Final answer:
Acetylcholine is a neurotransmitter synthesized from choline and acetyl-coenzyme A, involved in various cholinergic systems in the body, particularly at neuromuscular junctions.
Step-by-step explanation:
The missing term in the student's question is acetylcholine, which is an organic chemical that functions as a neurotransmitter in many types of animals, including humans. Acetylcholine is synthesized from choline and acetyl-coenzyme A. The parts of the body that use or are affected by acetylcholine are known as cholinergic. For example, acetylcholine binds at a motor endplate to trigger depolarization, which is a critical step in muscle contraction.
In the nervous system, different neurotransmitter systems are categorized based on their chemistry and the receptors to which they bind. The cholinergic system refers to the neurotransmitter system of acetylcholine, which includes its receptors and the enzyme acetylcholinesterase that breaks down acetylcholine. Amino acids like glutamate, glycine, and GABA serve as neurotransmitters in their specific systems—glutamatergic, glycinergic, and GABAergic—signaling through their own distinct receptors.