We are learning that neurotransmitters play a key role in controlling behavior, and understanding how they work has become a vital part of physiological psychology. Over a dozen neurotransmitters have been extensively studied, with many other substances suspected of serving in this capacity. Let's take a look at the major classes of neurotransmitters: acetylcholine, monoamines, amino acids, and neuropeptides. One of the first neurotransmitters to be identified was acetylcholine, which is involved in the activation of muscles as well as in arousal, memory, and motivation. Recent research suggests that the severe memory disruption in persons with Alzheimer's disease is associated with the degeneration of brain neurons that use acetylcholine (Quirion, 1993). The monoamine neurotransmitters include serotonin, dopamine, and norepinephrine. Serotonin is produced in the brain stem and is involved in sleep, sensory experiences, eating disorders, and mood. Research has suggested serotonin is involved in some types of hallucinations, and it has been implicated in the psychological disorder called obsessive-compulsive disorder (Pigott, 1996). Low levels of serotonin are associated with depression (Jacobs, 1994); thus antidepressant drugs such as Prozac help patients by raising the level of serotonin in the brain.