Neurotransmitters are molecules that communicate information between neurons and their target cells at chemical synapses. There may be hundreds of neurotransmitters, which can be categorized in several ways, such as by their molecular structure into amino acids, peptides, monoamines, and others.
Amino acids are organic molecules with amino (-NH2) and carboxylic acid (-COOH) groups. Peptides are polymers, or chains, of amino acids. Peptides are much larger molecules than all the other neurotransmitters, so sometimes they are just categorized as peptides or small molecule neurotransmitters. Monoamines, which are also called biogenic amines, are organic molecules with an amino group connected to an aromatic ring by a two-carbon chain. Some of the monoamines are also called catecholamines, which have a catechol group, which is benzene with two hydroxyl (-OH) side groups. There are many other types of neurotransmitters that are not amino acids, peptides, or monoamines.
Important amino acid neurotransmitters include glutamate, gamma aminobutyric acid (GABA), and glycine. Glutamate is the most common excitatory neurotransmitter of the nervous system, because it usually causes depolarization of target cells. GABA and glycine are the most common inhibitory neurotransmitters of the brain and the spinal cord, respectively, because they usually cause hyperpolarization of target cells. These neurotransmitters are involved in most functions of the nervous system.
Important monoamine neurotransmitters include serotonin, histamine, dopamine, epinephrine, and norepinephrine, the last three of which are also called catecholamines.
The monoamines play a role in many functions of the brain, including consciousness, attention, cognition, and emotion. Norepinephrine is also released from some autonomic neurons. Many disorders of the nervous system involve abnormalities of monoamine systems, and many drugs affect these systems.
Important peptide neurotransmitters include the group called the opioids, such as endorphin. The opioids play a role in many functions of the nervous system, particularly in the perception of pain.
An important neurotransmitter that is not in the other groups is acetylcholine. Acetylcholine participates in many functions of the nervous system, including being released by most autonomic neurons, as well as the neurons that synapse on skeletal muscle.