Myelin
Myelin is a lipid-rich biomembrane surrounding the axons of the neurons most vertebrate spirally and electrically insulated. Myelin was discovered in 1854 by the pathologist Rudolf Virchow ( 1821-1902 ) by light microscopy on tissue sections. He found in nerve fibers have a myelin sheath and suggested to call myelin (Greek myelòs = cord, brain ). The current term of myelin in biology and medicine goes back to detailed structural descriptions of the Parisian pathologist Louis -Antoine Ranvier in 1878. The essential for the function of axons annular recesses of the myelin sheath bear his name ( nodes of Ranvier ). In comparison to other biomembranes myelin has a very high lipid content (70%) and a relatively low protein content (30 %). Therefore, myelin appears in the macroscopic point of view knows why heavily myelinated regions are also referred to as " white matter " in the central nervous system, in contrast to the small myelinated " gray matter ". The fast -conducting sensory and motor axons of the peripheral nervous system are myelinated.
Function
The myelin sheath is used for electrical insulation of the axons of nerve cells. It is regularly interrupted along the axons of the Ranvier constriction rings. Only at the Ranvier constriction rings, action potentials arise. This structure allows saltatory conduction, which is significantly faster than continuous conduction non- myelinated fibers. Myelin is formed by cells: oligodendrocytes in the central nervous system, the peripheral nervous system, Schwann cells. That central nervous myelin is formed of cellular processes of the oligodendrocytes, has long been controversial and was first shown in 1962 by Mary Bartlett Bunge and Bunge Richard in electron micrographs. Myelin is often viewed as a particular feature of vertebrates. However, some invertebrate groups have functional and structural analogies.
Composition
Lipids
The lipid component (70%) consists of 25 % cholesterol, 20% galactocerebroside, 5% and 50% other phospholipids Galactosulfatid mainly phosphatidylethanolamine and lecithin.
Proteins
For the myelin -specific proteins are:
- Myelin basic protein or myelin basic protein (MBP)
- Proteolipid protein ( PLP/DM20 )
- Myelin - associated glycoprotein ( MAG)
- Connexin 32 ( Cx 32)
Central myelin
Peripheral myelin
- Protein zero (P0, MPZ ), Peripheral myelin protein -22 ( PMP -22)
Diseases
Multiple sclerosis is based on the destruction of myelin by the body's own immune system cells, thus represents a neurodegenerative autoimmune disease dar. This is also the Guillain -Barré syndrome, the case in which one's own immune system penetrates through the myelin sheath and nerve cells damaged or even severed. In contrast to multiple sclerosis, the cells repair themselves in this disease predominantly itself genetically inherited diseases which primarily affect the myelin in the central nervous system are called leukodystrophies. These include, inter alia, Pelizaeus- Merzbacher disease, Krabbe disease and X-linked adrenoleukodystrophy hereditary. A subgroup of hereditary neuropathies cause based on mutations in genes which are relevant for the myelin of the peripheral nervous system. Discussed is a role in the development of mental disorders such as schizophrenia beyond. Also in pernicious anemia, which is due to a lack of vitamin B12, it comes to the degeneration of the myelin sheaths and the resulting deficits.
Mouse mutants
Mice with specific defects in myelination are used for scientific investigation of this complex process. This allows a better understanding of the corresponding human hereditary diseases, leukodystrophies.
Historical
Of historical interest is a typical swelling of myelin in water to form wormlike forms. Another important feature of the Frankfurt physician Carl von Mette Home (1824-1898) discovered in 1858, the optical birefringence of myelin. From these properties, and based on our own experiments concluded the Karlsruhe physicist Otto Lehmann (1855-1922), the "Father of liquid crystals " that it is the myelin to "liquid crystals " is, precisely, a lyotropic liquid crystals in conjunction with solvent, here water form. Virchow has thus indeed observed for the first time a liquid crystal.
Swell
- Neurobiology
- Cell Biology
- Nervous tissue