Myelin sheath and myelination

Overview

To understand myelination, we must first understand the cellular structure of the nervous system. Recall that the nervous system is composed of two types of cells: neurons and neuroglia (also simply known as glia or glial cells). Neurons conduct signals throughout the nervous system, while neuroglia provide a supporting structural and metabolic role for neurons by protecting and nourishing neurons, as well as maintaining the surrounding interstitial fluid. This is why they are known as the “glue” of the nervous system (“glia” is Greek for “glue”).

Each neuron has four specialized regions to perform different functions:

• Dendrites receive incoming information. They are part of the receptive segment of a neuron.
• The cell body (also called soma or perikaryon) also receives incoming information and integrates information together. Depending on the type of neuron, various extensions or processes will extend from the cell body such as dendrites and an axon. As the cell body receives information, it is part of the receptive segment of a neuron.
• The axon then conducts information from the cell body of a neuron to the axon terminal. An axon makes up the conductive segment of a neuron.
• Axon terminals are the presynaptic component of a synapse, the site of intercellular communication, where a neuron transmits its signal to another neuron. Axon terminals are the transmissive segment of a neuron.

It is the axon of a neuron which is myelinated.

Before you go any further, why not test how well you know the different parts of a neuron and neuron types?

Myelination

Myelination is the formation of a myelin sheath.  Myelin sheaths are made of myelin, and myelin is produced by different types of neuroglia: oligodendrocytes and Schwann cells, where oligodendrocytes myelinate axons in the central nervous system, and Schwann cells myelinate axons in the peripheral nervous system. So which cells form myelin in the spinal cord? Since the spinal cord is part of the central nervous system, oligodendrocytes form this myelin. Functionally, oligodendrocytes and Schwann cells perform the same role, but structurally they are different.

Remember these cells and their location with the mnemonic "COPS" (Central - Oligodendrocytes, Peripheral - Schwann).

Myelin sheath function

Since the myelin sheath surrounds the axon, one of its functions is to separate the axon from surrounding extracellular components. Its main function, however, is to insulate the axon and  increase the velocity of action potential propagation. 

Myelin has properties of low capacitance and high electrical resistance which means it can act as an insulator. Therefore, myelin sheaths insulate axons to increase the speed of electrical signal conduction. This allows myelinated axons to conduct electrical signals at high speeds.

Nodes of Ranvier (gaps in myelination) contain clusters of voltage-sensitive sodium and potassium ion channels (approximately 1000 per µm2) whereas their distribution and numbers under myelin in the internodal axon membrane are spare. This creates an uneven distribution of ion channels, and the action potential in myelinated axons will “leap” from one node to the next in saltatory conduction. This type of conduction has important consequences:

• Increased conduction velocity
• Reduced metabolic cost of conduction as the amount of energy needed in myelinated fibers to conduct the impulse is less

The conduction velocity of an axon can be linked to the diameter. Myelinated axons are quite large in diameter, ranging from 1 - 13 µm. Unmyelinated axons on the other hand have a small diameter– generally less than 0.2 µm in the central nervous system and less than 1 µm in the peripheral nervous system. In unmyelinated axons, the conduction velocity is proportional to its (diameter)½ while the conduction velocity in myelinated axons increases linearly. This means that myelinated axons that are the same diameter as unmyelinated axons can conduct signals much faster.

Brain myelination

Myelination in the human brain is a continuous process from birth and is not mature until about 2 years of age. At this stage, motor and sensory systems are mature and myelination of the cerebral hemispheres is largely complete. There are, however, some processes which myelinate later in life: some thalamic radiations will be mature at about 5 - 7 years of age; and myelination of intracortical connections between association cortices continues into the 20s and 30s.

Brain myelination begins in utero, developing quite prominently from the 24th week of gestation. At birth, the myelination process continues to progress, and completes at about 2 years of age. It’s progression is predictable, and correlates with developmental milestones such as learning to walk.

Throughout the first year of life, myelin will spread throughout the whole brain in an orderly manner. Generally, myelination will start in the brainstem and progress to the cerebellum and basal ganglia, then will continue rostrally to the cerebrum, and rostrally from the occipital and parietal lobes to the frontal and temporal lobes. The progression typically follows the order from central to peripheral, caudal to rostral (inferior to superior), and dorsal to ventral (posterior to anterior). 

In the cerebrum, myelination progresses from the lower order cortices to higher order cortices. Primary cortical areas such as the primary motor cortex myelinate first, followed by secondary cortices, such as the premotor and supplementary motor cortices, and finally tertiary cortical areas such as the prefrontal cortex.

Now that you are familiar with myelin and myelination, why not take a broader look at histology of structures of nervous tissue via our fully customizable quiz!