Why is an action potential conducted in only one direction, from an axon hillock to an axon terminal? Why is an action potential conducted in only one direction, from an axon hillock to an axon terminal? Second messengers activate channels sequentially. The number of voltage-gated ion channels increases along the length of the axon. The membrane channels upstream are refractory and cannot open. The channels are progressively easier to open down the length of the axon.