Final answer:
The myelin sheath is composed of glial cells, specifically oligodendrocytes in the CNS and Schwann cells in the PNS. Its function is to insulate the axon for efficient transmission of nerve impulses.
Step-by-step explanation:
The myelin sheath consists of a glial cell. Specifically, it is formed by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). The myelin sheath acts as an insulating layer around the axon, allowing for more efficient transmission of electrical nerve impulses.