Final answer:
The limiting factor in the synthesis of acetylcholine is the speed at which choline is recycled. This process is important for maintaining proper functioning of neuronal transmission.
Step-by-step explanation:
The limiting factor in the synthesis of acetylcholine is the speed at which choline is recycled. ACh is broken down by the enzyme acetylcholinesterase (AChE) into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors. This recycling process is important for maintaining proper functioning of neuronal transmission.