Overview
Neurotransmitters are chemical messengers in the brain of all living species. Neurotransmitters work between a neuron and another cell, and are located in vesicles underneath the membrane on the presynaptic side of the synapse of a neuron. Neurotransmitters are released at the arrival of an action potential at the synapse of a neuron, however, they may also be released by graded electrical potentials (Wikipedia, 2008).
Different Neurotransmitters
Neurotransmitters can be identified by many different characteristics. Neurotransmitters can be broken up into amino acids, peptides, and monoamines. To date, scientists have only identified about ten neurotransmitters, however it is believed that there are many, many more (Wikipedia, 2008).
Acetylcholine is a neurotransmitter located in both the peripheral nervous system and the central nervous system. Acetylcholine is also the only transmitter in the somatic nervous system. Acetylcholine can activate muscles and the peripheral nervous system, and cause excitatory actions in the central nervous system.
Monoamine Transmitters are neurotransmitters that contain one amino group, which is connected to an aromatic ring through a two-carbon chain. This group of neurotransmitters are derived from aromatic amino acids such as tyrosine and phenylalanine. These include norepinephrine, dopamine, serotonin, and melatonin.
Amino Acids are glutamate, gamma aminobutyric acid (GABA), aspartate, glycine, and histamine (Wikipedia, 2008).
Purines include adenosine, ATP, GTP, and their derivatives.
Not only have these select types of neurotransmitters been discovered, but over 50 neuroactive peptides have been discovered, and new ones are discovered on a pretty regular basis.
Actions of Neurotransmitters
Glutamate is used in fast reflexes between the brain and spinal cord. It is also used in synapses that are “modifiable”, or capable of increasing or decreasing in strength. These synapses are assumed to be found in the memory-storage elements of the brain (Wikipedia, 2008).
GABA is used in fast inhibitory synapses in just about every area of the brain and plays a major role in alcohol intoxication.
Acetylcholine is a well-known neurotransmitter that is found at the neuromuscular junction connecting nerves to muscles. Acetylcholine also operates many different sections of the brain, but while using different receptors (Wikipedia, 2008).
Dopamine is critical to the reward system; it’s the neurotransmitter that makes us feel “happy”. Dopamine levels are also associated with Parkinson’s disease, as well as schizophrenia.
Serotonin is a complex neurotransmitter with many different effects on people. It plays the biggest role in bodily cycles, and temperature. Serotonin levels are modified by the amount of light you see, as well (Wikipedia, 2008).


References
  1. Wikipedia (2008) Neurotransmitters, http://en.wikipedia.org/wiki/Neurotransmitters
  2. ^ Neurotransmitter at Dorland's Medical Dictionary
  3. ^ //**a**// //**b**// Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone, page 474 for noradrenaline system, page 476 for dopamine system, page 480 for serotonin system and page 483 for cholinergic system.. ISBN 0-443-07145-4.