Correct Response: B. The myelin sheath is a protective covering, composed of proteins and lipids, that surrounds an axon. It speeds up the transmission of electrical impulses by allowing a current to flow only along certain segments of the axon that are exposed to extracellular space. Although water (A) helps in electrical conductivity, it is the myelin sheath that increases the speed of electrical impulses along axons. The temperature of the body cannot be said to significantly affect electrical resistance along vertebrate axons (C), since the body temperatures of cold-blooded and warm-blooded vertebrates vary widely. All organisms use sodium to propagate action potentials along axons (D).
