Final answer:
Acetylcholine used in parasympathetic transmission is synthesized inside synaptic vesicles. It is released during neurotransmission and interacts with receptors on the post-synaptic membrane. Acetylcholinesterase in the synaptic cleft breaks down ACh to terminate the signal.
Step-by-step explanation:
Acetylcholine (ACh) used in parasympathetic transmission is synthesized inside synaptic vesicles. The process involves several steps, starting with the synthesis of the protein in the neuron's cytoplasm and then the packaging of ACh into vesicles for storage and release during neurotransmission. When a nerve impulse arrives at the nerve terminal, ACh is released from the vesicles into the synaptic cleft and binds to ACh receptors on the post-synaptic membrane, completing the process of signal relay from one neuron to another or from a neuron to a muscle fiber at the neuromuscular junction. Acetylcholinesterase, the enzyme that breaks down ACh after the signal transmission is complete, is located in the synaptic cleft and is not free to diffuse away from the synapse. It ensures the termination of the signal by degrading ACh.