has many processes and can wrap many lengths of different axons simultaneously.
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