Neurons are the structural and functional units of the nervous system found in animals. They are responsible for bringing about the nervous coordination among the various organs found in the body. each neuron has a cell body and cellular processes. The cellular processes of neurons are interconnected via synapses. The information is carried by these processes in the form of changes in the membrane potential.
The proper conduction of nerve impulses requires the neurons to be insulated from the electrical influences of the surrounding environment. The concentration of different ions is different in the intracellular and extracellular fluids. This concentration difference should be maintained for the proper conduction of nerve impulses. It is done by insulating the nerve fibers with a myelin sheath.
Myelin sheath is a layer of lipids that surrounds the cellular processes of the neurons and provides them electrical insulation from the surrounding fluid. This lipid layer is impermeable to ions and thus, the membrane potential is not disturbed.
In this article, we will discuss the structure of myelin sheath, its biochemical nature, and the development of the myelin sheath around the neurons. We will also talk about the physiological benefits of the myelin sheath and some clinical conditions associated with it.
Myelin sheath is found around the cellular processes of both the central nervous system as well as the peripheral nervous system.
In the central nervous system, the myelin sheath is found around axons of the neurons present in the brain, spinal cord, and the optic nerve. Here, it serves to separate the various axons that packed together in the brain or spinal cord. The myelinated nerve fibers are found in the outer grey matter of the spinal cord whereas, in the case of the brain, the grey matter containing myelinated nerve fibers is found on the inner side.
In the peripheral nervous system, the myelin sheath is found around the axons and dendrites of the motor neurons. It also covers the central and peripheral processes of the sensory neurons. All the spinal nerves are covered by a layer of the myelin sheath.
One thing should be noted that the myelin sheath is not present in the form of a single continuous layer. Rather, it is present in the form of small segments. The nerve fibers are insulated by myelin sheath in the form of small segments. A non-insulated segment is present between two successive segments. This non-insulated segment is called the node of Ranvier. In the case of myelinated nerve fibers, a nerve impulse travels from node to node. This will be explained further under the heading of physiology.
The structure of the myelin sheath can be studied at two levels; histological structure as well as biochemical structure. Both of these structures are described below.
The histological structure of the myelin sheath can be studied under the light microscope after proper tissue preparation. Aldehydes such as formalin are used in the fixation of nervous tissue for light microscopy.
The prepared tissue slides are stained to be viewed under the light microscope. In the horizontal cut sections, myelin sheaths are apparent as the rings surrounding the axons in the nervous tissue. These rings appear pink as they take the hematoxylin stain. In the vertical cut sections of the nervous tissue, myelin sheath appears as vertical projections side by side with the long axons or nerve fibers.
The nuclei of glial cells that make the myelin sheath are also visible along with the myelin rings in some sections. These nuclei take the eosin stain and appear as pink dots.
Myelin sheath is mainly composed of lipids, proteins, and water. Water makes around 40% of the total mass of the myelin sheath. Lipis make around 60% to 75% of the total dry mass whereas the proteins make around 15% to 30% of the total biochemical content in the myelin sheath.
Lipids in Myelin Sheath
The major lipids present in the myelin sheath are galactocerebroside. They belong to the category of glycolipids. Other important lipids in the myelin include sphingomyelin and cholesterol. The sphingomyelin provides strength to the sheath whereas the cholesterol molecules are needed for its formation.
Due to the abundant lipids, myelin sheath appears as white on gross appearance. That’s why the region containing myelinated nerve fibers is called white matter.
Proteins in Myelin Sheath
The major protein present in the myelin sheath are as follows;
- Myelin basic protein (MBP)
- Proteolipid protein (PLP)
- Myelin oligodendrocyte glycoprotein (MOG)
- Myelin protein zero (MPZ)
In terms of the proteins, the composition of myelin in the central nervous system and the peripheral nervous system is slightly different. The myelin oligodendrocyte glycoprotein (MOG) is exclusive to myelin sheath found in the CNS. In the peripheral nervous system, this protein is replaced by the myelin protein zero (MPZ).
As we have studied the structure of myelin sheath, let us now discuss its synthesis. In this section, we will talk about the biochemical synthesis of various components found in the myelin sheath.
Synthesis of Myelin Proteins
The proteins found in the myelin sheath are made by the process of transcription and translation. The genes for these proteins are active in the cells that make myelin. These are the oligodendrocytes in the CNS and the Schwann cells in the PNS.
The genes are first transcribed to form mRNA which is then translated by the ribosomes present in the cytoplasm of these cells. The myelin proteins thus formed are transferred to the plasma membrane to be used in making myelin around the nerve fibers.
Synthesis of Myelin Lipids
The major lipids in the myelin sheath are the galactocerebrosides. They are made in the smooth endoplasmic reticulum of the cells that make myelin. A molecule of galactose is attached to the cerebroside backbone to make galactocerebroside.
Sphingomyelin is another important lipid found in the myelin sheath. It consists of sphingosine alcohol. A fatty acid is attached to the amino group of sphingosine via an amide linkage to make ceramide. Then, the addition of choline to the hydroxyl group of sphingosine converts it into sphingomyelin.
Development and Formation
In this section, we will talk about the process by which the myelin sheath is formed around the nerve fibers.
Myelin sheath is made by the glial cells found in the central as well as the peripheral nervous system. In the CNS, oligodendrocytes are responsible for synthesizing myelin sheath. On the other hand, in the PNS, the Schwann cells make myelin sheath around the spinal nerves. The process of making the myelin sheath around the nerve fibers is the same in both the nervous divisions.
The following steps are followed by glial cells in making myelin sheath around the nerve fibers:
- The glial cell wraps around the axon
- It starts rotating around the nerve fiber
- The cytoplasm and membrane of the glial cell makes consecutive layers around the nerve fiber
- The cytoplasm between these membrane layers tend to condense
- The overlapping inner layers of the membrane make the myelin sheath with the nucleus and cytoplasm being pushed to one side
The process of myelination begins during the late fetal period of embryonic life. The process continues until the end of the first year of life. myelin basic proteins are essential for the myelination process to begin. The glial cells involved in myelination are derived from the neural crest cells.
In this section, we will talk about the major physiologic roles played by the myelin sheath in the better performance of the nervous system.
Increased Speed of Conduction
This is the most important function performed by the myelin sheath in the myelinated neurons. The speed of impulse conduction is greatly increased due to myelinated fibers as compared to the unmyelinated neurons. It is because, in the unmyelinated fibers, a nerve impulse travels as a single wave throughout the fiber. However, in the case of myelinated nerve fibers, depolarization occurs only at the nodes of Ranvier. So, the impulse jumps from node to node, and its speed is much increased.
The reason for the node to node conduction is the absence of voltage-gated sodium channels in the regions of the internodes. These channels are needed for the depolarization of neurons. In the case of myelinated nerve fibers, voltage-gated sodium channels are only expressed at the nodes of Ranvier. Therefore, the electrical currents flowing in the cytoplasm cause depolarization only at the nodes.
As the nerve impulse has to travel from node to node, the nodes must be closely spaced for the better propagation of action potential. If the nodes are spaced apart, the action potential may die before reaching the next node.
Protection of Axons
Myelin sheath also protects the axonal nerve fibers. It protects them from any injury or damage.
Another important function of myelin is to provide electrical insulation to the nerve fibers. It preserves the ionic concentration on both sides of the membrane and maintains the electrical potentials. In this way, the axoplasm is saved from the electrical influences of the extracellular fluid as the impulse is transmitted without any disturbance.
Myelin sheath also has a role in providing nutrition to the nerve fibers. It is especially true in the case of the peripheral nervous system, where the myelin sheath is made by Schwann cells. These cells are involved in providing nutrition to the axonal nerve fibers.
The major clinical conditions associated with the myelin sheath are as follows.
It is the removal of the myelin sheath from the nerve fibers. It is seen in several neurodegenerative diseases that are of autoimmune nature. Some examples of such diseases include sclerosis, neuromyelitis, leukodystrophy, etc.
The loss of myelin sheath impairs the impulse conduction along the nerve fibers. The conduction of action potential may be completely lost.
This disease is characterized by blurred vision, pain in the eye, double vision or loss of vision at all. The patient may feel numbness or tingling in the legs, arms, chest, or face. In severe conditions, the disease may cause cognitive impairment, memory loss, uncoordinated movements, slurred speech, etc. The range of symptoms depends on the degree of demyelination and the nerve fibers that are affected.
Certain techniques are under study for mental repair. The current practice is to implant glial precursor cells in the nervous system. These cells will give rise to glial cells that will again begin the process of myelination.
- Myelin sheath is a layer of lipoproteins that surrounds the nerve fibers in the white matter of CNS as well as the PNS.
- When viewed under the light microscope, the myelin sheath appears as white rings around the nerve fibers.
- The biochemical composition of the myelin sheath involves water, lipids, and proteins.
- All the components of the myelin sheath are made in the glial cells. These include oligodendrocytes in the CNS and the Schwann cells in the PNS.
- The process of myelination begins in the late stages of embryonic development. It is completed in the first year of life after birth.
- Myelin sheath helps in the better condition of nerve impulses in the following ways:
- It increases the speed of impulse conduction
- It protects the axonal fibers from damage
- It provides electrical insulation to the nerve fibers
- It provides nutrition to the nerve fibers
- Demyelination is a condition when the myelin sheath is lost due to some autoimmune diseases. The patient presents with variable symptoms depending on the extent of demyelination.
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