<prompt> The myelin sheath is formed around the nerve fibers in the central nervous system by:AstrocytesMicrogliaOligodendrocytesProtoplasmic astrocytes  <response>: Oligodendrocytes: Ref: 63-G, 634-35-SL 80- Ganong 23rdFunctions of the neuroglial cells Astrocytes - Astrocytes twist around the nerve cells and form the supporting network in brain and spinal cord.Astrocytes form the basis of blood brain barrier and thereby regulate the entry of substances from blood into brain tissues. Microglia: - Phagocytic in functions Oligodendrocytes: - responsible for the formation of myelin sheath also provide support by forming a semistiff connective tissue between the nerve cells. 