Introduction

Neurons are specific cells of the Nervous system that are involved in transmitting signals from the brain and spinal cord to target cells of the body. It is estimated that the number of neurons in the human brain is about 80 billion. These neurons are classified into the following three types:

1. Sensory neurons
2. Interneurons
3. Motor neurons

Motor neurons are present in the peripheral nervous system. Moreover, the sympathetic and parasympathetic nervous systems also use motor neurons for the conduction of nerve impulses. Motor neurons receive the information from sensory neurons and convert it into action in muscles or glands. These motor neurons are of two types on the types of axons:

1. Myelinated Motor Neurons
2. Unmyelinated Motor Neurons

What is Myelinated Motor Neuron? 

When the axon is covered with a myelin sheath then the nerve fibre is known as a myelinated motor neuron. The myelin sheath is present at specific gaps and these gaps are known as nodes of Ranvier. The presence of a myelin sheath makes the neuron thicker and helps in faster nerve impulse conduction. For example, the motor neurons of the peripheral nervous system are myelinated but the neurons which form the autonomic nervous system (sympathetic and parasympathetic nervous system) are unmyelinated motor neurons i.e. the axons of these motor neurons don’t have a myelin sheath covering around their axons.

Read more about the Structure of a Motor Neuron

Structure of Myelinated Motor Neuron 

As myelinated motor neuron is a type of motor neuron, it possesses the basic single cell structure of a motor neuron. Motor neurons consist of all the classes of eukaryotic cell organelles such as mitochondria, cell membrane, ribosomes, and nucleus, etc. The structure consists of three basic components:

1. Dendrites
2. Cell body
3. Axon

Myelinated motor neurons are multipolar, which means they have a single axon and multiple dendrites in their structure.

Dendrites

The branch-like extension that is present at the end of a neuron is known as dendrites. These are the part of the neuron, where information is received from other neurons are is transmitted to the cell body as signals that are required for activating the cell. In all the neurons, the number of dendrites is about five to seven. But in some neurons, such are Purkinje neurons in the brain, the number of dendrites is more than a thousand.

Cell body (Soma) 

The cell body or Soma is an important part of the neuron structure. It is the part that contains all the organelles required for the proper functioning of the nerve fiber. It is therefore involved in controlling major functions of the cell. It is the location in neurons where protein synthesis occurs. With a cell body, a neuron can’t survive.

Axon 

An axon is a conductive segment (sometimes quite long) that arises from the cell body. It transmits the information received from the dendrites cell body to the axon terminal. The information is then passed to another neuron, muscle cell, or gland.

A neuron typically contains a single axon with multiple branches with many terminals for effective communication with several cells. The length of axons varies greatly they can be really long as well as short. For example, the axons that make up the sciatic nerve are really long and can be over 1 meter in length.

Axon terminals are pre-synaptic components of neuron structure. These are the site of intracellular communication, where the signals are transferred to different motor neurons. The location where the dendrites combine with axon terminals for transferring signals is called synapses. These are electrical and chemical junctions between different neurons for connecting them.

The axon is the component of the motor neuron that is myelinated (contains myelin sheath) due to which it becomes a myelinated motor neuron. The myelin sheath acts as a protection from the external environmental conditions that can possibly interfere with the nerve impulse conduction process.

Myelination in Myelinated Motor Neuron-Formation of Myelin Sheath 

The process of formation of a myelin sheath around the axon is known as Myelination. This is the basic difference between then myelinated and unmyelinated neurons. For the formation of myelin sheath myelin is needed and myelin is formed by neuroglia (Neuroglia which is also called glia or glial cells play an important role by helping neurons in conducting nerve impulses by providing the structural and metabolic machinery. The major functions that these cells perform for the neurons are the protection and nourishment of neurons. Moreover, they help to maintain the interstitial fluid present around the nerve cells).

Neuroglia, that are responsible for the formation of myelin, are of two types:

• Oligodendrocytes
• Schwann cells 

Oligodendrocytes

The star-shaped oligodendrocytes are also known as oligodendroglia. These are the neuroglia that are involved in the formation of the myelin sheath around the axons of the central nervous system. Several arm-like processes arise from the cell body of a single oligodendrocyte. Due to these multiple arm-like processes, oligodendrocytes can myelinate multiple axons by the formation of the myelin sheath around them. In contrast to Schwann cells, structures such as the cell body and nucleus of oligodendrocytes don’t attach to the myelin sheath. In the case of the Myelination by oligodendrocytes, the neurons contain Nodes of Ranvier but the distance between these nodes is more than those formed due to Schwann cells.

Schwann cells 

The other name for Schwann cells is neurolemmocytes. In the peripheral nervous system, the myelin sheath covering of the axons is formed by Schwann cells. These cells have a flat surface. During the process of Myelination, only one axon is myelinated by a single Schwann cell. So, multiple axons will be required for the myelin sheath formation around multiple axons ( this is due to the feature of Schwann cells that a single cell can form a lipid-rich layer around the axon in about 1mm of axon’s diameter).

The process of Myelination by Schwann cells is initiated during the infant developmental stage. The formation of the lipid-rich layer continues until a sufficient number of layers have formed around the axon. The nucleus and the cytoplasm of the Schwann cells are squeezed out during the process of formation of the myelin sheath. Due to this, the nucleus and the cytoplasm of the Schwann cells exist in the peripheral layer. The outermost layer in which the nucleus and cytoplasm are present is known as neurolemma.

Throughout the length of the axon, spaces in the form of gaps are present which are formed after the formation of the myelin sheath. These gaps are known are Nodes of Ranvier. In saltatory conduction, nerve impulses jump from node to node to reach the target site.

Function of Myelin Sheath 

Some of the major functions of the myelin sheath are listed below:

1. Myelin sheath serves are a separating layer for the axon from the surrounding cellular materials. But the basic function is to increase the speed of the nerve impulses for a faster response.
2. The low capacitance and high electrical resistance property of the myelin sheath make it an insulator. So, it performs the function of an insulating layer around the axon. This increases the speed of nerve impulse conduction. It’s due to the myelin sheath that motor neurons can conduct nerve impulses at a higher speed than unmyelinated neurons.
3. The gaps formed due to Myelination, Nodes of Ranvier, contain many sodium and potassium ion channels that can detect voltage (voltage-sensitive). These Nodes of Ranvier play a vital role in the Conduction of nerve impulses during saltatory conduction. Some of the benefits of saltatory conduction are below:

• Faster signal conduction.
• It reduces the amount of energy required for the process of signal transmission.

Location of Myelinated Motor Neurons

Motor Neurons are present in the cerebral cortex of the brain or brain stem, and spinal cord. The motor neurons of the cerebral cortex are involved in voluntary actions in the body. They have projections that can communicate with the effect organs to show a response due to a stimulus.

Types of Myelinated Motor Neurons- On basis of location

Myelinated motor neurons can be classified on the basis of location and different origins of their myelin sheath (oligodendrocytes or Schwann cells). There are two types of myelinated motor neurons on the basis of location:  

Upper Motor Neurons (UPNS)

The upper motor neurons are present in the central nervous system (brain and spinal cord). The motor neurons of the CNS are myelinated by oligodendrocytes.

Lower motor neurons (LMNs)

Lower motor neurons contain the motor neurons of the peripheral nervous system (the nerve fibers that transmit signals from CNS to glands, tissues, etc.). The myelinated motor neurons of the peripheral nervous system contain a myelin sheath that is formed of Schwann cells.

Functions of Myelinated Motor Neurons 

Motor neurons transmit nerve impulses from the brain and spinal cord to the effector glands and organs. They are involved in voluntary and involuntary responses. On the basis of functions, motor neurons are classified into three categories.

Types of Motor Neurons- On the basis of functions

1. Somatic Motor Neurons
2. General Visceral Motor Neurons
3. Visceral Motor Neurons

Somatic Motor Neurons

These motor neurons arise from the central nervous system and transmit signals to the skeletal muscles, that are involved in the movement. In the list of skeletal muscles, intercostal muscles, thigh and limb muscles, arm muscles, and many others that are responsible for the movement of bones are included.

General Visceral Motor Neurons

The term general visceral motor neurons are designated for the nerve fibers that transmit signals to the smooth muscles, cardiac muscles, and glands.

Visceral Motor Neurons

Visceral Motor Neurons are particularly involved in transmitting signals to organ-related muscles to stimulate them. Branchiomeric muscles are controlled by specialized visceral neurons. These muscles basically include the muscles of the face and neck.

Importance of Myelin Sheath of Motor Neurons

Myelin sheath is a fatty acid layer that protects the motor neurons and allows them to conduct nerve impulses more quickly than unmyelinated motor neurons. Myelin sheath is present in the motor neurons of both the central nervous system and peripheral nervous system. It contains proteins that can be attacked by the immune system resulting in autoimmune disease. This causes the destruction of the myelin sheath which will show symptoms of multiple sclerosis (MS).

In this autoimmune disease, the body’s T cells destroy the myelin sheath by attacking it. This disease causes results in plaque formation in the areas where myelin is destroyed. The plaque doesn’t allow the signals to reach the target cells. The disability in an individual due to this disease depends upon the number of nerve fibers attacked by the immune system. When the brain will not be able to send signals to the other areas of the central nervous system, then multiple sclerosis symptoms include:

• Fatigue
• Vision problem
• Difficulty in walking
• Body pain
• Weakness
• Mood swings and irritability
• Dizziness

Therefore, the myelin sheath is not only important for increasing the signal conduction in the myelinated motor neuron fibers but also acts as a protection from the external environment.

Features of Myelinated Motor Neurons

Some of the major features of Myelinated Motor Neurons are:

1. They contain a myelin sheath, a covering made up of myelin, that helps them to transmit impulses quickly and also provide protection.
2. Myelinated motor nerve fibers are white in color.
3. In myelinated motor neurons, nodes of Ranvier are present that are important for saltatory nerve impulse conduction which makes the process of transmitting signals faster than the unmyelinated motor neurons.
4. Myelinated motor neuron fibers have long axons so can transmit signals at farther distances.
5. In myelinated motor neurons, chances of loss of nerve impulse during conduction are less due to the presence of myelin sheath. Hence, the signal will reach quickly and more effectively to the target muscle or gland.
6. A myelinated motor neuron transmits the signals at about 100 m/s. So, a signal sent from the hindbrain will reach the foot in about 16 milliseconds and in 10 milliseconds to the hand.

So, myelinated motor neurons make the process of nerve impulse conduction much faster and protected in comparison to the unmyelinated neurons.

Frequently Asked Questions

What are myelinated motor neurons?

Myelinated motor neurons are those in which axons are enveloped by Schwann cells to form the myelin sheath. Nerve impulses in such neurons travel by jumping from one node to another.

What are Nodes of Ranvier in myelinated motor neurons?

Nodes of Ranvier or simply nodes are areas on axons of myelinated motor neurons that lack the myelin sheath. These myelin-free areas are conductors of nerve impulses. Nerve impulses in these axons travel from node to node. 

What type of cells are involved in the myelination of motor neurons?

Two types of cells are involved in the process of myelination; Oligodendrocytes and Schwann cells. Oligodendrocytes are responsible for the myelination of neurons found in the central nervous system. Schwann cells cause myelination in motor neurons present in the peripheral nervous system.

What are 2 types of myelinated motor neurons?

Myelinated motor neurons are of two types; upper motor neurons and lower motor neurons. Upper motor neurons are present in the CNS and are myelinated by oligodendrocytes. Lower motor neurons are a part of the PNS and are myelinated by Schwann 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.