|Structure of a nerve cell|
The myelin sheath (also myelin sheath , in ancient Greek μυελός myelos , German 'marrow' ) is a lipid-rich layer that surrounds the axons of some nerve cells in vertebrates . It consists of myelin , which is formed by glial cells ( oligodendrocytes in the CNS , Schwann cells in the PNS ) and wraps the axons in a spiral. The myelin sheath serves to reduce membrane conductance and membrane capacity and thus enables particularly rapid saltatory conduction .
In the CNS, processes originate from the soma of an oligodendrocyte, each enveloping a section of an axon. An oligodendrocyte can thus be involved in the sheaths of around 10–50 axons. A Schwann cell in the PNS wraps around a section of an axon, with many Schwann cells lying one behind the other when the long neurite is enveloped and thus becomes an axon.
During the formation of myelin sheaths, the myelogenesis or medullary maturation , the Schwann cells grow several times around the nerve cell process and wrap it up to fifty times with double layers of their cell membrane . The cell membrane layers come to lie directly against one another. Normally there is no longer any cytoplasm between two inner membrane surfaces, but they are fused together to form the main line or inner attachment line , where the basic myelin protein is located. The cell membranes are not fused at the points of the so-called Schmidt-Lantermann notches or myelin incisions . Here there are cytoplasmic bridges for the exchange of substances, which run through all the myelin layers of the myelin sheath as narrow oblique stripes that are visible with a light microscope and are connected to one another.
Nerve fibers with well-developed myelin sheaths are referred to as medullary or medulla-rich , and thus differentiated from medullary-poor fibers with less developed myelin sheath and unmixed fibers without myelin sheath. A strong myelin sheath protects the enveloped neurite mechanically and electrically isolates it from the surrounding environment; the greater distance between intracellular and extracellular fluid also lowers the specific membrane capacity.
Active excitation conduction is slower than passive (purely electrotonic ) excitation conduction . In the saltatory conduction of excitation, action potentials are only built up at the so-called Ranvier cord rings , where the thick myelin sheath of the axon is interrupted between two enveloping glial cells. The axon section between two such cord rings, each approximately 0.2 to 1.5 millimeters long, is called the internode . In these internodal segments, the isolating myelin sheath reduces leakage currents through the membrane, which is why the electrotonic propagation of the depolarization is possible with little loss over this long distance (larger membrane longitudinal constant ). The reduced membrane capacity accelerates the charge reversal of the membrane (smaller membrane time constant ).
When multiple sclerosis and other demyelinating diseases just this myelin sheath of axons is destroyed in parts - in the case of MS by autoantibodies such as anti- MOG - antibodies and anti- MBP antibodies. This process, which occurs during the disease flare-up, is known as inflammatory demyelination .
- Renate Wahrig-Burfeind (Ed.): True. Illustrated dictionary of the German language . ADAC-Verlag, Munich 2004, ISBN 3-577-10051-6 , pp. 590 .
- Ulrich Welsch : Sobotta textbook histology. Cytology, histology, microscopic anatomy . 2nd, completely revised edition. Elsevier, Urban & Fischer, Munich et al. 2006, ISBN 3-437-44430-1 , pp. 189 .
- Ulrich Welsch : Sobotta textbook histology. Cytology, histology, microscopic anatomy . 2nd, completely revised edition. Elsevier, Urban & Fischer, Munich et al. 2006, ISBN 3-437-44430-1 , pp. 186-188 .