Neurilemma and Myelin Sheath

All axons in the PNS (myelinated and unmyelinated) are surrounded by a continuous, living sheath of Schwann cells, known as the neurilemma, or sheath of Schwann. The axons of the CNS, by contrast, lack a neurilemma (Schwann cells are only found in the PNS). This is significant in terms of regeneration of damaged axons, as will be described shortly.

Some axons in the PNS and CNS are surrounded by a myelin sheath. In the PNS, this insulating covering is formed by successive wrappings of the cell membrane of Schwann cells; in the CNS, it is formed by oligodendrocytes. Those axons smaller than 2 micrometers (2 |im) in diameter are usually unmyelinated (have no myelin sheath), whereas those that are

Fox: Human Physiology, I 7. The Nervous System: I Text I I © The McGraw-Hill

Eighth Edition Neurons and Synapses Companies, 2003

The Nervous System: Neurons and Synapses 157

Schwann cell cytoplasm

Schwann cell cytoplasm

Unmyelinated

Schwann cell cytoplasm

Oligodendrocyte

Unmyelinated

Schwann cell cytoplasm

■ Figure 7.7 An electron micrograph of unmyelinated and myelinated axons. Notice that myelinated axons have Schwann cell cytoplasm to the outside of their myelin sheath, and that Schwann cell cytoplasm also surrounds unmyelinated axons.

larger are likely to be myelinated. Myelinated axons conduct impulses more rapidly than those that are unmyelinated.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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