Which term describes the white lipid covering of axons?

• A. Axon terminal
• B. Dendrite
• C. Myelin sheath
• D. Synapse

Answer: (C)

The white lipid covering of axons is the myelin sheath. This insulating layer is made by glial cells—Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system—and it wraps the axon in segments with gaps called nodes of Ranvier. The myelin sheath prevents current leakage and dramatically increases the speed of electrical signaling via saltatory conduction, so impulses hop from node to node rather than traveling continuously along the axon. The white appearance of myelin also explains why brain tissue with many myelinated fibers is referred to as white matter. An axon terminal is the sending end where neurotransmitters are released, a dendrite is a receiving branch of a neuron, and a synapse is the junction between neurons—none describe the lipid insulating covering of the axon.