Since the binding of neurotransmitters to the postsynaptic receptors is reversible, after these neurochemicals perform their role they must be eliminated from the synaptic cleft. Neurotransmitters can then bind to specific proteins that carry them back to the axon they came from in a process called neurotransmitter re-uptake. They can also be destroyed by specific enzymes, like acetylcholinesterase, an enzyme that destroys acetylcholine. Or they can simply diffuse out of the synaptic cleft.
