3. How do myelin sheaths increase the speed with which action potentials are propagated along an axon? (A) They increase the permeability of Na+ ions in the plasma membrane. (B) They amplify the action potential by increasing sodium influx along the entire distance of the neuron. (C) They insulate the axon, preventing K+ ions from exiting the neuron. (D) They cause action potentials to "jump" down the axon rather than travel in a continuous path along every site on the axon.