Nerve Agents
All nerve agents share very similar mechanisms. The nerve agent enters the body and inhibits the normal actions of acetylcholinesterase, a chemical whose function is to break down acetylcholine, which is responsible for muscular contraction. Therefore, nerve agents cause violent muscular spasms, including in the heart.

source: http://whyfiles.org/025chem_weap/4.html
The following nerve agents were developed just before or during World War II by German scientists but were never used in the war (click on the molecule to see more information):