Myelin in the Nervous System

Myelin is a protective covering around the neurons. The continuous covering of myelin around a single internode of neurons forms a sheath-like structure. This structure is called a myelin sheath. To understand the importance it is vital to know what is a myelin sheath. Myelin is made up of mostly lipids, sphingolipid concentration is maximal in myelin. It constitutes about 20% of protein. Myelination is the process of formation of the myelin sheath, across the nerve. Myelin is the enveloping of the plasma membrane, the key role of this sheath is to provide insulation and to the neuron that allows the transfer of action impulses from one neuron to another.

Structure of Myelin Sheath

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Myelin sheath is the discontinuous membrane-like structure around the axon. The junction between two nodes that are covered by the myelin forms the node of Ranvier. It is the presence of myelin that results in the saltatory movement of the signal. The myelin lamella is formed by fusion of the plasma membrane in glial cells, this fusion occurs at the inner leaflet of the glial cell, with no intervening cytoplasm. Glial cells are the cell of the nervous system. It is these cells that form the sheath, different types of glial cells form different myelin sheath in the central and peripheral nervous system. The outermost layer of the myelin sheath is known as neurolemma. Neurolemma has the nucleus and cytoplasm of the glial cell. It is important to note that only neurons of PNS (peripheral nervous system) have this feature. This is the reason behind the regenerative nature of neurons of PNS. The neurons of the CNS (central nervous system) are not regenerative, so any damage to the neuron results in permanent damage.

Composition of Myelin

Myelin is mainly made up of lipids, this is the reason behind decreased capacitance of the membrane. The major constituents of the myelin are as follows-

1. It contains 40% water of the total mass

2. It contains about 60 to 65% lipid, the protein content is about 20%

3. The myelin of CNS has a specifically high concentration of proteolipid protein (PLP), it is rarely present in the myelin of PNS.

4. Another protein named myelin protein zero (MPZ or P0) is present in PNS. It performs functions similar to the PLP of CNS.

5. Both of these proteins hold multiple concentric layers of the glial cell to form a  myelin sheath.

6. The major lipid constituent is sphingolipids, the hydrocarbon chain intertwining provides the structural integrity of the myelin sheath.

7. Galacto Cerebro Lipids are another class of lipids that are predominant in the neuron.

8. Sphingolipids and galacto cerebro lipid both belong to the class of glycolipids. These are the major component of the plasma membrane.

Cells of the Nervous System

To understand the function and mechanism of action of myelin sheath, it is important to understand the cell of the nervous system that produces myelin.

The nervous system is made up of two main categories of the cell, they are neurons and supporting cells. Neurons are the main cells that transmit the signal to the brain and the response back to the effector muscle or tissue. The signals are transmitted in the form of an electrical impulse or action potential. The supporting cells, the cell of CNS and PNS that contribute to the smooth functioning of the neuron, is the supporting cell that forms the myelin sheath layer around an axon. It is important to note that not all neurons are myelinated, such cells are called unmyelinated neurons. The supportive cell of the brain includes the glial cell. 

Glial Cell

They are also known as the neuroglial cell, they act as a supporting cell that helps maintain the functioning of the neuron. It is the glial cell that attributes the regenerative property of the neurons of the PNS. Neuroglial cells are not excitatory but proliferative in nature. 

There are the following types of the glial cell of the brain or central nervous system,

1. Microglial- They are specialized macrophages. They are of non-nervous origin, that is they originated from the mesodermal layer rather than ectoderm. They are the least abundant type of cells.

2. Astrocytes- They are the most abundant cell of the CNS, they provide nutrition and protection to the cell body of the neuron. The key role of these cells is to form a blood brain barrier.

3. Ependymal Cells- These are the cells that are responsible for the formation and circulation of the spinal fluid. They line the brain cavity. 

4. Oligodendrocytes- These are one of the most important cells of the CNS, they perform myelination of the CNS neuron. It refers to the process of formation of neuron myelin sheath. These cells, unlike Schwann cells, do not remain attached to the myelin sheath, thus no neurilemma is formed.

There are the following types of the glial cells of the peripheral nervous system (PNS)-

1. Satellite Cell - These cells are similar to the astrocytes of the CNS, they provide protection and nutrition to the cell body of a neuron.

2. Schwann Cell- These are one of the most important cells of the human body, they are similar to the oligodendrocytes of the CNS. They perform myelination of the neuron, the outermost layer of the myelin sheath is known as neurolemma. The neurilemma of the neuron formed by these cells contains the nucleus and cytoplasm of the cell, which attributes to the regenerative property of the neurons of PNS. It is important to note that one Schwann cell myelinates one internodal area between nodes of Ranvier. 

Myelination

Myelination is the process of the formation of a myelin sheath around the axon. It starts during embryonic development, from 24 weeks of gestation period. It is associated with the development of cognitive and motor skills. 

PNS myelination also starts during the embryonic developmental stage and can continue till adulthood. Myelination in PNS is performed by neuregulin 1 type III protein, which is expressed on the axon surface. This protein interacts with glial ErbB receptors, which has a pivotal role in Schwann cell differentiation and myelination. Unmyelinated neurons lack this protein. The myelination mechanism is still unclear in the CNS.

The Function of Myelination

It performs the following 2 key functions,

1. It insulates the axon which allows the rapid conduction of action potential

2. It separates the neuron from the extracellular matrix

Mechanism of Action

The neuron myelin sheath contains sphingolipids that contribute to the dielectric behavior of the cell. The addition of the myelin layer increases the surface area of the neuron, this increased surface area leads to higher resistance and low capacitance. That is, it increases the separation of cation and anion. Because of this increased resistance, there is a reduction in leak current across the transmembrane. On the other hand, the ion channels can only be found on nodes of Ranvier, which allows the action potential to travel in the saltatory manner (hopping), this combined results in faster conduction of the potential. 

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