Scientists have identified at least fifty neurotransmitters in the nervous system, and there may be several dozen more. There are four groups of neurotransmitters: 1) acetylcholine, 2) amino acids, 3) monoamines, and 4) neuropeptides.

Acetylcholine, perhaps one of the best-known neurotransmitters, is the most important neurotransmitter between motor neurons and voluntary muscle contraction. It has an inhibitory effect on heart muscle and excitatory effect on smooth muscles, through the effects on different types of acetylcholine receptors.

Amino acid neurotransmitters include glutamate and asparate. These neurotransmitters are some of the most potent excitatory neurotransmitters in the central nervous system. They are found in the brain.

There are two important groups of monoamines: catecholamines and indoleamines. Catecholamines include norepinephrine and dopamine. Serotonin, believed to be involved in sleep, mood, appetite, and pain, is an indoleamine.

Neuropeptides include somatostatin, endorphins, and enkephalins. Somatostatin is a growth-hormone inhibiting hormone. Endorphins and enkephalins suppress synaptic activity leading to pain sensation.