Final answer:
Acetylcholinesterase is released from the structure labeled '9', breaking down acetylcholine into choline and acetate to terminate the signal transmission effectively.
Step-by-step explanation:
Acetylcholinesterase is the enzyme responsible for breaking down the neurotransmitter acetylcholine (ACh) after it has been released into the synaptic cleft and has served its purpose in signal transmission.
The structure labeled '9' would be where acetylcholinesterase is located, which catalyzes the degradation of acetylcholine into choline and acetate. This enzyme ensures that acetylcholine does not continue to stimulate the postsynaptic neuron or muscle fiber, allowing the nerve signal to be precisely controlled and the muscle to relax after contraction.