Explain how the structure of a chemical synapse ensures that neural signals are transmitted in one direction only.

Synaptic vesicles containing neurotransmitter are found only in the presynaptic terminal, so the presynaptic neuron can release the signal.
Receptor proteins (ligand‑gated ion channels) for that neurotransmitter are located only on the postsynaptic membrane, so only the postsynaptic neuron can respond.
When the vesicles fuse, neurotransmitter diffuses across the synaptic cleft and binds to the postsynaptic receptors, generating a postsynaptic potential.
The presynaptic membrane lacks these receptors, so the postsynaptic neuron cannot trigger a signal back into the presynaptic neuron – the signal is therefore unidirectional.