Final answer:
The myelin sheath consists of concentric layers of plasma membrane with a high protein/lipid ratio, providing insulation for the nerve cell axon and facilitating faster transmission of electrical signals.
Step-by-step explanation:
The myelin sheath is a lipid-rich layer of insulation that surrounds the axon of a nerve cell. It is composed of concentric layers of plasma membrane that have an extremely high protein/lipid ratio. The lipids in the myelin sheath help to create an electrical insulator, allowing for faster transmission of electrical signals along the axon.