Myelinated and Unmyelinated Axons

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Myelinated and Unmyelinated Axons

Introduction

Nerve fibers or axons are a part of the nervous system which receives environmental signals and produce a response. For response, the nervous system uses nerve fibers for transmitting nerve impulses to the target cells. These nerve fibers are of three types:

  1. Sensory neurons
  2. Interneurons
  3. Motor neurons

These neurons are classified into two categories based upon the presence or absence of myelin sheath:

  1. Myelinated axons
  2. Unmyelinated axons

What are Myelinated Axons? 

Those axons which have a covering of myelin sheath are known as myelinated axons. These axons protected by myelin sheath are not easily damaged by the external environment and the rate of nerve impulse is also high in comparison to Unmyelinated axons.

What are Unmyelinated Axons? 

Those axons which are not protected by myelin sheath are known as unmyelinated axons. These axons are usually thinner, less than one micron in diameter. They are also known as non-myelinated axons. They conduct nerve impulses at a low speed and the chances of losing nerve impulse exist in the case of unmyelinated axons.

What is an axon? 

A neuron is made up of three basic components:

  1. Dendrites
  2. Cell body
  3. Axon

Axon is a thin long projection that is essential for transmitting nerve impulses from one neuron to the other. The length of axons in different nerve fibers is different. For example, the sciatica nerve has an axon that is approximately 1 meter in diameter. Without these axons, nerve fibers can’t conduct nerve impulses.

Structure of Myelin Sheath

Myelin

Myelin is a lipid-rich (fatty) substance that makes a covering around axons. This covering of myelin insulates the axons and protects them. It also increases the rate of nerve impulse conduction. There are spaces left in between the myelin sheath that form nodes of Ranvier (essential for speedy nerve impulse conduction).

Myelin formed around the axons of central nervous system neurons is different from that of peripheral nervous system neurons. In the central nervous system (brain, spinal cord, and optic nerve), the glial cells responsible for forming myelin sheath are oligodendrocytes. But in the peripheral nervous system, the glial cells which produce myelin sheath are Schwann cells. Due to myelin sheath, saltatory conduction of nerve impulse occurs in which nerve impulses jump from one node of Ranvier to another until they reach the target cells for producing a response.

Chemical composition of Myelin Sheath 

Although both the central nervous system myelin and the peripheral nervous system myelin plays the same role of insulating the axon both differ from each other slightly in composition and configuration. The myelin of the central nervous system appears to be white due to the presence of lipids in large quantities. Hence it is termed as white matter. Blood vessels supply oxygen and energy-rich compounds such as glucose to these axon fibers.

Chemically myelin sheath has the following substances:

  • Approximately 40% of water
  • 60-70% lipid
  • 15-25% protein

The proteins present are the myelin basic protein (MBP). It is abundant in the central nervous system where it has a very important role in the formation of the myelin sheath. The protein which forms myelin in the central nervous system is myelin oligodendrocyte glycoprotein (MOG). While in the peripheral nervous system, myelin protein zero (MPZ or PO) has a role in myelin synthesis. The primary lipid that forms myelin is a glycolipid called galactocerebroside. Besides, sphingomyelin chains are present, in a twisted manner, which further gives strength to the myelin sheath. Cholesterol is also a major component of myelin. In the absence of cholesterol, myelin is not formed.

Process of Myelination

Myelination is defined as the formation of the myelin sheath around the axon. This makes myelinated motor neurons different from the unmyelinated ones. The myelin sheath is formed by myelin and myelin is produced by neuroglia (Neuroglia are also known as glia or glial cells and help the neurons in conducting nerve impulses by providing structural and metabolic role. These cells provide support by protecting and nourishing the neurons and in addition to this they maintain the interstitial fluid present around the neuron).

Neuroglia, that form myelin in neurons, are of the following types:

  • Oligodendrocytes
  • Schwann cells

Oligodendrocytes

Oligodendrocytes also are known as oligodendroglia are star-shaped neuroglia that produces myelin sheath on the axons of the central nervous system. A single oligodendrocyte has multiple arm-like processes that arise from the cell body. These arm-like processes help these neuroglia to myelinate multiple axons by making a covering of myelin sheath around them. Unlike in Schwann cells, the cell body and nucleus of oligodendrocytes remains detached from myelin sheath. In the myelinated neurons of oligodendrocytes, nodes of Ranvier are present but they are present at a distance than those formed by Schwann cells.

Schwann cells 

Schwann cells are also known as neurolemmocytes. These are flat cells that are involved in the formation of the myelin sheath covering over the axons present in the peripheral nervous system. A single Schwann cell myelinates only a single axon. Therefore, for myelinating more than one axon of the peripheral nervous system, multiple Schwann cells are required (this is because a single Schwann cell makes a lipid-rich layer around the axon in about 1mm of axon’s length).

The myelination of the axons by Schwann cells starts in the fetal development stage. Schwann cells continue forming a lipid-rich membrane around the axon until there are sufficient layers formed around the axon. During the process of forming layers, the nucleus and cytoplasm of the Schwann cells squeezed out gradually. After myelination, the nucleus and cytoplasm of the Schwann cells are present in the outermost layer. This outermost layer, containing the nucleus and cytoplasm, is known as neurolemma.

Along the length of the axons, the gaps that are formed after the formation of the myelin sheath are called nodes of Ranvier. In these, the electrical impulses form and travel quickly by jumping from node to node. In comparison to this, in unmyelinated neurons, the electrical impulses have to travel throughout the cell membrane to reach the target which makes the process of signal transduction relatively slower.

Importance of Myelin Sheath 

The insulating feature of Myelin is important for the normal functioning of neurons. Motor neurons that are involved in movement such as walking need Myelin Sheath insulation for quick signal conduction. Sensory neuron functions, hearing, seeing, or sensation of pain, are also dependent upon the myelin sheath insulation.

As myelin sheath is covering around the axon, one of its functions is to serve as a separating layer for the axon from the extracellular components. However, it is mainly involved in increasing the velocity of nerve impulse conduction for a quick response.

Myelin has low capacitance and high electrical resistance and can act as an insulator. Therefore, the myelin sheath acts as an insulating layer to increase the speed of signal conduction. Due to myelin sheath, myelinated motor neurons can conduct nerve impulses at a higher speed than unmyelinated neurons.

Nodes of Ranvier, gaps formed due to myelination, have clusters of sodium and potassium ion channels that are voltage sensitive. These nodes of Ranvier are important in saltatory nerve impulse conduction in which the signal jumps from node to node along the entire length of the axon. This type of conduction due to myelin sheath gives the following benefits:

  • Increases the signal conduction speed.
  • Reduces the amount of energy consumed for conducting nerve impulses as the energy needed in myelinated motor neuron fibers is less than that for unmyelinated fibers.
  • The speed of signal conduction also depends upon the diameter of the axon. The diameter of the myelinated axons is large enough to facilitate a speedy nerve impulse conduction.

Examples of Myelinated and Unmyelinated Axons

Myelinated Axons 

Myelinated axons are required in cells where quick response is required. Myelinated axons are present in sensory neurons and motor neurons. But all the motor neurons are not myelinated. Some are unmyelinated as well. Motor neurons of the somatic nervous system are myelinated and the motor neurons of the upper motor neurons are myelinated. They are involved in conducting quick response signals to the muscles or glands as happens in the case of the reflex arc, where a fast response is required to avoid any injury.

Unmyelinated Axons

Unmyelinated Axons are present in the autonomic nervous system. Both the parasympathetic nervous system and sympathetic nervous system are a part of autonomic nervous system. These systems are formed by motor neurons whose axons are unmyelinated. In addition to these, the small axons neurons in the central nervous system are also unmyelinated axons.

Differences Between Myelinated and Unmyelinated Axons

Following are some of the differences between myelinated and Unmyelinated axons or nerve fibers:

Myelin Sheath

Myelin Sheath is present in the myelinated axons. A layer of myelin serves as the outer covering of the axons in these fibers. While in unmyelinated axons, the myelin sheath is not present around the axon. Myelin sheath is a fatty substance that acts as an insulation to protect the nerve fibers from the outside environment and also increases the speed of signal conduction.

Color

Myelinated nerve fibers can be easily differentiated from the Unmyelinated ones. The myelinated axons are white while the Unmyelinated axons are grey.

Nodes of Ranvier

Nodes of Ranvier are formed due to the gaps left in between the myelin sheath while forming covering over the axon. Myelinated axons have nodes of Ranvier while unmyelinated axons don’t possess nodes of Ranvier. These nodes of Ranvier are involved in rapid nerve impulse conduction.

Speed of transmission of nerve impulse

The speed of transmission of nerve impulses is more in myelinated axons than in unmyelinated axons. This is due to the reason that myelinated axons have nodes of Ranvier. Nerve impulse can jump from node to node in myelinated axons which favors a faster transmission speed. But in unmyelinated axons, nodes of Ranvier are not present due to the absence of myelin sheath. Therefore, the speed of nerve impulse is low in unmyelinated nerve fibers.

Location

The majority of the neurons in the central nervous system and peripheral nervous system have myelinated axons. This is because the most central nervous system and peripheral nervous system neurons require fast signal transmission such as neurons responsible for spinal reflexes. Unmyelinated axons are also present in the central nervous system and peripheral nervous system in the group c nerves. These nerves are involved in transmitting signals for secondary pain or itching.

Impulse conduction

The presence of a myelin sheath ensures signal transmission to the target cell because myelinated neurons do not lose impulse during conduction. Whereas, unmyelinated axons can lose nerve impulse during transmission.

Axon length

In myelinated axons, the axon length is more than that of the unmyelinated axons.

Thickness

The thickness of myelinated axons is more than the unmyelinated axons. Besides, myelinated axons produce collateral nerve fibers and unmyelinated fibers don’t produce collateral fibers.

Similarities Between Myelinated and Unmyelinated Axons 

They have some similarities which are below:

  1. Both are nerves and are a part of the nervous system.
  2. Both conduct nerve impulses as electrical signals.

Myelinated axons are better at the conduction of nerve impulses because they transfer the signal quickly and protects it too along the way to the target cells.

References

  1.  Lodish, H; Berk, A; Kaiser, C; Krieger, M; Bretscher, A; Ploegh, H; Amon, A (2000). Molecular Cell Biology (7th ed.). New York, NY: W. H. Freeman and Company. p. 695.
  2. Marieb, E. N., & Hoehn, K. (2014). Human anatomy & physiology. San Francisco, CA: Pearson Education Inc.