Scientists have identified more than 50 neurotransmitters but there are certainly more. The most common are acetylcholine and adrenaline. Acetylcholine was the first neurotransmitter to be discovered. It has an excitatory effect at the junction between nerves and muscles. Nerve pathways which use acetylcholine as a neurotransmitter are generally involved in memory, motivation and arousal.
Dopamine is another neurotransmitter.
A shortage of dopamine in the brain causes Parkinson's disease. Over-activity of the nerve cells that release dopamine seems to be related to schizophrenia. Serotonin is an important neurotransmitter that has received a lot of attention by the scientific community because it has been discovered that deficiencies in serotonin in the brain are associated with depression. Additionally, deficiencies of serotonin, or disturbances in serotonin transmission, seem to underlie many psychiatric problems such as bulimia and kleptomania.