Glial Cells and Myelination of Nerve Fibers

‭Glial Cells and Myelination of Nerve Fibers‬ ‭✪The feet of the astrocytes form a continuous layer which isolates the nervous tissue from the CSF‬ ‭or the nervous tissue from the blood so there is a juxtaposition of the extensions of the‬ ‭ependymocytes, the feet of the astrocytes to form a sort of wall which separate the neurons from the‬ ‭CSF.‬ ‭ In certain areas of the CNS, the cavities are in direct relation with the meninges: around‬ ✪ ‭the wall of the neural tube there is connective tissue which will become the meninges which are‬ ‭envelopes that surround the entire CNS. In the innermost envelope:the pia mother there has a lot‬ ‭of ships. When the nerve tissue has developed the lumen of the tube remains not everywhere as‬ ‭light, it remains at the level of the spinal cord in the form of a small central ependymeric canal. In‬ ‭the brain there are no canals but cavities, cisterns called the ventricles. When we look at the brain or‬ ‭spinal cord from the outside to the inside we see the meninges, nervous tissue and a cavity filled‬ ‭with CSF. In certain cases, for example at the roof level of 3th ventricle there is between the cavity‬ ‭and the meninges no nervous tissue so there is a juxtaposition in places precise of the pia mater and‬ ‭the ependymocyte layer. At this location, the ependymocytes will transform:‬ ‭ In their shape: these cells become cubic, lose their cilia and have at the level of the apical pole of‬ ✪ ‭multiple microvilli.‬ ‭in their function:cells have acquired the capacity to secrete CSF. This CSF is not not just a dialysis‬ ‭of the plasma which would have come from the vessels but it is a liquid produced by these cubic‬ ‭ependymeric cells and these areas are called the plexuses choroid = an assembly of transformed‬ ‭ependymeric cells and connective tissue vessels belonging to the pia mater.‬ ‭-Glial cells exclusively in the PNS:‬ -‭ ‬‭Schwann cells.‬ ‭-‬‭Satellite cells found in the spinal ganglia and which surround the neurons in the‬ ‭nerve/spinal ganglia.‬ ‭-‬ ‭-‬‭-The axon of the neurons constitutes the nerves or nerve fibers. A nerve is made up of‬ ‭several nerve fibers of several axons. These nerve fibers can be surrounded or not‬ ‭different types of sheaths. So nerve fibers are in 4 categories depending on whether‬ ‭there is a myelin sheath and whether there is a Schwann sheath.‬ ‭-‬‭Myelinated fibers‬ ‭-‬‭Unmyelinated fibers‬ ‭-‬‭With or without Schwann sheath.‬ ‭-Unmyelinated fibers without Schwann sheath‬ ‭‬ ● ‭These are axons which are immediately after the bare emergence cone, without sheath‬ ‭around the neuronal extensions therefore only a limit by the plasma membrane which surrounds the‬ ‭axon = axolemma. It's at the origingin of nerve fibers at the level of the CNS and therefore these‬ ‭unmyelinated extensions will travel a certain distance between other extensions of neurons or glial‬ ‭cells.‬ ‭-Unmyelinated fibers with Schwann sheath‬ ‭-‬‭The Schwann cells are elongated and arranged like an Indian thread one above the other and‬ ‭which will be linked to each other so at each end of the cell there are extensions which are‬ ‭juxtaposed with others of the adjacent cell to form a conduit inside which the axons will be‬ ‭found.‬ ‭-‬‭The Schwann cell has a wall hollowed out by invaginations which can be simple or complex.‬ ‭Invaginations in relation to the exterior through a sort of small tunnel named themesaxon. Inside‬ ‭these invaginations is the axon (called the cylindrical). So the axons are surrounded by the‬ ‭cytoplasm of the cell but not inside Schwann's cell.‬