Final answer:
Myelin is formed by oligodendrocytes in the CNS and Schwann cells in the PNS, not by axons or ependymal cells, to facilitate efficient transmission of electrical signals along neurons.
Step-by-step explanation:
Myelin is a lipid-rich sheath crucial for increasing the speed and efficiency of electrical signal transmission along the axons of neurons. In the central nervous system (CNS), oligodendrocytes are responsible for the formation of myelin. The peripheral nervous system (PNS) relies on Schwann cells to produce the myelin sheaths that envelop axons. Therefore, myelin is not formed by axons themselves or ependymal cells.
While oligodendrocytes can insulate multiple axon segments within the CNS, a single Schwann cell focuses on a single axon segment in the PNS. The integration of lipids and important proteins in the myelin sheath is essential for the maintenance of its structure and function in both nervous systems. Thus, Schwann cells and oligodendrocytes collectively contribute to the myelination process in the nervous system.