Nicotine mimics acetylcholine in the brain. acetylcholine is classified as a

That is a neurotransmitter that makes your muscles move

Acetylcholine which is an ester of acetic acid and choline is a type of neurotransmitter released by the parasympathetic nervous system (The one responsible for the rest and digest). Acetylcholine is uses in a variety of areas by the body, primarily muscles to induce movements, but there areas of the brain, that use it to stimulate certain emotions and reactions.

Acetylcholine is an incredibly vital neurotransmitter. Certain object we call "Nerve agents" like Satin, VX, and Soman are incredibly dangerous because they interfere with acetylcholine e release by actually overstimulating it. This massive flood of Acetylcholine by the agents we call cholinergics can induce muscle spasms, of every muscle in the body including of organs like the heart... Which is bad.