Nerve agent
Nerve agents work by blocking communication between the brain and the body.  Usually, a neurotransmitter (a chemical messenger) called acetylcholine gives the signal for the muscles to tighten up.  It also keeps them from relaxing.  However, when it is time for the muscles to relax, an enzyme called acetylcholinesterase destroys acetylcholine, and another neurotransmitter tells the muscles to relax.