Final answer:
Acetylcholinesterase (AChE) degrades acetylcholine in the synapse, preventing it from causing prolonged muscle contraction.
Step-by-step explanation:
The enzyme that degrades acetylcholine, preventing it from accumulating in the synapse, is Acetylcholinesterase (AChE). Acetylcholinesterase resides in the synaptic cleft and functions to break down acetylcholine (ACh) into acetyl and choline. This enzymatic action is crucial because it ensures that acetylcholine does not remain bound to ACh receptors, which would otherwise lead to unwanted extended muscle contractions.