Why do chemical impulses only go in one direction across a synapse?

Ca2+ channels are only in the membrane of the presynaptic neuron. ACh-containing vesicles are only within the presynaptic neuron. (ACh only produced within the presynaptic neuron, has choline and acetyl)APs only travel in one direction down axon, from dendritic spines to axon terminal (due to hyperpolarisation)Gated Na+ channels (with ACh receptors) are only in the membrane of the postsynaptic neuron.