Which of the following statements are true about the propagation of a nerve impulse? Select all the true statements.
a. In mammals, actions potentials propagate more slowly because they are insulated by a myelin sheath, b. The action potential can only travel in one direction along the axon. c. In propagation of a nerve impulse relies on changes in membrane potential more positive. d. The action potential triggers the opening of voltage-gated potassium channels.