Final answer:
The muscle relaxes if no new nerve signal arrives. However, the neurotransmitter from the previous stimulation is still present in the synapse. The activity of acetylcholinesterase helps to remove this neurotransmitter (Option A).
Step-by-step explanation:
When a muscle relaxes because no new nerve signal arrives, the neurotransmitter from the previous stimulation that is still present in the synapse needs to be removed.
The activity of an enzyme is crucial in this process. The correct answer to which enzyme removes the neurotransmitter is acetylcholinesterase.
This enzyme degrades acetylcholine, which is the neurotransmitter typically involved in neuromuscular junctions where nerve cells meet muscle cells.
The removal of acetylcholine by acetylcholinesterase allows the postsynaptic membrane to reset and be ready for the next signal.
If acetylcholinesterase is inhibited, it would lead to increased neurotransmission at acetylcholine synapses because the neurotransmitter would remain in the cleft to continually bind and unbind to postsynaptic receptors.
Hence, the correct answer is: Acetylcholinesterase (Option A).