Neurotransmitters are substances that allow neurons to communicate with each other in the synapse. There are, of course, many substances found in the synapse, yet not all of them are neurotransmitters. Before a substance is given this name, it must meet 4 criteria;

• The substance must be produced in a neuron
• It is present in the presynaptic end of a neuron (messenger), and in sufficient quantity to elicit an action in the postsynaptic neuron (receiver).
• When the same substance is administered from the outside, a similar effect should occur as the real substance does.
• At the site in the brain where the substance is active, a specific mechanism must be found that causes the breakdown of this substance

If it is decided that a substance is a neurotransmitter, then it can also be further subdivided into a category. Since there are so many neurotransmitters, it is therefore useful to make a subclassification in this way; 

1. Amino acids

This category includes the well-known neurotransmitters GABA and glutamate. Glutamate is involved in adjusting the connections between two neurons, and thus has an important role in learning and memory. GABA receptors are located all over the brain, and when sedative drugs are taken, these receptors become overactive, resulting in fewer stimuli being perceived.

2. Monoamines

This category includes the well-known neurotransmitters dopamine and serotonin. Dopamine is the most important substance in the brain's reward system, this makes you want to repeat a pleasant experience more often. In addition, dopamine is associated with Parkinson's disease and Schizophrenia. Serotonin is involved in the most basic human needs, such as regulating sleep, temperature, hunger and thirst. In addition, it has a role in regulating mood, and too little intake of serotonin is therefore associated with depression.

3. Peptides

This category includes substance P. This is a substance involved in the transmission of pain from nerve cells to the pain center in the brain.

4. Other

This category includes the neurotransmitter acetylcholine. When people control their muscles, the signals must first be transferred from the brain to the muscles. This happens through synapses that use this neurotransmitter.

With respect to neurotransmitters, we often talk about excitatory and inhibitory neurotransmitters. For example, the amino acid GABA is said to be an inhibitory neurotransmitter. However, if we study the reactions at the synapse more closely, we see that neurotransmitters function as messengers, but do not elicit a response in the next cell themselves. To say that these neurotransmitters are excitatory or inhibitory is therefore incorrect, it is always the combination of the neurotransmitter with the receptor it binds to that determines if it results in an excitatory or inhibitory response.

Author: Myrthe Princen (translated by Joyce Burger)