Difference Between Neurilemma and Myelin Sheath

Main Difference – Neurilemma vs Myelin sheath

Neurilemma and myelin sheath are two layers that surround the myelinated nerve fibers. The main difference between neurilemma and myelin sheath is that neurilemma is the plasma membrane layer of the Schwann cells whereas myelin sheath is the fatty acid layer that encloses the nerve fiber. Schwann cells supply the myelin for the peripheral nervous system whereas oligodendrocytes supply the myelin for the central nervous system. Both oligodendrocytes and Schwann cells are supporting cells in the nervous system. The myelin sheath is enclosed by the neurilemma in the peripheral nervous system.

Key Areas Covered

1. What is Neurilemma
      – Definition, Characteristics, Function
2. What is Myelin Sheath
      – Definition, Characteristics, Function
3. What are the Similarities Between Neurilemma and Myelin Sheath
      – Outline of Common Features
4. What is the Difference Between Neurilemma and Myelin Sheath
      – Comparison of Key Differences

Key Terms: Internodes, Mesaxon, Myelin Sheath, Myelinated Nerve Fibers, Neurilemma, Nodes of Ranvier, Oligodendrocytes, Saltatory Conduction, Schwann Cells

Difference Between Neurilemma and Myelin Sheath - Comparison Summary

What is Neurilemma

Neurilemma is the plasma membrane of the Schwann cells which surrounds the nerve fibers of the peripheral nervous system. Neurilemma is also called the Schwann cell sheath. Since Schwann cells are only found in the peripheral nervous system, the neurilemma is also present only in the peripheral nervous system. The neurilemma forms the outermost layer of myelinated nerve fibers and attaches the nerve fiber to the connective tissue layer in the nerve fiber called endoneurium.

Main Difference - Neurilemma vs Myelin Sheath

Figure 1: Neurilemma

Beneath the neurilemma, a thin layer of the cytoplasm and the nuclei of the Schwann cells can be found. The neurilemma is important for the regeneration of the nerve fiber. It also protects the nerve fiber. 

What is Myelin Sheath

Myelin sheath is the insulating covering of the nerve fibers, which is made up of lipids and proteins. The myelin sheath surrounds the axis cylinder of the neuron. The lipids and proteins form concentric layers by spiralization of the mesaxon. The mesaxon is a pair of plasma membranes of parallel Schwann cells, which encircles the axon of a neuron. The lipids include glycolipids, phospholipids, and cholesterol. The myelin sheath is found in intervals, and the interrupted portions are called the node of Ranvier. The portions of the nerve fiber which is surrounded by the myelin sheath are called the internodes. Each internode is myelinated by a single Schwann cell. The oblique clefts in the myelin sheath help to conduct nutrients to the deepest Schwann cells.

Difference Between Neurilemma and Myelin Sheath_Figure 2

Figure 2: Myelinated Nerve Fibers

The major function of the myelin sheath is to electrically insulate the nerve axon. The capacitance of the axon fiber is reduced at the internodes. Therefore, the conduction of the nerve impulses occurs only through the nodes of Ranvier. The ‘to hope or to leap’ conduction of nerve impulses in the myelinated nerve fibers is called the saltatory conduction. The cross sections of myelinated nerve fibers are shown in figure 2.

Similarities Between Neurilemma and Myelin Sheath

  • Neurilemma and myelin sheath are two layers that surround the axons of the myelinated nerve fibers.
  • Both neurilemma and myelin sheath are involved in increasing the speed of conduction of nerve impulses through the nerve fiber.
  • Both neurilemma and myelin sheath are formed by Schwann cells.

Difference Between Neurilemma and Myelin Sheath

Definition

Neurilemma: Neurilemma is the plasma membrane of Schwann cells that surrounds the myelinated nerve fibers.

Myelin Sheath: Myelin sheath is the insulating covering of the nerve fibers that increase the speed of conduction of nerve impulses.

Correlation

Neurilemma: Neurilemma is the outer most layer of a myelinated nerve fiber.

Myelin Sheath: Myelin sheath is covered by neurilemma.

Found in

Neurilemma: Neurilemma is only found in the peripheral nervous system.

Myelin Sheath: Myelin sheath is found in both central nervous system and the peripheral nervous system.

Composition

Neurilemma: Neurilemma is composed of the plasma membrane of Schwann cells.

Myelin Sheath: Myelin sheath is composed of proteins and lipids such as glycolipids, phospholipids, and cholesterol.

Formation

Neurilemma: Neurilemma is formed by the Schwann cells.

Myelin Sheath: Myelin is secreted by Schwann cells or oligodendrocytes.

Function

Neurilemma: Neurilemma is involved in protecting and regenerating nerve fibers.

Myelin Sheath: Myelin sheath increases the speed of the conduction of the nerve impulses through the nerve fiber.

Conclusion

Neurilemma and myelin sheath are two layers of a myelinated nerve fiber. Neurilemma is the plasma membrane of the Schwann cells, which insulate the axon. Beneath the neurilemma, the cytoplasm and the nuclei of the Schwann cells can be found. The neurilemma is important for the protection and regeneration of the nerve fiber. Myelin is secreted by the Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system. The axons of the nerve fibers are electrically insulated by the myelin sheath, increasing the speed of conduction of the nerve impulses by saltatory conduction. The main difference between neurilemma and myelin sheath is the composition and the function of each layer in the nerve axons.

Reference:

1. “Neurolemma.” Wikipedia, Wikimedia Foundation, 29 July 2017, Available here. Accessed 31 Aug. 2017.
2. Morell, Pierre. “The Myelin Sheath.” Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition., U.S. National Library of Medicine, 1 Jan. 1999, Available here. Accessed 31 Aug. 2017.

Image Courtesy:

1. “Gray632″ By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body (See “Book” section below) Bartleby.com: Gray’s Anatomy, Plate 632 (Public Domain) via Commons Wikimedia
2. “1211 Myelinated Neuron” By OpenStax (CC BY 4.0) via Commons Wikimedia

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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