Acetylcholine is both a central and peripheral neurotransmitter. Synthesized from dietary choline and acetyl coenzyme A, this neurotransmitter and similar substances play a central role in depression, therapies for Parkinson's and Alzheimer's diseases, and in chemical warfare against insects and humans.

View a second image of crystallized acetylcholine.

Originally referred to as "vagusschtuff", because it was found to be the substance released by the stimulation of the vagus nerve, altering heart beats, acetylcholine was one of the first neurotransmitters discovered. The synaptic action of acetylcholine on the receptor is terminated by enzymatic cleavage with acetylcholinesterase. The choline is then returned to the neuron terminal for resynthesis of acetylcholine.

Either excitatory or inhibitory, acetylcholine's actions are dependent on the type of tissue and the nature of the receptor with which it is interacting. Alkaloids, especially nicotine and muscarine, have specific receptors in the human central nervous system for acetylcholine transmission. Nicotine receptors are selectively found in the brainstem and spinal cord, while the muscarinic receptors are more diffuse in their distribution. Nicotinic responses are fast to initiate, of short duration, and usually excitatory. The neurotoxin from the cobra's venom, which binds specifically to the nicotinic receptor, assisted research scientists in the purification of the receptor protein.