3 Major Functional Classes of Neurotransmitters
The neurotransmitter “juice” squirted from the presynaptic to the postsynaptic neuron consists of neurotransmitter molecules. A variety of different types of neurotransmitters exist, each varying in its type of effect and time course on the postsynaptic cell. Neurotransmitters are grouped into three major functional classes:
- Fast, excitatory neurotransmitters: The most important neurotransmitters are the fast, excitatory neurotransmitters glutamate and acetylcholine. These communicate strong, immediate excitation from the presynaptic to the postsynaptic cell. Most long-distance neural projections in the brain are made by axons that release glutamate. All voluntary muscle contractions are caused by the release of acetylcholine by motor neurons onto muscle cells.
- Fast, inhibitory neurotransmitters: Because large interconnected systems made only of excitatory connections tend to be unstable and subject to seizures (strong, uncontrolled, sustained, and recurrent excitation), subtlety and balance in the nervous system is guaranteed by having inhibitory connections, using the fast inhibitory neurotransmitters gamma amino butyric acid (GABA) and glycine. Inhibitory neurotransmitters allow for computations like winner-take-all. For example, if most of the neurons are voting that you’re looking at an elephant but a couple here and there are saying, “If it has four legs, it could also be a donkey,” the inhibitory neurotransmitters suppress the “it could be a donkey” message.
- Slow neuromodulators that include both excitatory and inhibitory types: Most of the fast neurotransmitters are amino acids (other than acetylcholine). Modulating neurotransmitters include biogenic amines, such as dopamine and serotonin, and small proteins like somatostatin, substance P, enkephalins, or organic chemicals called catecholamines, such as epinephrine and norepinephrine. The actions of these transmitters are more hormonelike, producing slow but long lasting changes. Think of their action as setting the “mood” within the brain as a background for what the faster neurotransmitters are doing at shorter time scales. For example, you might be resting quietly while digesting a good meal (which is a good idea) when a lightning storm strikes, triggering the release of norepinephrine, the fight or flight neurotransmitter. Its action shuts down digestion and other internal homeostatic processes in favor of increasing your pulse and blood pressure and diverting blood flow to the muscles in preparation for moving quickly to safety.
Some neurotransmitters are thought to have originated evolutionarily as hormones. Hormones are typically secreted into the blood stream where they influence cells that are receptive to them all over the body. Neurotransmitters, released by axons, appear to have been a way nervous systems evolved such that one cell could talk specifically to just a few other cells — something like whispering a secret to a few of your best friends rather than shouting news to an entire auditorium.