Myelin is a fatty substance that surrounds many nerve cells. Since it is spirally wrapped around the nerve cells, the structure that is created is called a myelin sheath. Myelin sheaths are found both in the central nervous system, i.e.
in the brain, and in the peripheral nervous system, i.e. in all other nerves that are located in the human body. They surround the nerve cells, where a fast transmission of signals is necessary. These are, for example, the nerve cells that are responsible for executing movements. In the brain and spinal cord, too, rapid transmission is necessary, which is why there are myelin sheaths. Here, the entirety of the medullary sheaths is also known as the white matter.
Function
The myelin sheaths that surround the nerve cells are necessary for electrical insulation. In order to prevent new electrical impulses from constantly having to form along the nerve cell in order to transmit the signals, the myelin sheath insulates the nerve cell. This saves time and enables faster transmission.
Structure
A nerve cell (neuron) consists of three parts. The central cell body (soma), the dendrites, which receive and transmit signals from other nerve cells on one side of the cell body, and the axon with its terminal branches, where the signals are passed on to the next cells. The axon of the various nerve cells is very long, depending on its location in the body.
For example, the nerve cells that supply the legs have a length of up to one meter. Here it must be ensured that the signals are passed on along the axon very quickly so that, for example, a movement initiated by the brain is not carried out seconds later but immediately. For this reason, the axons have a myelin sheath that surrounds them.
In the central nervous system, i.e. in the brain and spinal cord, the myelin sheath is formed by so-called oligodendrocytes. This is simply the special name of the cells that form a spiral around the nerve cells. In the peripheral nervous system these cells are called Schwann cells.
But there they have the same function. Since axons can be very long, it is not enough for a single cell to wrap itself around this axon to isolate it. All along the axon, many of these cells wrap themselves around the axon.
Small gaps are formed between the sites where the axon is exposed. These spaces are about 1 micrometer long. They are called Ranvier’s lacing rings because the myelin sheath looks like it is laced together.
Only at these points is an electrical impulse (an action potential) triggered. Due to the insulation, this potential can be passed on through the myelin sheath by 1 to 1.5 mm until a new impulse is triggered at the next lacing ring. This phenomenon continues until the end of the axon. Here the impulse is then passed on to the next cell.
