What Is a Neurotransmitter?
By Kendra Cherry, About.com Guide
The neurotransmitters cross the synaptic gap to reach the receptor site of the other cell or neuron
Image from the Wikimedia Commons
 A neurotransmitter is a chemical messenger that carries, boosts and modulates signals between neurons1 and other cells in the body.
In most cases, a neurotransmitter is released from the axon terminal after an action potential2 has reached the synapse. The neurotransmitter then crosses the synaptic gap to reach the receptor site of the other cell or neuron. Then, in a process known as reuptake, the neurotransmitter attaches to the receptor site and is reabsorbed by the neuron.
Neurotransmitters play a major role in everyday life and functioning. Scientists do not yet know exactly how many neurotransmitters exist, but more than 100 chemical messengers have been identified. When neurotransmitters are affected by disease or drugs, there can be a number of different adverse effects on the body. Diseases such as Alzheimer's and Parkinson's are associated with deficits in certain neurotransmitters.
Types of Neurotransmitters
Neurotransmitters can be classified by function:
 Excitatory neurotransmitters: These types of neurotransmitters have excitatory effects on the neuron; they increase the likelihood that the neuron will fire an action potential.
Some of the major excitatory neurotransmitters include epinephrine and norepinephrine.
 Inhibitory neurotransmitters: These types of neurotransmitters have inhibitory effects on the neuron; they decrease the likelihood that the neuron will fire an action potential. Some of the major inhibitory neurotransmitters include serotonin and GABA
Some neurotransmitters, such as acetylcholine and dopamine, can both excitatory and inhibitory effects depending upon the type of receptors that are present.
They can also be categorized as one of six types:
 Amino acids: Gamma-aminobutyric acid (GABA) and Glycine Glutamate Aspartate.
Neuropeptides: Oxytocin,endorphins, vasopressin, etc.
Monoamines: Epinephrine, norepinephrine, histamine, dopamine and serotonin.
Purines: Adenosine, ATP.
Lipids and gases: Nitric oxide, cannabinoids.
The actual identification of neurotransmitters can actually be quite difficult. While scientist can observe the vesicles containing neurotransmitters, actually figuring out what chemicals are stored in the vesicles is not quite so simple. Because of this, neuroscientists have developed a number of guidelines for determining whether or not a chemical should be called a neurotransmitter:
 The chemical must be produced inside the neuron
The necessary precursor enzymes must be present in the neuron
There must be enough of the chemical present to actually have an effect on the postsynaptic neuron
The chemical must be released by the presynaptic neuron, and the postsynaptic neuron must contain receptors that the chemical will bind to
There must be a reuptake mechanism or enzyme present that stops the action of the chemical