Acetylcholine (ACh) is the most common type of neurotransmitter, and the most well understood. It's found in parts of the peripheral nervous system, spinal cord, and areas of the brain. In the peripheral nervous system, ACh activates muscles that help the body move. When Ach is released to the muscle cells, the muscle contracts. In the brain, ACh is involved in breathing, attention, arousal, motivation, etc. Obviously there are many problems that can occur if ACh is blocked (muscles can't contract). One example is the black widow spider uses venom that causes a flood of ACh into muscle cells and results in violent, uncontrollable muscle contractions, paralysis, and death for it's prey.