Like all the other animals, humans must interact with the external world. The environment is, in a certain sense, the center of the human being. If we would use a philosophical terminology we could say that the environment determines what we are.
Neurons allow our body to interact with the environment. Obviously, some neurons need to communicate with the others in a very short time, in order to allow the body to adapt fastly to the environment’s changes.
Different neurons have different characteristic
In the human body, there are different types of nerve fibers. Firstly, we have to separate two types: myelinated fibers and unmyelinated ones.
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Myelinated fibers have, as the name suggests, a structure around the axons of the neurons which form the fiber itself: it is the myelin sheath. This structure allows to transmit information very fastly from a neuron to another because the electrical pulse, which contains the message, doesn’t go forward “point by point” but “node to node”.
Let’s try to explain this phenomenon with a picture and it will be immediately clearer.
In the picture above we have an unmyelinated fiber on the left side, and a myelinated one on the right. As you can see, the pulse goes forward in two different ways.
In the unmyelinated fiber, it proceeds “point by point”. To clarify, when you stimulate a neuron and its membrane reaches the threshold potential, it can transmit a signal through its axon. Here Na+ ions enter in the membrane allowing the current to flow. This kind of conduction is called “continuous”.
On the other hand, when a fiber has its myelin sheath the conduction is faster. The myelin sheath works as the cases of an electrical wire. For instance, you can image the axon as an electrical wire but its case has many cuts: in this region, there won’t be the coating.
Those cuts, in the axon, have a name: Ranvier’s nodes. Myelin fibers have ion channels only in their Ranvier’s nodes and use “saltatory conduction”, from a node to another.
What is the myelinic sheath?
There are two different kinds of cells that form the myelin sheath. Firstly, in the peripheral nervous system, we find the Schwann’s cells and each of them cares only one neuron. The Schwann’s cell membrane rolling around the neuron forming a piece of the coating, from a node to another.
Secondly, in the central nervous system, we have another type of cell that has, more or less, the same meaning: oligodendrocytes.
Oligodendrocytes are bigger than Schwann’s cell and one of them takes care of more than one neuron, even forty at the same time.
The myelin sheath allows reaching incredible conduction’s speed
Myelin fibers are the highway of the human body. However, conduction’s speed of a fiber depends also on the thickness of the fiber we are considering.
The thickest myelin fibers can transmit the pulse at 120 m/s, equal to 432000 Km/h! We call those fibers Aα. They are involved in the transmission of sensorial and motor information. Aβ fibers are a bit thin and, thanks to their myelin sheath, have a speed of 33-75 m/s.
On the other hand, C fibers haven’t the coating. They transmit signals at 0.5-1 m/s. This kind of fiber has the function to transmit heat and a particular kind of pain.
As you can see, the myelin sheath has a highly important role in our body. When someone calls you, your response is almost immediate and it wouldn’t be possible without this structure. From an evolutionary point of view, when danger is coming an animal has to be reactive in a very short time and muscles have to react fastly to run away.
Multiple Sclerosis: damage to the myelin sheath
Multiple sclerosis is an example of what happens if the myelin sheath is damaged. In this pathology, the immune system reacts against its own cells, precisely those cells which form myelinic sheaths. Symptoms of SM depended on the cerebral regions which are damaged. The most common symptoms are: fatigue, visual problems but also sexual, urinary, intestinal and cognitive disorders, depression, coordination and verbal difficulties.