Final answer:
The most common neurotransmitter in the human body is acetylcholine (ACh), which is involved in muscle contraction and various central and peripheral nervous system functions.
Step-by-step explanation:
The most common neurotransmitter in the human body is acetylcholine (ACh). Acetylcholine is a pivotal neurotransmitter that binds at a motor end-plate to trigger depolarization of the muscle, leading to muscle contraction. Apart from its role at the neuromuscular junction, it's also involved in many functions in the central and peripheral nervous systems. While norepinephrine and epinephrine (also known as noradrenaline and adrenaline respectively) act both as hormones and neurotransmitters in the body, and dopamine is a key neurotransmitter in the brain affecting neurological processes, acetylcholine is recognized for its widespread and predominant neurotransmitter activity.