Explain why postsynaptic potentials are transient (i. e. short lived). In other words, what terminates the action of the neurotransmitter on the postsynaptic cell?

Answer: The action of a neurotransmitter at the postsynaptic cell is terminated by the following: enzymatic degradation, diffusion or reuptake into the presynaptic cell by specific transporters.

Explanation: After a neurotransmitter has bound to its receptor on the postsynaptic membrane and triggered a biological response, in order to avoid over stimulation of the receptors, the neurotransmitter is either degraded by specific enzymes or taken back to the presynaptic cell into the synaptic vesicles by action of specific transporters. This mechanism is to maintain a balance (homeostasis).