What is the Difference Between Myelinated and Unmyelinated Axons

The main difference between myelinated and unmyelinated axons is that myelinated axons have a large diameter, whereas unmyelinated axons have a small diameter. 

Myelinated and unmyelinated axons are two axon types that occur in nerve cells. They are important for the transmission of nerve impulses. 

Key Areas Covered

1. What are Myelinated Axons
– Definition, Structure, Function
2. What are Unmyelinated Axons
– Definition, Structure, Function
3. Similarities Between Myelinated and Unmyelinated Axons
– Outline of Common Features
4. Difference Between Myelinated and Unmyelinated Axons
– Comparison of Key Differences

Key Terms 

Myelinated Axons, Unmyelinated Axons

What are Myelinated Axons

The myelinated axons 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. Schwann cells secrete the myelin in the nerve fibers of the peripheral nerves. Oligodendrocytes secrete myelin in the central nervous system. The myelinated portions of the nerve fiber are internodes. The non-myelinated portions of the nerve fiber are the nodes of Ranvier.

Figure 1: Myelinated Axons

Furthermore, 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. 

What are Unmyelinated Axons

Unmyelinated axons are the nerve fibers that do not contain a myelin sheath insulating the nerve axons. In comparison to myelinated axons, unmyelinated axons show 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 also contain unmyelinated fibers. The olfactory nerves are also unmyelinated.

Figure 1: The Olfactory Nerve

Moreover, 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 Schwann cells. 

Similarities Between Myelinated and Unmyelinated Axons

• Myelinated and unmyelinated axons are two axon types in the nervous system.
• They are responsible for the transmission of nerve impulses through the neuron. 

Difference Between Myelinated and Unmyelinated Axons

Definition

Myelinated axons refer to axons that are covered by a myelin sheath, a multilayer of proteins and lipids. In contrast, unmyelinated axons refer to axons that are not protected by a myelin sheath.

Diameter

Myelinated axons have a large diameter, while unmyelinated axons have a small diameter.  

Axon

Concentric layers of Schwann cell plasma membrane surround the myelinated axons, while the cytoplasm of Schwann cells surrounds the axon of unmyelinated axons. 

Salutatory Conduction

Salutatory conduction is present in myelinated axons, while salutatory conduction is absent in unmyelinated axons. 

Speed of Transmission

Myelinated axons transmit nerve impulses at high speed, while unmyelinated axons transmit nerve impulses at low speed. 

The density of Voltage-gated Na Channels

Myelinated axons contain more voltage-gated Na channels, while unmyelinated axons contain less voltage-gated Na channels. 

Color

Myelinated axons are white in color, while unmyelinated axons are gray in color. 

Nodes and Internodes

Myelinated axons contain nodes and internodes, while unmyelinated axons do not contain nodes and internodes. 

Length of the Axon

Myelinated axons contain long axons, while unmyelinated axons contain short axons.

Examples

Peripheral nerves contain myelinated axons, while C fibers of the skin, muscles, viscera and olfactory nerves contain unmyelinated axons. 

Conclusion

In brief, myelinated and unmyelinated axons are two types of axons that occur in nerve cells. Long axons are myelinated, and they have a large diameter. They contain concentric layers of Schwann cell plasma membrane surrounding the axon. Therefore, salutatory conduction is present in myelinated axons. The transmission of nerve impulses occurs at high speed through the nodes of these axons. Also, they contain more voltage-gated Na channels. In comparison, unmyelinated nerve fibers contain the cytoplasm of Schwann cells surrounding the axon. Therefore, it does not form nodes and internodes on the axon. The speed of transmission of nerve impulses is low in unmyelinated axons. They have short axons with a small diameter. Unmyelinated axons are gray in color. Therefore, the main difference between myelinated and unmyelinated axons is their structure and function.

References:

1. Structure and function of myelinated axons. Patterning and Cell Type Specification in the Developing CNS and PNS (Second Edition). 
2. Arcilla CK, Tadi P. Neuroanatomy, Unmyelinated Nerve Fibers. [Updated 2023 Jan 2]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.

Image Courtesy:

1. “Neuron1” By NickGorton~commonswiki – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Head olfactory nerve” By Patrick J. Lynch – Own Work (CC-BY 2.5) via Commons Wikimedia