Myelin Sheath: Definition, Function & Purpose

The myelin sheath, also known as the myelin sheath or axon sheath, is a multilayered, lipid-rich, spiral-shaped structure that encases the axons of vertebrate nerve cells.

In the central nervous system (CNS), the myelin sheath is formed by oligodendrocytes. In the area of ​​the peripheral nervous system (PNS) from the Schwann cells. The development of the myelin sheath was an important evolutionary step in vertebrates. An enormous increase in the nervous system could be achieved without the diameter of the axons having to increase immeasurably.

Structure of a myelin sheath

Figure 1: Nerve cell structure.

Here you can see an image of a nerve cell with its most important components. This includes the myelin sheath and the axon. The next two figures take a closer look at the axon and myelin sheath.

Figure 2: Cross section of myelin sheath.

The myelin sheath is made of myelin. After each individual section, axons are spirally wrapped by Schwann cells. This happens with the help of the myelin layers. Axons can be wrapped with up to fifty such double layers. In the central nervous system, extensions grow out of the soma of the oligodendrocytes, which enclose the axons with the myelin sheath. Several axon sections can be wrapped by a dendrocyte at the same time.

The nerve fibers that run from the myelin sheaths are called myelinated, sheathed, myelinated, or myelinated. The myelination of the axons begins during pregnancy, continues during childhood and is further refined during puberty. After that, only a small amount of myelination takes place.

Figure 3: Myelination process.

The figure shows how a Schwann cell wraps around an axon in three steps. The Schwann cell becomes flatter and longer. The cell plasma is squeezed out of the inside of the layers, so that in the end there are only membranes inside.

Functions and tasks of the myelin sheath

insulation

A myelin sheath provides the neurites mechanical protection. At the same time, it functions as an electrical insulator for peripheral nerves, which is only interrupted at the nodes of Ranvier. This isolation function reduces the membrane capacitance and prevents leakage currents through the membrane. When two axons touch, action potentials from one axon could trigger further action potentials on the other axon. Kind of a knee-jerk reaction.

forwarding

Another function of the myelin sheath is to conduct action potentials. This transmission is accelerated many times over by myelination. The larger the diameter of the axon, the greater the conduction velocity. Therefore, in the course of evolution, the axons in invertebrates became thicker and thicker.

The resting potential is slightly negative and is maintained by ion channels and the sodium-potassium channels in the membrane. If the nerve cell receives a certain stimulus, it becomes depolarized. The electrical conditions change for a short time and the action potential arises via the voltage-gated sodium-potassium ion channels.

This means that the cumbersome transmission of stimuli is bridged by the continuous transmission of the action potential and is replaced by the erratic transmission of stimulus from node to node. The action potentials jump on the axon, i.e. from Schürring to Schürring, which considerably shortens the transmission speed.

The transmission speed then increases from around 1 to 2 m/s without a myelin sheath to up to 120 m/s in axons with a thick myelin sheath. The narrower the nodes, the faster action potentials can develop there.

Myelin is primarily found in animal species in which many axons have to conduct very quickly in a very small space (vertebrates). In most of these animal species, conduction velocity is facilitated by an increase in axon diameter.

Myelin sheath in the central and peripheral nervous system

In the central nervous system

In the CNS, the myelin sheath is formed by the oligodendrocytes (= part of the glial cells). In contrast to the Schwann cells, they do not envelop the axons with their cell plasma, but only with extensions. An oligodendrocyte always has several processes, which is why it can also myelinate several axons in comparison to Schwann cells.

In the peripheral nervous system

In the peripheral nervous system, the myelin sheath is formed by Schwann cells. The axon is myelinated by several Schwann cells. The nodes of Ranvier are located at regular intervals between the Schwann cells. The Schwann cells envelop the axon several times with their cytoplasm until the axon is electrically isolated from the cytoplasm. We’re talking about here myelinated nerve fibersbecause they do not have a myelin sheath.

The structure of the myelin in the CNS differs from that of the PNS. The myelin sheaths in the CNS are usually thinner than those in the PNS.

Myelin Sheath – Diseases

Multiple sclerosis (MS) is an autoimmune disease that causes inflammation in the nervous system. Lymphocytes that enter the CNS via the blood-brain barrier trigger such inflammatory reactions. As a result, the myelin in the myelin sheath is broken down.

The myelin sheath is an insulating protective cover for nerve cells. Axons, the processes and the nerve cells in general are thus directly damaged. Permanent damage and disabilities occur as the disease progresses. The disease can affect the entire nervous system, such as the optic nerve.

The causes of this damage are not yet fully understood. The activation of the T cells takes place in the immune system, but the trigger is also unknown. A hereditary predisposition, vitamin D deficiency and a previously unknown viral infection are discussed. There is currently no therapy with a chance of recovery.