Final answer:
Muscle relaxation is caused by the enzyme acetylcholinesterase decomposing acetylcholine. This enzyme breaks down acetylcholine into acetyl and choline, preventing extended muscle contraction.
Step-by-step explanation:
Muscle relaxation is caused by the decomposition of acetylcholine via the enzyme acetylcholinesterase. Acetylcholine (ACh) is broken down by acetylcholinesterase into acetyl and choline, preventing ACh from remaining bound to ACh receptors and causing extended muscle contraction. This enzyme plays a crucial role in ending the transmission of nerve impulses and allowing the muscle to relax.