Final answer:
The enzyme acetylcholinesterase inactivates the neurotransmitter acetylcholine (ACh), leading to a decreased electrical response in muscle cells.
Step-by-step explanation:
The enzyme acetylcholinesterase (AChE) present on the motor end plate membrane inactivates the neurotransmitter acetylcholine (ACh) by breaking it down into acetyl and choline. This prevents ACh from remaining bound to ACh receptors on the muscle cells, leading to a decreased electrical response of the muscle cells. The inactivation of ACh by AChE ensures that the muscle contraction is not prolonged and allows for proper muscle function.