Main Difference – Myelinated vs Unmyelinated Nerve Fibers
Myelinated and unmyelinated nerve fibers are the two forms of nerve fibers that are found in the nervous system. The main difference between myelinated and unmyelinated nerve fibers is that myelinated nerve fibers contain a myelin insulation whereas unmyelinated nerve fibers do not contain a myeline insulation. The myelin sheath is made up of lipids and proteins. Therefore, myelinated nerve fibers appear in white color while unmyelinated nerve fibers appear in gray color. The unmyelinated portions of the myelinated nerve fiber are called nodes of Ranvier. Most of the peripheral nerve fibers are myelinated, increasing the signal transduction efficiency through the axons.
Key Areas Covered
1. What are Myelinated Nerve Fibers
– Definition, Characteristics, Function
2. What are Unmyelinated Nerve Fibers
– Definition, Characteristics, Function
3. What are the Similarities Between Myelinated and Unmyelinated Nerve Fibers
– Outline of Common Features
4. What is the Difference Between Myelinated and Unmyelinated Nerve Fibers
– Comparison of Key Differences
Key Terms: Axis Cylinder, Endoneurim. Myelin Sheath, Myelinated Nerve Fibers, Nodes of Ranvier, Neurolemmal Sheath, Signal Transduction Efficiency, Unmyelinated Nerve Fibers
What are Myelinated Nerve Fibers
The myelinated nerve fibers are the nerve fibers that are insulated by a myelin sheath. Myelin is a fatty white substance, and myelinated nerve fibers are white in color. Most of the peripheral nerves are myelinated. The myelin in the nerve fibers of the peripheral nervous system is secreted by the Schwann cells. Oligodendrocytes secrete the myelin in the central nervous system. The myelinated portions of the nerve fiber are called internodes. The non-myelinated portions of the nerve fiber are called the nodes of Ranvier. The main function of the myelin sheath is to increase the electrical resistance through the nerve fiber. Therefore, the nerve impulse hops through the nodes of Ranvier through the nerve fiber. This type of transmission of nerve impulses is called saltatory conduction.
Figure 1: Saltatory Conduction
A myelinated fiber is composed of four layers: axis cylinder, myelin sheath, neurolemmal sheath, and endoneurium. The axis cylinder is the central core of the fiber. In the axis cylinder, the axoplasm is covered by the axolemma. The myelin sheath surrounds the axis cylinder. The thicker axons are composed of longer internodes and thick myelin sheaths. The neurolemmal sheath is the Schwann cell sheath, which surrounds the myelin sheath. This sheath is important in regenerating damaged nerves. The endoneurium is the connective tissue sheath, which covers Swann cells. The saltatory conduction of the action potential is shown in figure 1.
What are Unmyelinated Nerve Fibers
Unmyelinated nerve fibers are the nerve fibers that do not contain a myelin sheath insulating the nerve axons. When compared to myelinated nerve fibers, unmyelinated nerve fibers show a slower conduction of nerve impulses through the nerve. The unmyelinated nerve fibers are gray in color. Most of their axons are short. The peripheral postganglionic autonomic fibers are a type of unmyelinated nerve fibers. The C fibers of the skin, muscles, and viscera are also unmyelinated fibers. The olfactory nerves are also unmyelinated.
Figure 2: Myelinated and Unmyelinated Nerve Fibers
Unmyelinated nerve fibers contain Swann cells as a series on the nerve fiber. But, these Schwann cells do not spiral the mesaxon around the nerve fiber. The endoneurium encloses a single layer of the Schwann cells. Both myelinated and unmyelinated nerve fibers are shown in figure 2.
Similarities Between Myelinated and Unmyelinated Nerve Fibers
- Both myelinated and unmyelinated nerve fibers are made up of axons of the nerve cells.
- Both myelinated and unmyelinated nerve fibers present in the central nervous system as well as the peripheral nervous system.
Difference Between Myelinated and Unmyelinated Nerve Fibers
Definition
Myelinated Nerve Fibers: Myelinated nerve fibers are the nerve fibers that are insulated by a myelin sheath, allowing the faster conduction of the action potential along the nerve fiber.
Unmyelinated Nerve Fibers: Unmyelinated nerve fibers are the nerve fibers that do not have a myelin sheath.
Myelin Sheath
Myelinated Nerve Fibers: Myelinated nerve fibers contain a myelin sheath around the nerve fiber.
Unmyelinated Nerve Fibers: Unmyelinated nerve fibers do not contain a myelin sheath.
Color
Myelinated Nerve Fibers: The myelinated nerve fibers are white in color.
Unmyelinated Nerve Fibers: The unmyelinated nerve fibers are gray in color.
Nodes of Ranvier
Myelinated Nerve Fibers: The myelinated nerve fibers consist of nodes of Ranvier.
Unmyelinated Nerve Fibers: The unmyelinated nerve fibers do not consist of nodes of Ranvier.
Speed of Signal Transduction
Myelinated Nerve Fibers: Since transmission occurs only through nodes of Ranvier, the speed of transmission of nerve impulses is high in myelinated nerve fibers.
Unmyelinated Nerve Fibers: Since unmyelinated nerve fibers do not contain myelin insulations, the speed of the transmission of the nerve impulses is low.
Location
Myelinated Nerve Fibers: Most peripheral nerves consist of myelinated nerve fibers.
Unmyelinated Nerve Fibers: The small-axon neurons in the central nervous system and postsympathetic nerve fibers in the peripheral nervous system are unmyelinated nerve fibers.
Length of Axons
Myelinated Nerve Fibers: Typically, the nerve fibers with longer axons are myelinated.
Unmyelinated Nerve Fibers: The short axon nerve fibers are unmyelinated.
Loss of Impulse during Conduction
Myelinated Nerve Fibers: The myelin sheath prevents the loss of the impulse during conduction in myelinated nerve fibers.
Unmyelinated Nerve Fibers: The unmyelinated nerve fibers can lose the nerve impulse during conduction.
Conclusion
Myelinated and unmyelinated nerve fibers are the two types of nerve fibers in the nervous system. The myelinated nerve fibers contain a myelin sheath, surrounding the axons of the nerve cells. Myelin is secreted by the Swann cells or oligodendrocytes, which wrap the axons of the nerve cells. But, Schwann cells or oligodendrocytes in unmyelinated nerve fibers do not produce a myelin sheath. The saltatory conduction in the myelinated nerve fibers increases the speed of conduction of the nerve impulses. The main difference between myelinated and unmyelinated nerve fibers is the presence or absence of a myelin sheath in each type of nerve fibers.
Reference:
1.“Classification of nerve fibers.” LinkedIn SlideShare, 13 May 2014, Available here. Accessed 31 Aug. 2017.
Image Courtesy:
1. “Propagation of action potential along myelinated nerve fiber en” By Helixitta – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Myelinated unmylinated neurons” By Nick Gorton – here (CC BY-SA 3.0) via Commons Wikimedia
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