Final answer:
Myelin is formed by glial membranes, specifically by oligodendrocytes in the CNS and Schwann cells in the PNS, and it is crucial for insulating neuronal axons and facilitating rapid signal transmission.
Step-by-step explanation:
Myelin, which insulates many neuronal axons, is formed by d. glial membranes. The myelin sheath is a lipid-rich layer of insulation that surrounds an axon, and is essential for the proper functioning of the nervous system. This sheath is created by two different types of glial cells: oligodendrocytes in the Central Nervous System (CNS) and Schwann cells in the Peripheral Nervous System (PNS), both of which facilitate the rapid transmission of electrical signals along the axon.
The process of myelination involves the wrapping of the glial cell's own membrane around the axon, contributing to the nerve's function akin to how insulation covers electrical wires, ensuring faster and more efficient nerve signal transmission.