23. Why are action potentials usually conducted in one direction? (A) The axon hillock has a higher membrane potential than the terminals of the axon. (B) Voltage-gated K+ channels cause membrane potential overshoot after the action potential. (C) The brief refractory period prevents reopening of voltage-gated Na+ channels. (D) Voltage-gated channels for both Nat and K+ open in only one direction.